YEARBOOK OF PHYSICAL ANTHROPOLOGY 41:93–136 (1998)

Environmental Hypotheses of Hominin Evolution

RICHARD POTTS Human Origins Program, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0112 KEY WORDS climate; habitat; adaptation; variability selection; fossil humans; stone tools

ABSTRACT The study of human evolution has long sought to explain major adaptations and trends that led to the origin of Homo sapiens. Environmental scenarios have played a pivotal role in this endeavor. They represent statements or, more commonly, assumptions concerning the adap- tive context in which key hominin traits emerged. In many cases, however, these scenarios are based on very little if any data about the past settings in which early hominins lived. Several environmental hypotheses of human evolution are presented in this paper. Explicit test expectations are laid out, and a preliminary assessment of the hypotheses is made by examining the environmental records of Olduvai, Turkana, Olorgesailie, Zhoukoudian, Combe Grenal, and other hominin localities. Habitat-specific hypotheses have pre- vailed in almost all previous accounts of human adaptive history. The rise of African dry savanna is often cited as the critical event behind the develop- ment of terrestrial bipedality, stone toolmaking, and encephalized brains, among other traits. This savanna hypothesis has been countered recently by the woodland/forest hypothesis, which claims that Pliocene hominins had evolved in and were primarily attracted to closed habitats. The ideas that human evolution was fostered by cold habitats in higher latitudes or by seasonal variations in tropical and temperate zones also have their propo- nents. An alternative view, the variability selection hypothesis, states that large disparities in environmental conditions were responsible for important episodes of adaptive evolution. The resulting adaptations enhanced behav- ioral versatility and ultimately ecological diversity in the human lineage. Global environmental records for the late Cenozoic and specific records at hominin sites show the following: 1) early human habitats were subject to large-scale remodeling over time; 2) the evidence for environmental instabil- ity does not support habitat-specific explanations of key adaptive changes; 3) the range of environmental change over time was more extensive and the tempo far more prolonged than allowed by the seasonality hypothesis; and 4) the variability selection hypothesis is strongly supported by the persistence of hominins through long sequences of environmental remodeling and the origin of important adaptations in periods of wide habitat diversity. Early bipedality, stone transport, diversification of artifact contexts, encephaliza- tion, and enhanced cognitive and social functioning all may reflect adapta- tions to environmental novelty and highly varying selective contexts. Yrbk Phys Anthropol 41:93–136, 1998. ௠ 1998 Wiley-Liss, Inc.

TABLE OF CONTENTS A Paleoenvironmental Primer ...... 95 Marine oxygen isotope analysis ...... 96 Ocean dust records ...... 98 Ice cores ...... 99

௠ 1998 WILEY-LISS, INC. 94 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 Sedimentary environment ...... 99 Lake sediments ...... 99 Loess sequences ...... 101 Fossil pollen sequences ...... 101 Stable isotope analysis of soils ...... 101 Faunal analysis ...... 104 Summary ...... 104 A Framework for Analyzing Adaptation ...... 104 Environmental Hypotheses of Hominin Evolution ...... 106 A concise history ...... 106 Challenges of the savanna ...... 106 Intrinsic and extrinsic approaches ...... 106 Turnover hypotheses ...... 108 Adaptive evolution in hominins ...... 109 Habitat-specific hypotheses ...... 109 Short-term variability hypothesis ...... 111 Long-term (variability selection) hypothesis ...... 111 Test implications ...... 112 Habitat-specific hypotheses ...... 112 Short-term variation (seasonality) hypothesis ...... 113 Variability selection hypothesis ...... 113 Environmental Records at Early Human Localities ...... 114 6.0 to 2.5 million years ago ...... 114 Eastern and southern Africa ...... 114 Discussion ...... 116 2.3 to 1.7 million years ago ...... 118 Turkana and Olduvai basins ...... 118 Discussion ...... 119 1 million to 250,000 years ago ...... 122 Olorgesailie basin ...... 122 Zhoukoudian, the Loess Plateau, and Bose basin, China ...... 122 Discussion ...... 125 140,000 to 50,000 years ago ...... 125 Combe Grenal, France ...... 125 Discussion ...... 127 Environmental Perspectives on Adaptive Change ...... 128 Conclusions ...... 130 Acknowledgments ...... 131 Literature Cited ...... 132

Regarding the study of human evolution, can be linked to in situ evidence of homi- it may be remarked that everybody talks nins.1 Usually, finding fossils and dating about the environment, but nobody does them have overshadowed any effort to deter- much about it. The environment of human evolution has been a source of speculation 1Although it will take getting used to, the term hominin is now preferred in place of hominid to indicate the group of bipedal apes since Darwin. Analyses of animal bones, to which humans belong (genera Homo, Australophitecus, Paran- pollen, and sediments have rendered a gen- thropus, and Ardipithicus) (Begun, 1992; Andrews, 1992; An- drews et al., 1996, Delson and Tattersall, 1997). The systematics eral portrait of Pliocene and Pleistocene of apes, including humans, now recognizes the growing body of evidence that humans and chimpanzees are sister taxa. Within habitats. Yet field projects are rarely de- the superfamily Hominoidea (apes) is the family of great apes signed specifically to recover high-resolution (Hominidae). The hominid family includes the subfamily of African apes (Homininae), or hominines. The tribe rank of data on the ecological settings inhabited by Hominini, then, is reserved for humans and related bipeds since early humans. Thus, there is a paucity of the split from the chimpanzee lineage. Because early Pliocene bipeds no longer comprise a valid subfamily, I use the informal fine-scale spatial and temporal data that term australopiths to refer to them. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 95 mine habitat contexts. A related problem is may contribute decisive data to the study of that most projects still focus on surface hominin evolution. collecting, with unavoidably poorer tapho- A final aim is to offer a brief primer to help nomic and contextual control than obtain- anthropologists comprehend the barrage of able by digging. It is, however, only with paleoenvironmental methods, data, and in- exact stratigraphic relating of environmen- terpretations published almost weekly in tal data, fossils, and artifacts that answers journals such as Nature, Science, Quater- may emerge about the range of habitats nary Research, and other scientific periodi- associated with particular species, when key cals. A basic familiarity with isotope data, expansions of habitat use occurred, or other deep-sea cores, pollen analysis, loess climate substantive questions related to hominin curves, orbital parameters, and the main adaptive change. sources of paleoenvironmental evidence and As fossil discoveries have sparked more habitat change is essential for understand- than a century of ideas on the phylogenetic ing the contexts in which hominins evolved. history of hominins, so research on the con- To the extent that context is important—and texts of fossils and artifacts may come to Darwinian and later evolutionary biology stimulate a long phase of careful work on the established its fundamental role in organis- ecological history of hominins. The immedi- mal evolution—the field of paleoanthropol- ate goal is to make this endeavor as system- ogy is, in this view, on the brink of novel atic, empirically based, and comparative as ideas and data sets concerning the how and possible. Clear definition of hypotheses and why of hominin evolution. Tremendous ad- their test implications can help serve this vances in the environmental sciences are goal. forcing these developments to occur. The first purpose of this paper is to define pivotal hypotheses about the context of early A PALEOENVIRONMENTAL PRIMER human evolution and to lay out their test implications. Second, I will present some of An intricate system of heat and moisture the best-known environmental sequences circulation determines Earth’s climate and from hominin sites as a preliminary series of has strong influence over the distribution of tests. Environmental hypotheses can be di- environmental zones and gradients. This vided into two types: those concerning adap- enormous heating and plumbing system tation and others concerning species turn- links the atmosphere, oceans, and land. It is over. Although the latter will be discussed affected by orbital cycles that determine the briefly, our focus is on environmental hypoth- planet’s variable dose of solar radiation. eses as they relate to adaptive evolution. Volcanic activity and uplift of the crust also This topic is of marked interest because impact it. The basic processes and factors adaptive explanations relate directly to the that therefore contribute to this circulation processes of natural selection and therefore system are help to discern how important evolutionary transformations took place. ● the amount and yearly pattern of solar Historically, the search for a convincing radiation, determined largely by the plan- narrative of human evolution has stimu- et’s distance, orbital shape, and axial tilt lated strong interest—and guesswork— relative to the sun; about the adaptive history of hominins. Thus, ● the amount of solar radiation absorbed essentially every account of hominin evolu- and reflected by the atmosphere and by tion describes or assumes something about the ground surface; the milieu of human ancestors and the envi- ● atmospheric processes, including evapora- ronmental problems that uniquely human tion, cloud formation, and rainfall; features supposedly solved or overcame. Our ● ocean and wind currents, two related pat- focus here on environmental hypotheses with terns of circulation that redistribute heat an adaptive flavor requires a framework for and moisture in a biased manner around analyzing adaptation. So a third goal of this the globe; paper is to outline such a framework, which ● the size and distribution of ice caps, land will show how paleoenvironmental studies masses, and major bodies of water, all of 96 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 which create complex feedback with atmo- nishes a record of glacial ice volume (which spheric processes; contains the sequestered 16O) and tempera- ● land uplift, which may interrupt or deflect ture.4 Planktonic species of forams give infor- ocean and atmospheric circulation cur- mation about sea-surface conditions, while rents and may create rain shadows that benthic forams provide data about the deep- bias the distribution of rainfall near high- ocean environment, which is considered to lands; and reflect a wider record of ice (evaporation) ● volcanism, which alters climate by the and temperature history. This history is eruption of airborne particles, and re- reconstructed by measuring ␦18O in foram gional landscapes by the deposition of skeletons found in cores drilled from the lava or tephra (e.g., ash, pumice). deep sea. ␦18O curves are often thought to A series of methods has been developed to provide a measure of global climate, but this measure the interactions and effects of these idea only refers to ocean temperature and processes on Earth’s environments. All of ice volume, a history that may be decoupled these methods are applicable to the period of from environments on land (e.g., deMenocal human evolution; indeed, some of their most et al., 1993). fruitful applications have been in the con- Figure 1 depicts the average curve for text of paleoanthropological research. benthic forams from 70 Ma to the present. According to this curve, substantial global Marine oxygen isotope analysis cooling of oceans took place during this Interaction between the ocean and atmo- period, with especially marked ice-sheet for- sphere lies at the heart of oxygen stable mation over the past 3 myr or so. By looking isotope analysis, a technique that has revolu- at specific ocean cores, however, one sees tionized paleoclimate studies since the 1950s. that this average cooling/drying trend is 18 During evaporation, the stable isotope Ois actually composed of many oscillations, the 16 enriched in ocean water as the lighter Ois amplitude of which has risen over the past 6 2 preferentially released into the atmosphere. myr (Fig. 2A). Sets of oscillations occurred In building their carbonate skeletons, cer- within warmer and colder periods, forming a tain marine microorganisms, such as fora- sequence of oxygen isotope stages (odd- minifera, absorb oxygen isotopes in propor- numbered stages represent warm times; tion to the surrounding seawater. An 18O even refer to cold). Isotope stages 1–100 -enriched ocean will thus result in a higher ratio of 18Oto16O in the microscopic shells. cover the past 2.5 myr. As depicted in Figure This transfer from ocean to foraminifera is 2A, oscillation was particularly extreme over also affected, though, by temperature. For the past 700 ka. In this period, single excur- 18 every 1°C drop in water temperature, the sions of the ␦ O curve (about 100 kyr long) proportion of 18Oto16O within the foram encompassed 1.5‰ or more, which exceeded shells is increased by about 0.22 parts per the total average change since the late mil (‰). Thus the oxygen isotope ratio— Miocene (ϳ1.4‰). Foram records back to the ␦18O (delta 18-O)—in foraminifera is a mea- late Oligocene (see Fig. 2B) indicate that the sure, or proxy, of both temperature and total range of ␦18O variation within 1 myr evaporation.3 spans was fairly stable up through the During glacial periods, oxygen (biased to- Miocene. The total variation in each span ward 16O) that has evaporated from the sea consistently ranged between 0.3 and 0.8‰ becomes locked up in ice sheets as precipita- up until 5–6 million years ago. At that time, tion falls on land. Thus, foram ␦18O fur- the amount of variation increased signifi- cantly and continued to rise to the present 2This process leading to isotope enrichment is called fraction- (Potts, in press a). As we will see, different ation, in which the ratio of stable isotopes is altered during the hypotheses about hominin evolution are sup- transfer of an element (in this case, oxygen) from one medium to another. 3␦18O measurements reflect the relative concentration of the two isotopes and are expressed as ‰ variation from a standard isotopic reference, measured on a belemnite from the Pee Dee 4For periods in Earth’s history when ice sheets were not Formation (Cretaceous age), South Carolina. present, the isotope ratio is primarily a measure of temperature. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 97

Fig. 1. Oxygen isotope curve (average) based on composite data from benthic foraminifera, 70 Ma to present (Miller et al., 1987). ported depending on whether one focuses on lion years, the dominant cycles are approxi- the overall cooling trend (Fig. 1) or the mately 100 kyr and 41 kyr long and a dual oscillation (Fig. 2). cycle of 23 kyr and 19 kyr. These cyclicities Study of the frequency and strength of match variations in Earth’s orbit around the oscillation, using a technique called spectral sun, an idea proposed by Croll (1875) and analysis, shows that ␦18O fluctuation occurs then expanded by the mathematician Mi- at certain periodicities. For the past 1 mil- lankovitch (1941). A complete eccentricity 98 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 cycle (the circular-to-elongate shifting of Earth’s elliptical orbit around the sun) takes approximately 100 kyr. Obliquity (the chang- ing tilt of the planet’s axis of rotation rela- tive to the sun) varies between about 21.4° and 24.4° on a cycle 41 kyr long. Precession of the equinoxes, due to Earth’s axial wobble, changes the timing of the seasons relative to the distance from the sun on a cycle lasting about 21 kyr. Over this cycle, the northern hemisphere is tilted toward the sun at suc- cessively different points in Earth’s orbit. These cycles affect the total amount of incoming solar energy (insolation), and they interact to create complex fluctuation in insolation, which is remarkably registered in the shells of forams and the oxygen- isotope curve. It is assumed, therefore, that these orbital variations have a prevailing influence over global climate, including over- all cooling during the late Cenozoic and the rate and range of oscillation. An important milestone of Cenozoic climate change was the onset of high-latitude glaciations start- ing around 2.8 Ma, an event that has been linked to events in hominin evolution. Ocean dust records Dust plumes arising from continents re- sult from strong seasonality of rainfall and

Fig. 2. A: Oxygen isotope curve 6 Ma to present, based on benthic foraminifera (Shackleton, 1995). B: Range of variation in oxygen isotope values in 1 million year inter- vals from late Oligocene (27 Ma) to present. Based on data from Miller et al. (1996), Wright and Miller (1992), Woodruff et al. (1981), Shackleton (1995), and Prentice and Denton (1988). (Reproduced from Potts, in press a, with permission of the publisher). Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 99 in the winds that carry the dust to the ocean. (Broecker and Denton, 1990; Taylor et al., Aeolian dust accumulations found in ocean 1993; Dansgaard et al., 1993). Based on sediment cores thus provide a record of these findings, it has been argued (e.g., long-term change in seasonality and prevail- Calvin, 1996) that hominin adaptability ing wind patterns. These records may reflect evolved in response to large, short-term the history of continental vegetation cover, fluctuations in middle or high latitudes. since large areas of open vegetation and Evidence of such abrupt variation, how- desert enhance the chances of erosion and ever, is absent in the Antarctic ice cores wind transport of land detritus. (Jouzel et al., 1993), suggesting that the In a detailed study of cores off the west oscillation events may be largely a northern and east coasts of Africa, deMenocal (1995; hemisphere phenomenon. A partial explana- deMenocal and Bloemendal, 1995) has shown tion is that rapid cooling may correspond that African climate (and vegetation cover) with periods of iceberg discharges (or raft- was highly variable from early Pliocene to ing) into the North Atlantic (known as Hein- the present (Fig. 3). Spectral analysis (right rich events); by adding fresh water to the side of Fig. 3) indicates a change in oscilla- sea, these discharges may suddenly reduce tion at around 2.8 Ma, from a dominant the heat-conveying effect of one of the most frequency of 23 to 19 kyr (precession) to one important ocean circulation systems, the of 41 kyr (obliquity). Other shifts occur at North Atlantic Deep Water, thereby causing 1.7 and 1.0 Ma, at which point the 100 kyr abrupt cooling of 5–8°C (Bond et al., 1992; frequency begins to dominate. These find- Stauffer et al., 1998). Deep-sea data from ings suggest that African climate variation the North Atlantic show that such abrupt was tuned to orbit-related changes in insola- cooling-warming cycles occurred regularly tion. These shifts represent periods of mark- back to at least 1 Ma (Raymo et al., 1998). edly increased aridity at 2.8, 1.7, and 1.0 Ma; the origin of certain hominin species in Sedimentary environment Africa after 2.8 Ma, therefore, may reflect Basic geologic analysis at hominin sites the rise of an arid-adapted biota (deMeno- includes field recording and mapping of sedi- cal, 1995). However, as noted by deMenocal ments, describing major depositional envi- and Bloemendal (1995), these dates primar- ronments (lithofacies), and logging strati- ily signal substantial rises in the overall graphic change. Certain types of deposits, variance, or amplitude of fluctuation, rather such as caliches, and the presence of particu- than a permanent shift toward a drier, more lar minerals are indicative of specific envi- open environment (Fig. 3). ronmental conditions (e.g., aridity, rainfall). Other deposits, such as widely distributed Ice cores tephra, permit ready correlation between Accumulation of ice in glaciers, ice sheets, distant locations. Correlation of stratigraphic and ice shelves results from precipitation of sections over several kilometers enables the water evaporated largely from the ocean. reconstruction of paleogeographic settings Thus, ␦18O analysis of cores drilled from on a basinwide scale. Such reconstructions major ice bodies yields a high-resolution have proved powerful in interpreting the record of climate, which is the inverse of adaptive contexts of hominins at Olduvai, deep-sea ␦18O. Particularly informative re- Turkana, and other localities (Hay, 1976a; cords into the middle Pleistocene have been Brown and Feibel, 1991). obtained from Greenland and Antarctica. Lake sediments One significant finding from the Greenland cores is evidence of extremely rapid oscilla- Lakes may provide habitats for diatoms or tions during certain time intervals. Known other environmentally sensitive organisms as Dansgaard-Oeschger events, these oscilla- (e.g., ostracods). Analysis of these organisms tions last around 1,000 years; during these can yield a precise history of lake depth, events, fluctuations between interglacial salinity, and alkalinity—factors that are warmth and severe cold take place over a strongly influenced by climatic and tectonic century or even a decade in some cases variations. Study of diatom species in the Fig. 3. Continental dust (%) measured in the ocean core from Site 661, off the west coast of Africa, 4 Ma to present. Percentage aeolian dust was calculated using magnetic susceptibility, a measure of the concentration of magnetic particles typically found in wind-blown dust. The power spectrum (right) is an analysis of variance in the oscillation time series. This drilling site is one of seven on which deMenocal’s analysis is based (deMenocal and Bloemendal, 1995; deMenocal, 1995). Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 101 Olorgesailie Formation, for example, has and shrubs) and nonarboreal (herbs) pollen helped to establish the history of an ancient (abbreviated AP and NAP, respectively). Per- lake basin inhabited by hominins (Owen centage AP reflects the density of tree cover, and Renaut, 1981; Potts, 1994) and to cali- though this provides only a rough measure brate rates of environmental change faced in low latitudes since tropical trees have by the toolmakers (see below). lower pollen production than grasses and other NAP. Divisions of NAP often include Loess sequences grasses (Gramineae), aquatics (such as Ty- Thick sequences of alternating loess (wind- pha and Cyperaceae), and the C/A group blown sand) and soil (paleosols) occur (Chenopodiaceae/Amaranthaceae). The first throughout vast regions of Asia and Europe. indicates the relative density of grass cover, Loess layers represent times when vegeta- especially when aquatic taxa are factored tion cover was sufficiently low to allow out. The last group can be a good indicator of buildup of aeolian material. The soils, on the aridity in African settings, since most of the other hand, are often organic-rich due to C/A plants come from saline soils and dry thick vegetation cover at the time of soil streams. Environmental interpretation of formation. Loess sequences of north-central pollen spectra is based on modern analogues China have been especially well studied and and thus requires careful comparison of past provide a detailed record of environmental and present samples and preservation condi- change. Over the past 2.5 myr, at least 44 tions. The lack of accurate analogues for major shifts from cold to warm conditions past climates, particularly glacial condi- have been documented. tions, can sometimes be offset by incorporat- Environmental interpretations are based ing other evidence (e.g., in Europe, the repre- on the degree of loess and soil weathering, sentation of fossil beetles, which are fossil pollen and fauna, carbonate content, climatically sensitive). The sum of certain and magnetic susceptibility. The latter AP taxa is often used as a climate index, method measures the concentration of mag- although the validity of this approach has netic grains, which is especially high in fine been questioned. In East Africa, fossil pollen dust blown from distant sources. This fine analysis has yielded a sequence of general component dominates during warm, wet pe- vegetation and climate reconstructions for riods, while relatively coarse grains from localities such as Olduvai, Turkana, and local sources tend to be deposited during Hadar; while in Europe the coring of peat cold, dry eras. Thus, high susceptibility mea- bogs has enabled high-resolution analysis of surements are found in the soils and low precipitation and temperature based on pol- ones in the loess layers. These measure- len (Bonnefille, 1995; Guiot et al., 1989, ments offer a record of oscillation in climate 1993; Woillard, 1978). Figure 5 illustrates a and vegetation. Figure 4 illustrates a sec- detailed series of climatic fluctuations indi- tion of the Luochuan loess sequence, in cated by pollen spectra from the sediment which strong fluctuations between cold, open core at Les Echets, France. steppe and warmer, wooded to forested con- ditions have been documented (Liu, 1988; Stable isotope analysis of soils Kukla and An, 1989). Because they cover Under certain conditions, buried soils, or huge continental areas, loess-soil deposits paleosols, preserve organic residues and car- offer a potentially powerful method of deter- bonate deposits that bear a stable-isotopic mining environmental sequences in temper- signal of past vegetation. Interpretation of ate latitudes occupied by early hominins. vegetation is based on the photosynthetic chemistry of plants and the signature it Fossil pollen sequences leaves in soils. Most plant species undergo Pollen is deposited as ‘‘pollen rain,’’ one of two distinct photosynthetic pathways; samples of which can be characterized by one involves three carbon atoms and the the percentages of different vegetation com- other four (thus C3 and C4 plants). C4 plants ponents, known as a pollen spectrum. The are virtually all grasses adapted to hot, dry basic components are usually arboreal (trees conditions and low CO2 concentrations in 102 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998

Fig. 4. Sequence of loess layers (Ln) and paleosols (Sn) from Luo- chuan, north-central China, 1.1 Ma to present, and environmental in- terpretation based on the degree of loess/soil weathering, magnetic sus- ceptibility, pollen, and carbonate analysis. 1–3, weakly, moderately, and strongly weathered loess, re- spectively; 4, black loam; 5, carbon- ate cinnamon soil; 6, cinnamon soil; 7, leached cinnamon soil; 8, dark cinnamon soil (Liu, 1988). Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 103

Fig. 5. Estimates of mean an- nual precipitation and tempera- ture, expressed as deviations from present values (800 mm and 11°C) plus confidence intervals, 140 ka to present, based on pollen recovered from the Les Echets pollen core, eastern France (Guiot et al., 1989).

13 18 the atmosphere. C3 plants include woody Combined analysis of ␦ C and ␦ Oofan- species, herbs, and grasses adapted to cool cient soils, therefore, can allow quite specific growing seasons and shade. The two catego- interpretation of the complex relationship ries have nonoverlapping distributions of between soil and atmospheric chemistry, veg- ␦13O (ratio of carbon isotopes 13C/12C). In etation, and temperature (e.g., Cerling and temperate zone soils, therefore, 13C can indi- Hay, 1986; Cerling, 1992; Cerling et al., 1997; cate change in climate or atmospheric CO2. Ambrose and Sikes, 1991). Isotopic study of In tropical lowlands, this measure may also widely exposed paleosols has also enabled indicate vegetation structure along the gra- detailed comparison to modern landscape dient from heavily wooded to open grassland analogues, which may be extended to recon- habitat. In addition, the oxygen isotope ratio struct the mosaic of wooded, grassy, and wet- can be measured in soil carbonate and is land habitats in strata bearing early human correlated with mean annual temperature. artifacts (Sikes, 1994; Sikes et al., in press). 104 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 Faunal analysis tion (Amundson, 1996; Endler, 1986). The first is a nonhistorical approach, which de- Relative abundance of different animal species permits a general characterization fines an adaptation solely in terms of the of habitat, assuming that certain taxonomic current, observed benefits that a particular groups correlate with specific habitats in trait offers to an organism (Bock, 1980). The both the past and present (e.g., Vrba, 1975; process of origin (natural selection) and thus Potts, 1988; Shipman and Harris, 1988). A the history of a given trait are, by this different array of methods uses functional definition, less relevant to deciding whether morphology to test for definitive anatomical a trait is an adaptation. A nonhistorical indications of environment. This approach definition of adaptation, then, concerns sim- allows an ecomorphic (as opposed to a taxo- ply the contribution of a trait to current nomic) characterization of the biota and fitness (Fisher, 1985). associated habitats without assuming par- The second is a historical approach, which ticular taxon-habitat correlations. Methods focuses not only on fitness but also on the of ecomorphic analysis have increasingly process by which adaptation occurred. Ac- been applied to the faunas of early hominin cordingly, an adaptation is a trait generated sites (e.g., Andrews, 1989; Plummer and by natural selection due to its fitness ben- Bishop, 1994; Kappelman et al., 1997; Reed, efits. This approach focuses, then, on the 1997). history of a trait. It allows the possibility of studying and citing the selective causes that Summary underlie a particular adaptation. With homi- Development of these and other methods nin evolution, we are very much interested of environmental interpretation has pro- in history. Thus, the second approach is gressed impressively over the past several adopted here. For one thing, current benefit decades. The contexts in which these meth- (the basis for defining an adaptation in the ods can be applied include deep-sea cores, nonhistorical approach) has no meaning peat bogs, soils, and many diverse types of when looking at characters of extinct homi- sediment. The methods can in many cases go nins. In an evolutionary perspective, more- beyond a general history of climate and over, current benefit even in living humans habitat to offer a quite detailed picture of the does not always give a true picture of a adaptive contexts faced by early hominins. trait’s original advantage or the conditions under which it evolved. In a historical analy- A FRAMEWORK FOR ANALYZING sis, then, adaptation is the result of natural ADAPTATION selection, and the analysis of adaptation The process of adaptation, natural selec- entails an assessment of how the prevalence tion, involves the relationship between an of a trait in the fossil record may have organism and its surroundings. An organ- resulted from this process. ism’s environment (at least aspects that But how does one recognize natural selec- affect survival and reproduction) influences tion, or at least its results, in the fossil the traits that persist, increase, and spread record? The first order of business is to through a population. Traits positively af- define a trait, feature, phenotype, or charac- fected by the process of biased reproduction ter (all considered synonyms here) or even a are called adaptations. The analysis of adap- complex of traits or a definable morphologi- tations thus may afford insights into both cal or behavioral pattern. The definition of a the process of natural selection and the feature or overall phenotype provides the environmental context in which evolution basis for examining function, benefit, and has occurred. The purpose of this section is evolutionary setting. to define an analytical framework that shows In a historical approach, an analysis of how paleoenvironmental data from hominin adaptation must also be based on the pro- sites may contribute to the study of adaptive cess of natural selection itself. As summa- evolution in early humans. rized by Lewontin (1970), the process has The field of biology is divided between two three necessary and sufficient conditions: main approaches to the concept of adapta- variation, heritability, and differential fit- Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 105 ness. For selection to operate, the units of TABLE 1. A 13 part framework for the analysis evolutionary change (e.g., specific pheno- of adaptation typic features) must vary. The variations 1. How is a particular feature (morphological struc- ture, behavior, phenotypic system or complex) must be inherited (e.g., the phenotypes must manifested? What is its structure, or what are its relate to specific genetic variations). And the characteristics? variations must differ in their representa- 2. What is the function of a particular feature? What is it designed to do? What selective advan- tion in later generations. tages does the feature offer to an organism? These three conditions furnish an initial 3. What ‘‘currencies’’ of advantage or success does the feature provide to the organism? An advan- framework, but many other factors enter tage in survivorship? A direct reproductive into an analysis of adaptation. Differential advantage? Does the feature raise the organism’s fitness occurs presumably due to certain efficiency or economy (Steudel, 1994)? Ulti- mately, how does the feature contribute to the benefits that one variant offers compared to genetic fitness of the individual? others (alternative phenotypes). So it is es- 4. Besides its beneficial function, does the feature have any costs to the organism? What are those sential to determine what those benefits costs? were (or are), especially how a trait’s struc- 5. What is known about the feature’s heritability? tural design or pattern of expression pro- 6. What is known about the feature’s development (growth and expression during the life cycle)? vided a selective advantage. Each beneficial What are the necessary environmental param- feature may also have its share of costs, eters for proper development? 7. Does the feature (e.g., a particular behavior) which cannot outweigh the benefits if a have any repercussions (costs or benefits) for feature is to increase via natural selection. other organisms? In other words, what was the The adaptive evolution of a feature is often competitive and/or social context in which the feature evolved or currently functions? assumed to take place in a competitive set- 8. In the context of natural selection, when did the ting, and its increase may have serious feature originate? In what environmental con- text or conditions did it evolve? What advantages repercussions for other individuals within did the feature offer within that original context? that setting, which could affect the increase Did the feature represent a response to a specific and spread of the feature. environment or to diverse surroundings? Do the original advantages and environmental context Furthermore, benefits, costs, and competi- still apply to the organism that possesses the tion all take place in relation to particular feature? 9. What were the options or alternative phenotypes environments. This is one reason the analy- (e.g., the ancestral condition) available to the sis of adaptive evolution involves environ- organism when the specific feature evolved? mental reconstruction. The stability or dy- 10. Historically, is the feature a by-product of adap- tive change in another trait (in which case it is namics of environmental change may also not an adaptation), or did the feature evolve due have an impact on natural selection. The to specific benefits of a task it performs or con- tributes to? different time scales of environmental fluc- 11. Does the feature serve its original function, or is tuation may affect natural selection and, as it an exaptation (co-opted for some other function we will see, may determine the kind of traits [Gould and Vrba, 1982])? 12. Over what time scale does the feature assist the that are ultimately represented in a popula- organism? Daily? Seasonally? Does it offer resil- tion. ience to environmental variations faced over a lifetime? Does it offer flexibility to a population Finally, a growing number of evolutionary over longer periods of environmental change? biologists believe that lineages themselves 13. Are there lineage-level adaptations—that is, fea- possess adaptations, referring to character- tures of a lineage (rather than of an individual) that enhance its ability to spread, persist, or istics (e.g., geographic range, habitat vari- diversify when compared with other, related lin- ety, dietary versatility) that go beyond the eages? If natural selection can take place at the species level, what kinds of features might be makeup of an individual organism (Vrba, involved in this process? 1989; Jablonski, 1987). Since these charac- teristics may impact the survival of popula- tions over time, they may also affect the account phenotype definition, the operation success of particular features originated by of natural selection, and the factors that conventional, individual-level natural selec- affect differential fitness. tion. The analysis of adaptation is tricky largely A basic framework for the analysis of because the questions posed in Table 1 are adaptation, then, can be conveyed by a se- difficult to answer and any answers put ries of questions (Table 1) that take into forth are usually controversial. As debate 106 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 about the evolution of the human chin dem- tion of Man, Elliot Smith (1924:40) de- onstrates (Gould, 1977), even the first step— scribed the terrestrial domain of early hu- defining a trait subject to natural selection man ancestors as ‘‘the unknown world (Table 1, questions 1–3)—can create contro- beyond the trees’’—a place of novel food versy, partly because traits can emerge as a sources, quite distinct from the ‘‘land of by-product of adaptive modification (ques- plenty’’ of the apes. Dart (1925) took this tion 10). Besides these correlated effects, idea to dramatic heights. To convey the developmental (e.g., embryological) con- selection pressures that drove human evolu- straints are ubiquitous, which place bound- tion, Dart drew an analogy to the southern aries on trait variation, the raw material African veldt. The early australopiths, he acted on by natural selection, and thus on argued, confronted many dangerous preda- adaptation itself. Structural design, the ben- tors, a relative scarcity of water, and fierce efits, and the costs of morphological traits competition for food, all of which he ulti- can be considered with some success by mately encapsulated in the killer-ape sce- functional anatomists. But specialists inter- nario (Dart, 1953). Later views, such as the ested in past behavior (e.g., australopith dietary hypotheses of Robinson (1954, bipedality, late Pliocene stone flaking) usu- 1961a,b) and Jolly (1970), also explicitly ally have greater difficulty evaluating de- related hominin origin to a shift from for- sign properties, frequency of occurrence, ested/wooded ‘‘apelike’’ habitat to an open heritability, and the advantage over alterna- savanna setting. tive behaviors. In fact, very little is known Intrinsic and extrinsic approaches. If about the heritability of even simple struc- indeed human evolution followed a progres- tures and behaviors unique to humans. The sive path from apelike to humanlike, it could heritability of characters (e.g., megadontia) also be reconstructed by extrapolating up that distinguish extinct hominins, more- from the anatomical, behavioral, and ecologi- over, is largely inaccessible except perhaps cal characteristics of apes and other nonhu- by analogy to living mammals that possess man primates and extrapolating down from similar traits. ethnological data on human foragers. The Given these difficulties, the study of early paper ‘‘Ecology and the Protohominids’’ by hominin adaptation is largely left with the Bartholomew and Birdsell (1953) made ex- analysis of context (Table 1, question 8). The plicit this approach to modeling human evo- questions of time, place, and environmental lution. This paper was the first to draw setting are essential to all evolutionary attention to key theoretical aspects of early analyses of adaptation. The selective history human ecology, including population den- of a trait is reflected in part by when, where, sity and dynamics, territoriality, foraging, and the conditions or surroundings under energetics, body size, and sociality. Yet no which that trait first became apparent in the environment was reconstructed, and no ideas fossil record and began to characterize a were offered about the interaction of early particular lineage or clade. The precise defi- humans with specific environmental vari- nition of hypotheses relating past environ- ables. Implicit in their discussion was that ments to the emergence of unique hominin the environment of hominin evolution was traits, therefore, helps to galvanize research already generally known. It involved, at on early human evolution. first, increasingly arid, savanna conditions in Africa, followed by increasingly cool, gla- ENVIRONMENTAL HYPOTHESES cial conditions in northern latitudes. OF HOMININ EVOLUTION Despite their emphasis on ecology, Bartho- A concise history lomew and Birdsell (1953) provided an out- Challenges of the savanna. Over much line for an intrinsic account of hominin of the twentieth century, scientists thought evolution whereby adaptive change resulted that human evolution entailed a simple tra- largely from responses within populations jectory from apelike to humanlike and that over time without regard to change in exter- this process was promoted by the challenges nal conditions. All they required was the of an open savanna. In his book The Evolu- basic scene, a relatively open savanna. Their Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 107 argument followed Darwin’s (1871) basic East African rift (Isaac, 1976; Bishop, 1976). scenario: bipedality (ground living) and small It is interesting to note that Bishop’s paper canine teeth implied that the earliest homi- demolished the ‘‘myth of a Pliocene drought’’ nins used tools, which permitted an ex- (p. 40) championed by Dart, which had panded diet, including animal foods; thus, fueled the view that early human evolution hunting in bands and changes in sociality was a response to a sharp shift from moist occurred as one adaptive change led to oth- forest to arid savanna. Moreover, the papers ers. Washburn (1960) refined this type of concerning Miocene sites showed that early explanation, and placed tools at the center of apes lived in a complex mosaic of environ- an evolving system in which each novel ments, including closed and relatively open adaptation fostered others in an intricate vegetation. These papers were among the series of feedback loops. ‘‘Tools, hunting, first to hint at the intricate environmental fire, complex social life, speech, the human history related to ape and human evolution. way and the brain evolved together to pro- Isaac (1976:133) ended his article by noting duce ancient man of the genus Homo’’ (Wash- the paucity of contextual studies and com- burn, 1960:63). Again, Washburn’s intrinsic pared the existing information base about feedback model was not without context. If East African paleoenvironments to an ‘‘ex- bipedal tool use initiated the cause-and- posed tip of an iceberg.’’ effect process of hominin evolution, life in While most anthropologists took an intrin- the open plains provided the crucial, initial sic approach, geologists and vertebrate pale- setting (Washburn, 1960). ontologists showed strong interest in paleo- Washburn’s influence and interest in in- environmental data and the reconstruction trinsic accounts (as opposed to extrinsic of early hominin settings. Thus, in Howell accounts, which interpret evolutionary and Bourlie`re’s (1963) volume African Ecol- events in relation to environmental change) ogy and Human Evolution, almost all of the may explain why many paleoanthropolo- 19 papers discussed the importance of con- gists in the 1950s to 1970s paid little atten- textual data (the main exception was that by tion to environmental context. In two classic DeVore and Washburn). The papers (e.g., by volumes on hominin evolution (Washburn, Butzer) included cautions about simplistic 1961, 1963), none of the 35 papers focused linkages between higher latitude glacial se- primarily on environmental settings. and quences and lower latitude environmental 5 only seven considered the subject. This history. Other papers surmised that com- approach was echoed in the influential four plex tectonic and climatic events were inter- volume set Perspectives on Human Evolu- woven in creating the environmental history tion published between 1968 and 1978 of Africa (e.g., papers by Bishop and by de (Washburn and Jay, 1968; Washburn and Heinzelin). Several papers showed evi- Dolhinow, 1972; Isaac and McCown, 1976; denced that Pleistocene Africa underwent a Washburn and McCown, 1978). The only complex series of habitat changes between articles on paleoenvironmental context were moist and dry and between forested and in Volume 3 (six of its 21 chapters). Two open (e.g., papers by Moreau, Monod, Grove dealt with Miocene faunas and floras (Van and Pullan, and de Heinzelin). Articles by Couvering and Van Couvering, 1976; An- Biberson and by Howell and Clark were drews and Walker, 1976), two others consid- particularly successful in placing Pleis- ered environmental evidence from specific tocene stone technologies and behavior in hominin-bearing basins (Olduvai, Omo) specific sedimentary, vegetational, and cli- (Hay, 1976b; Howell, 1976), and two chap- matic contexts. ters highlighted the general context of the Later volumes by Bishop and Clark (1967), Butzer (1971), and Bishop (1978) continued 5Four papers (by Chance, Oakley, Hallowell, and Sauer in the to emphasize the importance of environmen- 1961 volume) reiterated the general view that the earliest tal (sedimentary, geomorphological, tapho- ancestors inhabited the African savanna while later hominins lived in an ice-age context. Three others (by Mayr, Dobzhansky, nomic, and ecological) settings for under- and Harrison and Weiner in the 1963 volume) discussed environ- standing hominin evolution. More than half ments as a theoretically important determinant of natural selection, adaptive change, and speciation in hominins. of the articles published in Butzer and Isaac’s 108 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 (1975) After the Australopithecines and pulse refers to a concentration of speciation nearly one-third of the contributions to Jol- and extinction events (i.e., turnover) in a ly’s (1978) Early Hominids of Africa focused brief period of time as a result of environmen- specifically on the climatic, geologic, and tal change. According to Vrba’s original hy- paleoecological contexts of hominin sites. pothesis, many different African mamma- By the early 1980s, a wide range of envi- lian clades were involved in a turnover ronmental data was becoming available due episode at 2.5 Ma. Synchronous change in to a surge in paleoclimate studies and de- multiple groups was caused by a shift from tailed work by field geologists at fossil and warm, moist conditions to a cooler, drier, and archeological sites. Laporte and Zihlman more open habitat. This environmental event (1983) made a strong case for the idea that was forced by global cooling, manifested as a environment was a major driving force in sharp transition (18O enrichment) in the early human evolution. In their view, adap- deep-sea isotope record. The hypothesis thus tive change in apes, including Pliocene homi- posits a strong causal link between hominin nins, represented responses to either new evolution and global climatic events. Accord- environments or physical access to existing ingly, the origins of Homo and of the robust environments (e.g., via land bridge). They australopiths at about 2.5 Ma were consid- argued that global cooling reduced tree cover ered part of the turnover pulse. Since a over large regions, and African uplift and speciation pulse in grazing bovids occurred rifting created a rain shadow effect (drying) at this same time (e.g., Vrba 1985, 1988), in the eastern part of the continent. These Homo and Paranthropus were also deemed factors caused the spread of the savanna part of the Pliocene arid, open-grassland mosaic, marked by strongly seasonal rain- biota of Africa. Vrba et al. (1989) posited two fall, discontinuous distribution of trees, and other turnover pulses, about 5 Ma and 0.9 heat-adapted, drought-resistant vegetation. Ma, which also appeared to coincide with Echoing earlier authors, Laporte and Zihl- periods of global cooling and continental man (1983:106–107) posited that the ‘‘chal- drying. The origin of the Hominini, on the lenges of the savanna-mosaic’’ caused one hand and the extinction of robust austra- ground-dwelling in a prehominin ape and lopiths, coupled with the dispersal of H. bipedality in the earliest hominins. Two- erectus into Eurasia, on the other were legged walking enabled mobility and carry- thought to be correlated with these environ- ing of food and water over long distances as mental shifts. resources became more widely (patchily) The original hypothesis has been re- distributed. Finally, they noted that the vamped in two main ways. Since the mid- ‘‘origin of hominids [sensu stricto] is not an 1980s, paleoclimatologists have broadened isolated evolutionary event, but rather is the age estimate of Pliocene global environ- part of the overall radiation of African homi- mental change. Based on the deep-sea oxy- noids; nor is it an unusual ecological occur- gen isotope record (Fig. 2A), for example, a rence. The hominids are only one of several major shift in global ice sheets began as mammalian groups that include the pigs early as 2.8–3.0 Ma, with the most signifi- and bovids that were able to exploit the cant change between 2.8 and 2.4 Ma (e.g., savanna-mosaic habitat that had become Prentice and Denton, 1988). The deep-sea widespread by the end of the Miocene’’ dust record (see Fig. 3) also indicates a (Laporte and Zihlman, 1983:108). major change in cyclicity beginning around 2.8 Ma. Thus, the pulse in the late Pliocene Turnover hypotheses. At about the same turnover pulse hypothesis has been broad- time, Vrba (1980, 1985, 1988; also Brain, ened; the target interval has been extended 1981) began to present an important idea, from a point in time (2.5 Ma) to a span as known as the turnover pulse hypothesis, wide as 400 kyr. According to this broader which expanded these same themes—cli- pulse model, key events in hominin evolu- matic forcing of evolution, a Pliocene aridity tion and concerted turnover in other mam- shift, and coincident change in hominins mals should be evident from 2.8 to 2.5 (or and other mammalian lineages. Turnover 2.4) Ma rather than randomly distributed Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 109 throughout the entire 3.5 myr span of the port the turnover pulse idea (Vrba, 1995b; Pliocene (Prentice and Denton, 1988). In Wesselman, 1995), though an analysis of Vrba’s (1995b) view, the turnover pulse in suids and hominins argues strongly against Africa began 2.8 Ma but became apparent in the hypothesis (White, 1995). the fossil record only between 2.7 and 2.5 The pulse hypothesis has been questioned Ma. This is the first way in which the on other grounds, however, and countered hypothesis has been altered. by what may be called the prolonged turn- The second way involved refining the test over hypothesis (Behrensmeyer et al., 1997; implications of the turnover pulse model McKee, 1996). Building on earlier observa- and broadening its theoretical basis. Build- tions (Feibel et al., 1991), a recent study by ing on a synthetic view called habitat theory, Behrensmeyer et al. (1997) focused on the Vrba (1992, 1995a) posits that the record of a Turkana-Omo basin since this area offers turnover pulse should depend on the breadth the richest and best calibrated record of of resources and habitats used by organ- African fossil mammals in the crucial span isms. First and last appearances will be between 3 and 1.8 Ma. Taking sampling synchronized or spread out according to the variation (rises and falls in fossil samples ecological tolerances of different lineages. A over time) into account, this study found no global cooling event causing the spread of statistically significant pulse in either first savanna between 2.8 to 2.5 Ma thus should or last appearances at 2.5 Ma or between 2.8 have caused turnover in specialized organ- and 2.5 Ma. Instead, turnover in this inter- isms first and in more generalized organ- val was unremarkable, and mammalian lin- isms later. Furthermore, it should cause a eages displayed a prolonged period of turn- larger number of first appearances in arid- over especially between 2.5 and 1.8 Ma. adapted taxa (e.g., hominins and grazing Moreover, a combination of xeric and mesic bovids) and more last appearances in warm, species was found to persist from 3 to 2 Ma. woodland-adapted species. Behrensmeyer et al. (1997) argued that a Vrba has been assiduous in testing the complex vegetation mosaic coupled with wide turnover pulse idea. Many appearances and fluctuation over time would explain both the disappearances of bovid lineages in the Afri- persistence of species of diverse ecological can fossil record are clustered around 2.5 tolerances and the prolonged period of turn- Ma (Vrba, 1985, 1988, 1992, 1995b). Grazing over. Whether similar results might apply to bovids (alcelaphines such as wildebeest, the whole of Africa has yet to be determined. hartebeest, and topi and antilopines such as gazelles) contributed strongly to the first Adaptive evolution in hominins appearances. This supports the idea that Although the turnover pulse idea has at- aridity and grassland expansion were the tracted much attention, environmental hy- environmental causes of bovid lineage turn- potheses pertaining to adaptive change re- over. Vrba (1985, 1988) has also emphasized main poorly developed. Hypotheses of this Wesselman’s (1984) conclusion (based on a type differ from turnover models. They at- study of micromammals from Omo, Ethio- tempt to describe how environmental con- pia) that forest taxa prevalent at 3 Ma gave text impinged on survival and natural selec- way to xeric-adapted, open vegetation taxa tion and in turn influenced the emergence of between 2.4 and 2.5 Ma. In the volume key adaptations. Since adaptive change may Paleoclimate and Evolution, with Emphasis be only loosely coupled to speciation (El- on Human Evolution (Vrba et al., 1995), dredge, 1989), these hypotheses do not neces- papers concerning paleoclimate and vegeta- sarily relate directly to the pattern of species tion tend to confirm that large environmen- turnover. tal change occurred between 2.8 and 2.5 Ma. Many of these papers, however, stress the Habitat-specific hypotheses. The most evidence for strong fluctuation at this time prominent narratives of hominin adaptive rather than a simple cooling or aridity shift. evolution are habitat-specific. Each of these Conclusions drawn from African bovids, mi- scenarios points to a particular type of envi- cromammals, and hominins are said to sup- ronment and sets forth the reason certain 110 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 adaptations arose to meet the special de- TABLE 2. Recent advocates of the savanna hypothesis mands of that setting. Some scenarios sim- Who What and when Result ply name the particular type of habitat in Vrba Pulses of savanna Turnover (in Homo which hominins were assumed to live (e.g., S. Stanley expansion = and robust austra- the savanna), while others may specify a G. Denton >2 lopiths) M. Prentice Aridification Novel adaptations to particular direction of environmental change > open conditions (e.g., the onset of dry or cold conditions). In Global cooling (ϳ5.0, (e.g., terrestri- both cases, it is implicit that natural selec- 3.0, 2.8 to 2.4, 1.7, ality, stone tool- and 1.0 Ma) making) tion had a consistent effect over time (i.e., directional selection). This process served to improve adaptive opportunities and favored certain innovations as ways of solving the savanna was caused by steplike episodes of challenges posed by the special habitat or global cooling starting in the late Miocene. trend. Vrba (1995b:406), for example, refers to the The discovery of human fossils and arti- onset of ‘‘the massive overall cooling trend’’ facts with ice-age fauna and strata, for ex- of the late Pliocene. Intense global cooling ample, led to the idea that advances in led to pulses of aridification in Africa, which human evolution were forged in the harsh caused savanna expansion. The shift from settings of Pleistocene Europe. This view dense woodland to savanna had a strong may be termed the ice-age hypothesis. An effect on adaptive characteristics, including updated version of this view is as follows: bipedality in early Australopithecus, hyper- cool steppe and cold periglacial conditions in trophy of the chewing apparatus in Paran- mid-latitude Eurasia posed a very challeng- thropus, and various developments in Homo, ing environment to early human popula- such as encephalization and dependence on tions, and advances in hominid social and toolmaking (Stanley, 1992; deMenocal, 1995; cognitive abilities helped to solve these prob- Prentice and Denton, 1988; Vrba et al., 1989; lems of survival. Accordingly, the onset of Vrba, 1988). ice-age habitats (due to global cooling) re- Although the savanna hypothesis has sulted in the evolution of key human adapta- gained recent support, it has also been tions. strongly contested. Some authorities advo- As already noted, the savanna hypothesis cate a contradictory model—the woodland/ is another habitat-specific standard and has forest hypothesis—which argues for the im- dominated thinking about the earliest phases portance of closed vegetation in early of hominin evolution. According to this idea, hominin evolution. Early australopiths, ac- adaptation to drier and increasingly open cording to some interpretations, were closely environments was the hallmark of hominin associated with wooded environments, exhib- evolution from late Miocene through at least ited significant arboreal activity, and should the early Pleistocene. Savanna was the con- be considered adapted to closed habitats text that incited the emergence of fundamen- (Clarke and Tobias, 1995; Berger and To- tal human traits—terrestrial bipedality, bias, 1996). Fossil pollen from Makapans- larger brains, stone toolmaking, meat-eat- gat, for example, suggests that A. africanus ing, and associated foraging behaviors such inhabited conditions approximating a tropi- as hunting. Essentially all textbooks prior to cal forest (Rayner et al., 1993). At Aramis, 1996 explained the origin of these traits in the oldest known hominin, Ardipithecus terms of adaptation to relatively open sa- ramidus, is associated with evidence of a vanna, defined by the presence of grass, relatively tree-dominated setting (WoldeGab- discontinuous trees, overall aridity, and sea- riel et al., 1994). Stable isotope studies, sonal rainfall (e.g., Klein, 1989; Wolpoff, moreover, show that the shift to C4-domi- 1980). nated environments (open, grassy, heat- Recent support for the savanna hypoth- adapted vegetation) occurred later than pre- esis has risen from the ranks of paleoclima- viously thought, as late as 1.7 Ma, with little tologists and paleontologists (Table 2). The evidence of consistently open savanna until shared view is that emergence of dry, open after 1.0 Ma (Cerling et al., 1991; Cerling, Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 111 1992; Kingston et al., 1994; Sikes et al.,in The short-term perspective pays attention press). This means that open grassland, a to variations experienced by an organism key ingredient of the savanna hypothesis, over its lifetime, especially seasonal fluctua- was by no means a regular feature of the tion, disease, and change from year to year. East African landscape by the late Pliocene This view, which we may call the seasonality or even the early Pleistocene. Comparison of hypothesis, places emphasis on how an or- forest and open-habitat chimpanzees offers ganism adjusts to alternative environments another interesting perspective. Tool use, during its lifetime, such as warm-cold or hunting, food-sharing, greater social coopera- rainy-dry periods. Short-term variation is tion, and other traits usually associated especially marked in certain regions, particu- with human evolution appear to be more larly savannas, higher-latitude biomes, or strongly displayed by closed-habitat chimps. any other habitat prone to large moisture or On this basis, Boesch-Achermann and Boe- temperature seasonality. Strong seasonal- sch (1994) have suggested that woodland or ity, furthermore, brings about interannual forest may have provided the initial setting variation. The rainy season may be extreme in which hominins evolved. The savanna in one year but fail in another; the difference hypothesis has thus been criticized by pri- between winter and summer temperature mate researchers, paleontologists, and envi- may be large in some years but much less in ronmental scientists, many of whom favor others. The influence of short-term habitat varia- replacing it with a woodland/forest-adapta- tion on savanna organisms, including early tion model. humans, is discussed by Foley (1987). Foley A related idea—the riparian-woodland notes that although early hominins occupied scavenging model—also underlines the im- wooded-to-open savanna, the most signifi- portance of closed habitat to early humans. cant aspect of this type of habitat is its In the modern Serengeti, animal carcasses marked seasonality. Driven by monthly vari- are found to be seasonally abundant in ance in rainfall, significant shifts occur wooded areas near water. This observation within each year in the abundance and led Blumenschine (1986, 1987; Cavallo and availability of resources. Savanna organ- Blumenschine, 1989) to suggest that a scav- isms are adapted not to the variation per se enger niche could exist in riparian wood- but to the pattern of variation. It is the lands. Thus, during Oldowan times, marrow- expected cyclicity of resources and their eating toolmakers may have occupied the availability that is important (Foley, 1987). wooded zones near water, where they could Short-term hypotheses of adaptive evolution exploit scavenging opportunities, which were thus underline repeatable variations, which far less available in open settings. The ripar- represent regularities in the annual (or life- ian-woodland model, like other versions of time) adaptive setting of an organism. the woodland/forest hypothesis, supposes The difference between seasonality and that woodlands offered a vital challenge or other hypotheses, then, can be subtle. Habi- opportunity—in this case, carcasses that tat-specific views lay stress on the ever- could be exploited with the help of stone present, or consistent, adaptive problems in tools—which grasslands did not. a given setting. The short-term variability idea emphasizes habitat variation that an Short-term variability hypothesis. The organism is likely to experience and adapt to ideas considered thus far see adaptation as a in every generation. response to the consistent challenges or tendencies posed by a specific type of habi- Long-term (variability selection) hypoth- tat. According to a different type of explana- esis. In contrast to seasonality and habitat- tion, important adaptive changes may arise specific ideas is the variability selection in response to environmental variation. Vari- hypothesis. According to this view, environ- ability hypotheses can be divided into short- mental variation is accentuated over the term (e.g., seasonal variation) and long-term long term and plays a critical role in the (i.e., variability selection). adaptive process. Environmental change in- 112 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 volves large, episodic shifts in adaptive set- In VS, therefore, the selective effect is tings over many hundreds of thousands of integrated over time. This sets up the situa- years. As a result, a lineage of organisms tion in which habitat-specific traits end up may face multiple, substantial disparities in eventual losers to features that in the past selective environment over time. This idea proved successful in novel surroundings. follows from the previously coined term vari- This unusual process thus improves adapta- ability selection (Potts, 1996a,b, in press a). tion to long-term environmental dynamics. The variability selection hypothesis thus (See Potts (in press a) for a more detailed states that certain adaptations have evolved discussion and response to challenges.) due to large environmentally caused incon- By contrast, habitat-specific and seasonal- sistencies in selective conditions. This dispar- ity explanations of evolution usually assume ity in the Darwinian optimum favors the the operation of directional selection, in buildup of complex mechanisms for dealing which the fitness results tend to be consis- with unexpected, episodic change. This pro- tent over the long term. They focus on a cess of adaptive evolution therefore en- specific setting, trend, or properties of varia- hances an organism’s capacity to thrive in tion that recur from generation to genera- tion. The VS hypothesis is thus easily distin- novel conditions. guished from both habitat-specific and Variability selection (VS) calls attention seasonality hypotheses. to the large remodeling of landscapes, vegeta- tion, animal communities, and regional hy- Test implications drology over long time frames. The long- Each of these hypotheses has testable term sum of habitat variability far exceeds implications—that is, a set of expectations seasonal or other types of variations encoun- regarding tered by individuals within their lifetime. This recurrent, wide-scale revision of envi- ● the general paleoenvironmental record of ronment can thus have an unusual impact the late Cenozoic; on the adaptive properties of a lineage. The ● the precise association of hominins and seasonality hypothesis implies that sea- habitats; sonal (or, more broadly, generation-scale) ● change in the breadth of habitats in which change can explain the emergence of adap- hominins lived; tive flexibility. The VS hypothesis, by con- ● the context in which hominin adaptations trast, seeks the evolutionary cause of versa- first became apparent and continued to tility in longer intervals of more dramatic thrive; and change in an organism’s survival regime. ● the functions or utility of particular adap- The VS hypothesis requires that different tive features and trait complexes. individuals in different generations of the Habitat-specific hypotheses. Global and same lineage experience marked inconsisten- regional environmental records should show cies in adaptive setting. As it is magnified that a particular type of habitat (e.g., wood- over time, the disparity in adaptive milieu land, savanna, glacial cold) was either main- shapes an overall selective environment that tained over a prolonged period or developed cannot be predicted from the more consis- over time as a trend. Hominin fossils or tent selective environments of shorter peri- artifacts, moreover, should be associated ods. Selective disparity ultimately favors consistently with that particular type of genes that build mechanisms of adaptive past environment or at least a relatively versatility. It promotes a process of adapta- narrow habitat range. The influence of a tion to novelty, including settings never pre- particular habitat type on adaptation can viously encountered. The resulting adapta- best be demonstrated in a situation in which tions do not anticipate future conditions (a alternative habitats were also present (and violation of natural selection theory); rather, in which hominins were rare or absent). their evolution stems from a prior history of The main expectation therefore is that major, periodic revision of the adaptive set- hominins lived in and were especially at- ting. tracted to a particular set of environmental Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 113 conditions. Accordingly, adaptive features observations that differentiate it from other should be well suited to those conditions and hypotheses. Long-term global and regional should have first appeared in that specific records should show large-scale environmen- type of habitat. Functional analysis should tal fluctuation. The degree of environmental therefore show that those features had prop- change should become larger over long time erties (e.g., shape, size, biomechanical, be- periods, greatly exceeding the scale of sea- havioral, or ecological properties) that were sonal or lifetime fluctuation. The emergence particularly beneficial in the environment of key hominin adaptations, moreover, specified by the hypothesis (e.g., arid sa- should coincide with spans of enhanced envi- vanna, woodlands, glacial cold). ronmental fluctuation. In addition, these adaptations should promote versatile behav- Short-term variation (seasonality) hy- iors and responsiveness to habitat diversity pothesis. The main expectations of this and change. hypothesis, distinguishing it from the oth- The VS hypothesis also implies a set of ers, are that 1) hominins lived in settings of expected observations regarding hominin- high seasonality, 2) key adaptations reflect environment associations in the geologic the scale of habitat variation typically faced record. These expectations are best pre- over a lifetime, and 3) large-scale fluctua- sented in a comparison with habitat-specific tions have occurred on seasonal, decade, and explanations, as summarized in Table 3. century time scales during spans of major Comparison of these two sets of expecta- adaptive change. tions leads to an important point. Paleocli- Short-term variation may be quite ex- mate records clearly demonstrate that dra- treme. According to climatologists Crowley matic environmental oscillation occurred and North (1991:11), ‘‘the seasonal cycle is over the past 6 million years (Figs. 2–4), a the largest climate change we know of—the finding that appears to support the VS hy- change in temperatures over North America pothesis. Change does not mean, however, from winter to summer is far greater than that the selective environment of hominins glacial-interglacial changes in mean annual (or other organisms) was similarly altered. A temperatures of the Pleistocene.’’ By this given species may track the geographic shift- reasoning, seasonal cycles represent the wid- ing of key resources as its environment is est range of environmental change to which modified. In this way, an organism can stabi- organisms adapt in both temperate (Calvin lize its selective milieu rather than adapt to 1996) and tropical latitudes (Foley 1987). novel variations.7 Thus, the external environ- Greenland ice-core records further indi- cate that huge deviations in climate may ment (such as climate) and the selective occur abruptly on a decade-to-century time environment of an organism may sometimes scale—thus within the lifetimes of humans vary independently (Brandon, 1990). For and other long-lived organisms. It is pos- this reason, habitat-specific explanations of sible that such rapid oscillation—and dra- hominin evolution are quite plausible, for matic shifts on an even shorter cycle, such as they imply that early human populations El Nin˜ o—characterized the period of human continued to exploit certain favored habitats evolution (Calvin, 1996).6 Thus, short-term despite overall shifts in global and regional variation can account for the range of set- environment (Table 3, expectation 1). Alter- tings to which hominins had to adapt. Ac- natively, hominins may have mainly been cordingly, large interannual variations in affected by the overall trend, the average high seasonality regions should dominate change in environment, and the trend was a the environmental record of hominin evolu- more important selective factor on hominins tion. than the variance (range and pace of fluctua- tion) (Table 3, expectation 2). Variabilityselection hypothesis. The VS hypothesis suggests a series of well-defined 7Conversely, change in an organism’s selective environment (e.g., competitors, predators, disease vectors) may occur indepen- 6The El Nin˜ o cycle, however, may be only 5,000 years old dent of climate, tectonics, or other geologic factors that alter (Sandweiss et al., 1996). habitats. 114 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998

TABLE 3. Test expectations concerning the geologic association between hominins and paleoenvironments: Habitat-specific versus variability selection hypotheses Habitat-specific hypotheses Variability selection hypothesis Expectation 1 Over long periods, hominins were tied to Expectation 4 The oldest hominins occupied a diverse a specific habitat type—for example, array of habitats (responsible for the open savanna (savanna hypothesis), emergence of initial adaptations); over relatively closed habitat (woodland/ time, hominins occupied an ever-wider forest hypothesis), cold steppe or peri- diversity of environments as they glacial setting (ice-age hypothesis) evolved new means of adaptive versa- Expectation 2 Over long periods, hominins became asso- tility (correlated with widening envi- ciated with a new type of habitat (i.e., ronmental extremes manifested over habitat use changed in a specific direc- the long term) tion following the environmental trend Expectation 5 Early in time, habitat use by hominins of the period) (e.g., increasing hominin focused on a specific vegetational set- presence in open conditions supports ting (or other well-defined environ- the savanna hypothesis, even if earlier ment) but became more diverse later on hominins were in woodland settings) as the range of environmental fluctua- Expectation 3 In a given stratigraphic interval (within a tion increased basin), hominins are correlated with a Expectation 6 In any given stratigraphic interval, homi- specific habitat type (requires demon- nins are found in a specific environ- strating the contemporaneity of dif- mental setting; but in a long strati- ferent habitat types); over time, this graphic section (over long periods of hominin-habitat correlation was either habitat change), the sum total of maintained or permanently shifted to a hominin environments is highly new habitat type diverse (i.e., hominins persisted through widely fluctuating environ- ments within a locality)

The VS hypothesis differs here. It claims rica, Ardipithecus ramidus (Aramis), Austra- that external variance strongly impacted lopithecus anamensis (Kanapoi, Turkana), the selective environment of hominins. It and Au. afarensis (Hadar, Laetoli, Turkana); asserts that variable conditions created a and in southern Africa, Au. africanus wide disparity in adaptive settings over (Makapansgat, Sterkfontein). Specimens time. This disparity then created an overall sometimes attributed to the Hominini are selective environment that fostered adap- known from Lothagam and Tugen Hills (Hill tive change in hominins via the process of and Ward, 1988) but lack diagnostic charac- variability selection. ters related to bipedality or canine reduc- The most important factor in testing these tion. No hominins have yet been recovered expectations or any environmental hypoth- from late Miocene and early Pliocene beds at esis is the detail and precision of data re- Kanam (Kanam and Homa Formations), lated to paleoenvironments and their asso- though new work provides an outline of the ciation with hominins (fossils or artifacts). environmental sequence there. Taking all With this in mind, I turn to four time the localities into consideration, Table 4 intervals in which evidence of hominins shows that no single environmental recon- and/or paleoenvironments is reasonably de- struction applies to the oldest recognized tailed in certain localities or regions, suffi- members of the hominin clade or to the cient at least for an initial test of the hypoth- geographic regions where they have been eses defined here. found. Kanapoi, Tugen Hills, Lothagam, Aramis, ENVIRONMENTAL RECORDS AT EARLY and Kanam possess sediments between 6 HUMAN LOCALITIES and 4 million years old. Large mammals and 6.0 to 2.5 million years ago isotopic data from Kanapoi, Tugen Hills, Eastern and southern Africa. Table 4 and Lothagam suggest that a mosaic of open summarizes environmental reconstructions and closed vegetation typified East Africa of ten hominin localities older than 2.5 Ma. during this interval and that a relatively These represent the likely contexts of early stable mosaic might have persisted for a australopiths. A diversity of species, how- long time. According to Kingston et al. (1994), ever, inhabited these locales: in eastern Af- a heterogeneous setting composed of a mix Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 115

TABLE 4. African environments from late Miocene to Pliocene, prior to 2.5 million years ago Hypothesis (conditions Locality Age (Ma) Interpretation of early bipedality) References1 Sterkfontein 3.5 to 2.5 Closed vegetation Climbing adaptation (forest) 1, 2 Makapansgat 3.3 to 3.0 Many competing views Fossil mammals and arthropods Savanna, woodlands, or forest 3–7 Pollen: forest/bush = grass/shrub Fluctuation 8 Pollen: forest (earliest australopiths) Adaptation to forest 9 Omo-Turkana 4.0 to 2.5 Dramatic floral change in gallery forest Forest mosaic or fluctuation 10–13 Dry to wooded savanna plus gallery forest Adaptation to the mosaic 14 Hadar 3.9 to 3.0 Forest/bush = open country = grassy/ Fluctuation 15–17 plain = forest/bush Laetoli 3.8 to 3.5 Open savanna with seasonal aridity Adaptation to savanna 18–21 Woodlands and wooded savanna Adaptation to woodland 22 Kanapoi 4.2 to 4.0 Mixed vegetation Adaptation to the mosaic 23 Dry, open vegetation (micromammals) Adaptation to savanna 24 Aramis 4.4 Forest-woodland Adaptation to forest 25 Tugen Hills 5.0 to 2.5 Mixed grassy woodland Adaptation to the mosaic 26 Kanam 6.1 to 3.5 Closed = open = closed vegetation Fluctuation 27 Lothagam 7.9 to 4.7 Mixed open woodland Adaptation to the mosaic 28, 29

1 References: 1, Clarke and Tobias, 1995; 2, Berger and Tobias, 1996; 3, deGraff, 1960; 4, Cooke, 1978; 5, Kitching, 1980; 6, Vrba, 1988; 7, Reed, 1997; 8, Cadman and Rayner, 1989; 9, Rayner et al., 1993; 10, Bonnefille and DeChamps, 1983; 11, Bonnefille, 1984; 12, Bonnefille and Letouzey, 1976; 13, Williamson, 1985; 14, Brown, 1981; 15, Johanson et al., 1982; 16, Gray, 1980; 17, Bonnefille, 1995; 18, Hay, 1981; 19, Hay, 1987; 20, Leakey and Harris, 1987; 21, Bonnefille and Riollet, 1987; 22, Andrews, 1989; 23, Leakey et al., 1995; 24, Winkler, 1995; 25, WoldeGabriel et al., 1994; 26, Kingston et al., 1994; 27, Potts et al., 1997; 28, Leakey et al., 1996; 29, Harris and Cerling, 1995.

of C3 and C4 plants prevailed in central stratigraphic resolution give evidence of a Kenya over the past 15.5 myr. These data temporal oscillation between 30 and 70% C4 suggest long-term environmental stability, grasses. The data suggest a mixed C3/C4 and that spatial habitat diversity (rather plant community but with nonoverlapping than long-term fluctuation) played a strong variations through time (Potts et al., 1997). role in the origin of bipedality. Since hominin fossils have yet to be found in With regard to Kanapoi (4.2 to 4.0 Ma), this sequence, it is unclear whether bipeds Leakey et al. (1995) similarly suggest that ever encountered the range of habitat change the oldest known bipeds inhabited a spatial implied by the isotopic data. mosaic of vegetation. Interpretation of the At the Ethiopian site of Aramis (4.4 Ma), mammal microfauna, however, suggests that fossil wood, abundant seeds of the woodland/ Kanapoi’s environment was open and rela- forest-dwelling Canthium, and the mix of tively dry around 4 Ma. Although trees were fossil vertebrates (rich in primates and present, extensively wooded tracks did not tragelaphine bovids) all point to a more apparently occur (Winkler, 1995). closed habitat (woodland or forest) than is The bovid, elephant, and equid fauna from generally recognized at the Kenyan sites Lothagam (7.9 to 4.7 Ma) supports the mo- (WoldeGabriel et al., 1994). The paleobotani- saic interpretation. The fossils suggest a cal evidence, which seems particularly im- mixture of riverine forest, woodland, and portant, derives from a carbonate horizon open grassy patches, although the assem- just beneath the pedogenically altered silts blage is time-averaged over a lengthy pe- that contain Ardipithecus ramidus and other riod, making it difficult to discern the vari- fossil vertebrates. ance between stratigraphic units. Stable Later, at Laetoli (3.8 to 3.5 Ma), the isotopic study of soil carbonates also implies geologic clues (aeolian tuffs, calcite-rich soils, a mixed habitat at Lothagam, a prevalence soil-weathering products) largely point to a of C3 plants but with C4 grasses also present dry savanna, where vegetation was at least (Leakey et al., 1996; Harris and Cerling, seasonally insufficient to prevent wind trans- 1995). port of sand-sized ash particles (Hay, 1981, At Kanam (6.12–3.5 Ma), fossil verte- 1987). Interpretation of the fossil pollen brates relate to a diverse, spatially mosaic samples agrees with this open savanna- environment. Isotopic data plotted with fine grassland interpretation (Bonnefille and Ri- 116 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 ollet, 1987). However, a different reading of originally buried or, based on taphonomic the evidence, largely the faunal remains, analysis, in which the hominins may actu- suggests a more closed, wooded community ally have lived and died. At a locality like of mammals, similar to woodland habitats of Makapansgat, paleoenvironmental data may the Serengeti region today (Andrews, 1989). come from the same stratigraphic member Around the same time, 4 to 3 million years as fossils of Australopithecus, but the amount ago, Hadar and the Omo-Turkana basin of time-averaging represented by any mem- offer evidence of a diverse range of habitats. ber may be very long. Analysis of a single Stratigraphic analysis of fossil wood, fruit, stratigraphic unit may yield an average and pollen (Bonnefille, 1984, 1995; William- environment largely irrelevant to the actual son, 1985) and large mammals (Gray, 1980; environments (and their variance) encoun- Johanson et al., 1982) suggest that consider- tered by early humans. At Laetoli, surface able fluctuation in climate, vegetation, and collecting of nearly all of the fossils means available water sources (basin hydrology) that 300,000 years of fossil animals are occurred over the long term in these areas treated as if they reflect a single community without clear sign of an aridity trend. or environment. Arguments about the best The wide range of environmental interpre- or most accurate reconstruction of the Laetoli tations of Pliocene sites, both in eastern and paleoenvironment are useless without bet- southern Africa (Table 4), depends on the ter stratigraphic and analytical control over type of geologic evidence, time period, and the data. This same problem applies to even the taxonomic group of fossil animals Hadar, Kanapoi, Tugen Hills, and most lo- and plants that are assessed in one study or calities. another. This range includes apparent sup- The evidence reviewed here nonetheless port for a savanna interpretation and for its suggests a preliminary interpretation. The antithesis, in which tropical forest or wood- body of data does not consistently favor land is considered the favored environment either the savanna or forest adaptation of early bipeds. At Makapansgat, for ex- model, even though data from individual ample, pollen data examined by Rayner et localities or stratigraphic levels have been al. (1993) suggest that A. africanus was used to defend both interpretations. The adapted specifically to forested habitat. Yet presence of Ar. ramidus in closed vegetation an earlier study of fossil pollen from this (at Aramis) coupled with Au. afarensis in same site emphasized the ‘‘fluctuating cli- more open settings later in time (at Hadar matic and vegetational conditions’’ in which and Laetoli) might suggest that the latter this species lived (Cadman and Rayner, 1989: evolved the capability of inhabiting more 112). open environments, while Ardipithecus was confined to closed woodlands (WoldeGabriel Discussion. Late Miocene and early et al., 1994). This interpretation would favor Pliocene environments in Africa provided the savanna hypothesis (Table 3, expecta- the context of the oldest bipedal apes. Infor- tion 2). This view can be valid, however, only mation about these environments is ulti- if eventually more than one Ar. ramidus mately pertinent to the origin of bipedality locality is found and closed habitat is deter- itself. This issue continues to be one of the mined to have been the domain of other most problematic in the study of hominin populations of this species. evolution. The combined evidence from all localities The largest problem in testing environ- indicates that in general the oldest homi- mental hypotheses with data from these nins were associated with a diverse range of localities concerns the poor connection be- habitats—forest, woodland, open savanna, tween hominin fossils and paleoenviron- and mosaic habitats. Lacking stratigraphic ments. Linkage in time and space between resolution in most of these sequences, it is these two types of data is imprecise. Rarely still evident that hominins experienced a do samples for paleoenvironmental analysis wide range of adaptive conditions at differ- come from the exact loci and sedimentary ent sites and at different times. Notwith- environment in which hominin fossils were standing the comparison between Aramis Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 117 and Hadar/Laetoli, there does not seem to be dense stands of trees and larger tracts of a dominant environmental trend in the data. open vegetation. Responsiveness to this type At some sites, there are hints of long-term of environmental disparity was, over time, a fluctuation in vegetation and climate. Com- vital aspect of locomotor function. The loco- bining all the localities also seems to sup- motor system of the earliest hominins, by port this idea on a broad regional scale. this interpretation, enabled them to accom- Hence, I have argued previously that early modate to the habitat variance. The original hominin bipedality may best be explained as functional benefit of bipedality resided in a response to variability selection (Potts, the versatility it afforded as local, prehomi- 1996a,b, in press a). Beyond the terrestrial nin populations faced a changing series of evidence, the deep-sea isotopic record shows vegetation types over time. As a result, the a dramatic rise in global environmental early australopiths possessed a locomotor variability between 6 and 5 Ma relative to system that persisted for 2 million years or the variability in each 1 million year inter- more, adaptable to the diverse settings that val back to 27 Ma (Potts, in press a) (see Fig. arose in the hominin geographic range be- 2B). The rise is enhanced even more be- tween 6 and 2.5 Ma. tween 4 and 3 Ma. As Table 4 shows, habitat In brief, expectations 1 and 3 (Table 3) reconstructions are also sufficiently varied concerning a habitat-specific interpretation to be consistent with a rise in environmental seem to be contradicted even by the poor- fluctuation, essential to the VS hypothesis. resolution data available. Temporal and spa- The key to this interpretation, though, tial resolution of hominin-environment asso- rests on the original functions of bipedal ciations is too poor either to support or reject behavior over the period of its origin. Of expectation 2 (Table 3). The savanna hypoth- what use was australopith bipedality in the esis (as an explanation of bipedality) would environments in which it evolved and was be favored if it could be shown, eventually, later maintained? Based on utility, it makes that 1) prehominins tended to occupy rela- sense that a novel form of terrestrial locomo- tively closed vegetation, 2) the earliest homi- tion arose during forest reduction and sa- nins tended to live (and die) in less closed vanna expansion. The function of early aus- vegetation and, later still, in fairly open tralopith locomotor structure has, however, habitats, and 3) locomotor and positional been controversial. According to one camp, functions of early australopiths were (in early bipeds were committed to terrestrial contrast to the argument above) largely bipedality; their skeletal structure was de- dedicated to terrestrial two-legged walking. signed to conduct this form of locomotion The VS hypothesis, by contrast, does not effectively (e.g., Lovejoy et al., 1973; Latimer appear to be contradicted by any aspect of and Lovejoy, 1990; Ohman et al., 1997). the evidence. However, the sample size of According to the other camp, the overall sites, the resolution of time, and the preci- skeletal structure of Pliocene hominins indi- sion of fossil-environment associations are cates the importance of arboreal activity. not adequate to test this hypothesis effec- Terrestrial bipedality was not particularly tively. The oldest bipeds may have inhabited effective, and the behavior of these hominins a wide range of habitats (Table 3, expecta- was largely geared to life in the trees (e.g., tion 4). In any one time or place, they may Stern and Susman, 1983; Susman and Stern, have occupied a fairly specific habitat which 1991). The debate largely centers, then, on varied according to the available vegetation the degree to which early australopiths were (Table 3, expectation 6). The data, though, committed to either arboreal climbing or do not appear to support expectation 5 (Table terrestrial bipedal striding. 3), the expansion of habitat diversity over That this controversy persists, however, time. suggests a different adaptive explanation. Despite global and some local evidence of The two viewpoints can be reconciled to a fluctuation, it is quite conceivable that a certain degree by challenging the either/or mosaic of open and closed vegetation (pro- question. Environmental fluctuation led, at moted by seasonal variation) favored the different times and places, to settings with origin of bipedality, including a mixture of 118 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 terrestrial and arboreal activities in early proto-Omo River. Artifact concentrations Pliocene hominins. The key question here were as low as before, and the choice of raw concerns the stability of the open/closed material was still quite localized, with trans- mosaic over time. Was the mosaic itself port distances possibly much less than 25 adequate to account for the range of environ- km (Rogers et al., 1994). The main difference ments to which early Pliocene hominins is that archeological sites were stationed in were adapted? Or was a more extensive at least three different contexts. One set- range of variability, expressed over longer ting, most similar to earlier times, was at time periods, crucial to the evolutionary the intersection between braided and axial process? This, of course, is not necessarily drainages. The main river itself, some dis- an either/or issue, as both spatial and tempo- tance from the braided tributaries, was used ral variability may have played important as a second setting; unlike the earlier pe- roles. The existing data from the 6 to 2.5 Ma riod, the proto-Omo river itself was appar- time span, however, do not appear to imply ently a useful source of lithic raw material. habitat stability. The third setting occurred at the lake mar- gin, where alluvial fans from the western 2.3 to 1.7 million years ago highlands drained into the lake. According Turkana and Olduvai basins. Domi- to Rogers et al. (1994), archeological re- nated by fluvial settings for most of its mains in this context are the only known history, the Turkana basin offers a rich Plio-Pleistocene lake-margin sites in the Tur- dataset for assessing environmental change kana basin. The fluctuating lake persisted associated with early humans of the latest to about 1.7 Ma (Brown and Feibel, 1991). Pliocene. In this period, Turkana (including Within the interval 1.8 to 1.7 Ma, a major the Omo region) underwent a series of major shift in the isotopic composition of paleosols paleogeographic changes (Feibel, 1988; Fei- has been noted, which signifies a likely bel et al., 1991; Brown and Feibel, 1991). expansion of open C4 plant habitat within During the same time, hominin toolmakers the Turkana basin (Cerling et al., 1988; considerably altered their landscape interac- Cerling, 1992). This vegetation shift is re- tions. This is evident in the archeological corded at the top of the Olduvai geomagnetic record from the growing diversity of paleoen- subchron, currently estimated to be 1.757 vironments in which stone tools were dis- Ma (Cande and Kent, 1992). carded (Rogers et al., 1994). By 1.7 Ma, another major paleogeographic About 2.3 Ma, a large, meandering river, shift had occurred. The central lake disap- like the Omo today, occupied the axis of the peared, replaced by an unstable fluvial sys- basin. The river was fed by braided streams tem. A very dynamic series of landscapes that descended from the eastern and west- ensued. Environments dominated by an axial ern highlands. Archeological sites, which river alternated with others consisting of consisted of low-density artifact clusters, intricate expanses of braided streams. This were focused where the braided and main pattern of fluctuation, filling in of channels, river systems met. Proximity to both aspects followed by further carving of the flood- of the fluvial system was apparently critical plains, was influenced by massive volcanic in inducing hominins to produce and deposit activity in the region. Dramatic change oc- stone tools. Rocks, which were obtained curred as volcanic ash blanketed the terrain from the braided, marginal drainages, need occupied by toolmakers. Archeological sites not have been transported by toolmakers occur in channel, gravel bar, overbank, and any great distance, as stream conglomerates floodplain settings—a far greater diversity were available within several tens of meters than in any earlier period, which seems to up to a few kilometers from the artifact sites follow from the temporal fluctuation and (e.g., Lokalalei site: Kibunjia, 1994). greater spatial division of the landscape By about 1.9 Ma, the landscape had (Rogers et al., 1994). changed dramatically. The center of the The Olduvai basin offers a second impor- Turkana basin was occupied by a sizable yet tant dataset for the latest Pliocene. During fluctuating lake fed from the north by the this period, the basin was lake-dominated; Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 119 many archeological remains are preserved hydrology and terrain took place in response in lake-margin settings, a point of consider- to tectonic and climatic influences (Hay, able contrast to Turkana. The sediments of 1976a). Bed I and lower Bed II at Olduvai (Fig. 6) are considered to span perhaps 50–100 kyr, Discussion. Two important developments bracketed between dates of 1.9 and 1.7 Ma in this time span were the origin of Homo (Hay, 1976a; Walter et al., 1991). A small ergaster (early African H. erectus), appar- 8 alkaline lake (7–25 km wide) persisted ently by 1.88 Ma, and expansion of the throughout this time; thus, hydrological ranging patterns of the toolmakers. With change was not as dramatic as in the longer body size and limb proportions similar to Turkana sequence. Sedimentary, isotopic, modern humans, H. ergaster was an endur- faunal, and pollen studies nonetheless point ance-oriented biped whose skeleton ab- to impressive shifts in climate and vegeta- sorbed high mechanical stresses probably tion (Hay, 1976a; Cerling, 1992; Cerling and related to locomotion (Ruff et al., 1993). Hay, 1986; Sikes, 1994; Butler and Green- With the Nariokotome skeleton (KNM-WT- wood, 1973, 1976; Jaeger, 1976; Potts, 1988; 15000) as a guide, body shape was linear Bonnefille and Riollet, 1980; Bonnefille, and adapted to heat stress, typical of mod- 1995). ern humans from hot, dry, tropical areas. Since it was not well adapted to relatively Three significant transitions are apparent closed, wet environments, H. ergaster may in the environmental record of Bed I and have marked a transition in the climatic lower Bed II (Fig. 6). Wet, marshy, and adaptation of early hominins (Ruff and wooded conditions low in Bed I (approxi- Walker, 1993; Ruff, 1991). mately 1.8 Ma) gave way to aridity and a The Nariokotome skeleton (approximately more open setting near the top of this unit. 1.50–1.55 Ma) postdates the dramatic shift In basal Bed II, there was a return to more to an arid, hot climate signaled in the Tur- moist and wooded habitats. Finally, the Le- kana and Olduvai sequences at about 1.75 muta Member at the top of lower Bed II Ma. In this regard, the climate-sensitive signals the prevalence of dry conditions with shape of this particular individual makes sparse vegetation coverage. Near the end of sense. Stature estimates based on other the Olduvai geomagnetic event (approxi- fossils from Turkana further suggest that mately 1.76 to 1.75 Ma), recorded near the the transition to tall, linear body form (x ϭ base of the Lemuta Member, there was a 170 cm, n ϭ 6) had occurred by 1.70 Ma, marked shift in soil ␦13C and ␦18O (Cerling whereas individuals who lived about 1.88 and Hay, 1986). Evidently, this reflects a Ma (represented by fossils ER 1472, 1481, dramatic change in the percentage of C 4 and 3728) were significantly shorter (x ϭ plants—from 20–40% up to 60–80%, repre- 147 cm, n ϭ 3) (Ruff and Walker, 1993). In senting an open, grassy environment. This general, then, evidence for an adaptive shift shift may be correlated with the similar one in body form is well correlated with or at Turkana (Cerling, 1992). The aridity con- immediately follows evidence of a major ditions of the Lemuta Member are esti- shift to hot, open habitat. mated to have lasted perhaps 50 kyr (Hay, The first appearance of H. ergaster, how- 1976a). ever, is perhaps 130 kyr older than this Stone tool sites—both concentrations and climatic event. An arid interval slightly older scatters of artifacts—occur throughout the than the major environmental shift seems to sequence. This suggests that Oldowan tool- be recorded in upper Bed I Olduvai (Fig. 6). makers had the means to adjust to the Pollen analysis suggests that this environ- changes they faced. Averaged over the se- ment supported mainly small herbaceous quence, there was one major environmental plants rather than trees (Bonnefille and transition every 33–50 kyr, which is within Riollet, 1980). Yet this environment is no the range of the 41 kyr periodicity related to orbital obliquity. Later, in upper Bed II to Bed IV times, the lacustrine system was 8Based on occipital specimen KNM-ER-2598 from the KBS Member of the Koobi Fora Formation (Rightmire, 1991; Wood, disrupted, and numerous changes in fluvial 1991). Fig. 6. Composite record of environmental change in Bed I and lower Bed II, Olduvai Gorge (northern Tanzania), approximately 1.85 to 1.75 Ma, based on interpretation of sedimentary, mineralogical, polynological, stable isotopic and faunal evidence. The presence of hominins is indicated by Oldowan archeological levels. See text for references. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 121 older than 1.8 Ma (Walter et al., 1991), still marrow from large animals (Ͼ30 kg) (e.g., after the first appearance datum of H. ergas- Bunn and Kroll, 1986; Potts, 1988; Blumen- ter. One explanation is that the adaptive schine, 1995; Monahan, 1996). shift in body form was decoupled from specia- As noted above, these changes took place tion, evident also in the climatic adaptation at Turkana in a dynamic series of environ- of modern humans. If, on the other hand, ments. During a period of landscape instabil- further discoveries of postcranial bones indi- ity (approximately 1.9 to 1.5 Ma), toolmak- cate that body form changed in this species ers became less tethered to local rock sources before 1.8 Ma, the adaptive shift preceded and particular habitats. They carried stones the most notable grassland expansion discov- farther and made clusters of tools in more ered so far in the East African Plio-Pleis- diverse areas of the landscape. The favored tocene record. areas included both old and new habitats The second key development in the period (e.g., near the axial channel and on fluvial 2.3 to 1.7 Ma is reflected by archeologically floodplains). The evidence thus points to an visible behaviors related to habitat use and expansion in landscape use rather than a stone transport. A significant problem here directional shift. concerns the temporal and geographic over- lap of potentially multiple species of toolmak- The toolmakers of Bed I Olduvai tended to ers. It is not possible, except by special use stone sources within the confines of the pleading to unsupported assumptions, to lake floodplain. Transport distances rarely say which species was/were responsible for exceeded 3 km (Leakey, 1971; Hay, 1976a). the stone tools deposited in any given stra- After a series of environmental fluctuations, tum or place. Overlap of possibly three lin- the toolmakers of middle Bed II expanded eages of Homo, as well as the presence of their range to depositional environments Paranthropus, underscores the problem, and both within and beyond the immediate lake- at present there is no obvious solution. margin area. Stone sources outside the lake Thus, in this period of the Plio-Pleistocene, margin were also newly exploited. These paleoanthropologists can do no more than developments at Olduvai, then, parallel make observations about generic stone tool- those in the Turkana basin. makers—overall shifts in archeological pat- At both localities, toolmakers were active terns independent of how specific behaviors through substantial shifts in environment were apportioned to lineages. This state of and were present in arid, moist, warm, cool, affairs is less than ideal for evolutionary more wooded, more open, lake-dominated, analysis. But observations of this sort do and fluvial-dominated phases. This suggests allow important changes in behavioral strat- that Oldowan toolmakers could accommo- egies to be identified, which may be correlat- date to periodic shifts in climate, basin able with the rise of new taxa. Even when hydrology, and vegetation. The persistence they are not correlated, generic behavioral of hominin toolmakers and their eventual shifts are important to identify in an analy- expansion into a greater range of habitats sis of hominin evolutionary history. are consistent with the VS hypothesis. In During the late Pliocene, Oldowan flaking this light, the transport of resources over was conservative. The main behavioral longer distances, occupation of a wider habi- changes concerned the environments in which stone tools were used and left. In the tat range, and intensification of animal food Turkana and Olduvai basins, stone trans- exploitation may represent means of dealing port distances increased slightly, and arti- with large changes in the distribution of facts were left in a much greater diversity of habitats, water, food, and other necessities. environments by the end of this period com- If this interpretation is correct, these behav- pared to its beginning (Rogers et al., 1994; ioral changes can be considered adaptations Potts, in press b). In addition, the processing to environmental instability rather than to of animal tissues intensified. Bones from any one specific type of habitat (e.g., wood- archeological sites manifest obvious damage land) or directional trend (e.g., increasingly as a result of hominins obtaining meat and arid savanna). 122 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 1 million to 250,000 years ago 3. Members 8 and 9 (780 to 662 ka). Three Olorgesailie basin. Located in the rift major shifts in the depositional and hydro- valley of southern Kenya, Olorgesailie is a logical regime of the basin are recorded in Pleistocene lake basin well known for dense this period of about 118 kyr. accumulations of Acheulean stone bifaces 4. Members 10–12 (662 to 601 ka). In a span (Isaac, 1977) and a well-calibrated record of of 61 kyr, three dramatic environmental fossil mammals and environmental change changes occurred involving the disappear- from about 1.2 Ma to 49 ka (Isaac, 1978; ance, reappearance, and expansion of the Owen and Renaut, 1981; Potts, 1989, 1994, lake. 1996a; Deino and Potts, 1990; Tauxe et al., 5. Members 13 and 14 (601 to 493 ka). In 1992; Potts et al., in press). The main por- this section, covering an estimated 108 tion of the Olorgesailie Formation, compris- kyr, two major environmental shifts are ing 14 members, is dated between Ͼ992 ka inferred, which correspond to the two and 493 ka, a span of about 500,000 years. members originally recognized by Shack- The geologic sequence consists of lake diat- leton (1978). omites, associated floodplain and fluvial sedi- Artifacts occur throughout the entire sec- ments, and tephra. tion. There is no consistent association be- Figure 7 depicts a composite stratigraphic tween the density of stone artifacts (a par- section of the Olorgesailie Formation (based tial measure of the level of hominin activity) on observations by R.M. Shackleton, G.L. and specific kinds of depositional environ- Isaac, A.K. Behrensmeyer, and R. Potts). It ment or hydrological regime. Toolmakers also indicates the stratigraphic position and did not occupy the basin continuously, but relative concentration of stone artifacts as they left evidence of their activity in fluvial determined by recent research in the region. and lacustrine settings and at times when The illustration shows, in general terms, the the lake was large, small, or not apparent at periods of stability and change in the Olorge- sailie basin and enables an estimate of the all. Analysis of paleosol stable isotopes by N. rate of major environmental revisions in Sikes (Sikes et al., 1997, in press, and in different parts of the stratigraphic section. preparation) has also documented change in The datable sequence can be divided into vegetation, including a grass-dominated five main intervals. Several significant open-savanna setting in upper Member 1 changes are obvious in each interval (see and more closed, bushland conditions in Fig. 7 for details). later members. Overall, vegetation was more open than in the period prior to 1 Ma, as 1. Members 1–5 (992 to 974 ka). Covering documented at other East African sites. approximately 18 kyr, five major remodel- Based on isotopic data from Olorgesailie and ings of the landscape can be documented Olduvai, Cerling (1992) infers that grass- after the lake became established (Mem- lands became a persistent part of East Afri- ber 1). These changes affected basinwide can environments sometime after 1 Ma, hydrology in ways that would have al- perhaps as late as 600 ka. Even within this tered the distribution of water-depen- more open setting, extensive and recurrent dent plants and animals. alteration of hydrology and landscape fea- 2. Member 6/7 (974 to 780 ka). This part of tures was the dominant environmental sig- the section records three major landscape nal of the Olorgesailie Formation. Modifica- changes. Toolmakers interacted with a tions occurred in episodes several thousands series of fluvial, fluctuating lake-margin to tens of thousands of years apart, elicited and stable, vegetated environments. From by climatic, tectonic, and volcanological the dated pumice in Member 5 to the causes. Brunhes/Matuyama boundary at the top of Member 7, a span of approximately Zhoukoudian, the Loess Plateau, and 194 kyr is represented. Most of this time Bose basin, China. Outside of Africa, is taken by upper Member 7 soil forma- hominin localities with lengthy stratigraphic tion (Potts et al., in press). records (Ͼ500,000 years) and well-cali- Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 123

Fig. 7. Major environmental changes recorded in 14 members of the Olorgesailie Formation (southern Kenya), 992 to 493 ka, based on lithology and diatoms. The concentration of stone tools within particular strata is indicated in relative terms: high, medium, and low. brated environmental sequences are ex- graphic units, though sedimentation ap- tremely rare. One of the few that come close pears to have been largely uninterrupted. to meeting these two criteria is Zhoukou- This section ranges from older than 780 ka dian, the famed Peking Man (Homo erectus) at the base (layers 13–18) to about 230 ka cave, southwest of Beijing in northern China. (based on uranium series analysis) near the A section over 40 m thick at Zhoukoudian top (layers 1–3). Between 1927 and 1982, Locality 1 has been divided into 18 strati- hominin fossils and stone artifacts were 124 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 recovered from layers 10 through 2, covering ered from the loess deposits, hominin locali- an estimated time range of 460 to 230 ka. H. ties such as Lantian, Dali, and Dingcun are erectus fossils are known from layers 2, 4, 8, situated in the loess region. and 10. Definite stone artifacts derive from As reviewed earlier, China’s loess sequence layers 3–10, with the exception of layer 5 furnishes a record of environmental change (Black et al., 1933; Jia and Huang, 1990; over the past 2.4 myr. Extensive fluctuation Binford and Ho, 1985). in vegetation, moisture, and temperature A variety of views has been offered about characterized this region of north-central the paleoenvironments of Zhoukoudian China (Fig. 4). Over the past 1 million years based on Locality 1. Mammalian fossils exca- or so, loess accumulated uninterrupted for vated from each layer provide a rudimen- 40 kyr or more on at least six occasions, tary dataset but demonstrate convincingly beginning approximately 1,020 ka, 830 ka, that substantial environmental change oc- 650 ka, 480 ka, 180 ka, and 75 ka (Kukla and curred during the period of H. erectus’s An, 1989). (According to data illustrated in presence in or near the cave. According to Xu Fig. 4, the earliest of these episodes may and Ouyang (1982), species that signal a have entailed considerable fluctuation.) The cool, grassland habitat outnumber forest loessic intervals, which represented dry, rela- taxa in layers 11 and 10. Layers 9 and 8 are tively treeless habitat, were interspersed decidedly different: grassland rodents are with soil formation periods in which dry to absent, and forest mammals dominate the moist forest prevailed. assemblage, indicating a change to a warm, By contrast, southern China is generally humid environment. Slightly colder condi- believed to have had an extremely stable tions prevailed in the time of layers 7 and 6, Pleistocene climate. Red lateritic soils occur and a warm, humid fauna reappeared in the throughout much of this region and into span of layer 5. Layer 4 contains a cold- Southeast Asia and India. The laterites range weather mammalian fauna, with a predomi- from a few meters to over 10 m in thickness, nance of grassland taxa. Finally, a cool, and their rather uniform appearance has grassland-forest mosaic prevailed in layer 3 been taken to imply a static Pleistocene times. Xu and Ouyang (1982) consider this environment (e.g., Teilhard de Chardin et sequence to match up with ␦18O stages 12 al., 1935; Movius, 1969). through 8, which implies that the conditions This characterization, however, appears at Zhoukoudian were affected by the phases of global cooling-warming and evaporation to be incorrect. Based on recent work in the reflected in the deep-sea oxygen isotope Bose basin, Guangxi Autonomous Region, curve. Based on Zhoukoudian sediments, an 8 m thick laterite containing stone tools fossil pollen, and fauna, Liu (1983) supports (dated approximately 730 ka) exhibits strong this idea and notes that cold-warm fluctua- cyclicity of clay, heavy minerals, and other tions intensified during the middle Pleis- sediments (possibly loess) when examined tocene. Jia and Huang (1990), however, dis- on a microstratigraphic scale (10 cm inter- agree with this conclusion. Their view of the vals) (Huang and Potts, in preparation). The mammalian fauna is that a single cool-warm- Pleistocene red soils of south China, in fact, cool cycle is indicated between layers 11 and appear to have formed as part of a climatic 1 and that in general all levels included a cycle linked to the loess-paleosol cycles to mixture of warm- and cool-adapted taxa. the north (Zhu et al., 1995). Furthermore, According to Jia and Huang (1990), most of analysis of fossil pollen obtained from cores the mammalian remains from Zhoukoudian drilled in the South China Sea indicates belong to species that were very adaptable to recurrent north-south migration of tropical climatic change. and subtropical vegetation zones through To the south and west of Zhoukoudian is southern China during the Quaternary the Loess Plateau, on which thick loess depos- (Wang et al., 1991). Together, these lines of its are distributed over an area of 274,000 evidence suggest that middle and late Pleis- km2 (Liu, 1988). Although no hominin re- tocene inhabitants of South China, like homi- mains have, to my knowledge, been recov- nin populations in other parts of the world, Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 125 faced a significant range of environmental periods of time. In each record examined variability over time. here, long-term modifications presented a much more varied, complexly changing se- Discussion. Several evolutionary changes ries of adaptive conditions at different times occurred in hominins over the past 1 million than any one population would have faced years. An increase in relative brain size was over seasonal or interannual periods. These among the most prominent from an adaptive points are at odds with the expectations of standpoint, and it is interesting to examine the seasonality and habitat-specific hypoth- how environmental change might relate to eses and are consistent with the idea of it. Rapid encephalization in hominins oc- variability selection (Potts, in press a,b). curred during the middle Pleistocene, be- 140,000 to 50,000 Years Ago tween 780 and 130 ka (Ruff et al., 1997; Aiello and Wheeler, 1995). There is growing Combe Grenal, France. The rock shelter evidence, moreover, that cognitive and be- at Combe Grenal, excavated by Bordes (Bordes havioral competence expanded in hominins et al., 1966), provides one of the longest late during this same interval. This evidence Pleistocene sequences in Europe and the may include hearths and control of fire most complete record of environmental (Kretzoi and Dobosi, 1990), manufacture of change in southwestern France between 140 well-designed spears (Thieme, 1997), neuro- and 50 ka (Laville et al., 1980; Mellars, logical control over speech (Kay et al., 1998), 1996). Nearly 13 m thick, the sediments occupation of high latitudes (Waters et al., represent the fill of a small cave, which were 1997), and possibly ways of enabling island divided during excavation into 64 archeologi- habitation (Morwood et al., 1998). It is un- cal layers. Figure 8 illustrates two datasets, clear what parts of this cognitive/behavioral the percentage of arboreal pollen and rela- package were possessed by early humans of tive abundance of two cervid species, that Olorgesailie, Zhoukoudian, or central China. serve as measures of climatic conditions. But as these localities offer among the best These are displayed in relation to the lithic stratigraphic records for this period, they industries deposited in the shallow cave by provide a way to assess the environments Middle Paleolithic toolmakers. Although Bor- faced by certain hominin populations as des (1961) believed the diversity of indus- brain size and behavior evolved. tries reflected different cultural traditions, None of these records lends support to others interpret the variations in terms of habitat-specific explanations of hominin different activities, different phases of stone adaptive change. Substantial environmen- reduction, or temporal phases of a single tal shifts, local to global in scale, occurred cultural tradition. It is generally believed over the past 1 million years. Large-scale that populations of Neanderthals were re- changes were manifested over periods of sponsible for these varied stone tool assem- 104–105 years. At Olorgesailie, 16 major blages. revisions of basin hydrology, paleogeogra- The Combe Grenal record documents ex- phy, and depositional environment occurred tensive changes in environment, which are between 992 and 493 Ma, an average of summarized by Mellars (1996). about 31 kyr between each transition. The 1. Corresponding to ␦18O stage 6, layers 64 distribution is uneven, however, with mean to 56 reflect severe glacial conditions. An intervals of about 3.6, 20, 39, 54, and 65 kyr extremely cold, dry, open climate is sig- between transitions for different parts of the naled by sedimentary frost cracking with stratigraphic section. Since depositional little chemical weathering, large percent- rates were neither constant nor continuous ages of reindeer (92–97% of all ungulates (e.g., erosional phases at the base of M6/7 [not represented in Fig. 8B]), and consis- and M9), these estimates do not represent tently low levels of tree pollen, most of actual periodicities. Yet they do imply that which is pine. large-scale revamping of the terrain and 2. A hiatus is then represented by heavy environment, greatly exceeding seasonal weathering and soil formation of earlier types of oscillations, took place over lengthy deposits. As no new sediments were laid 126 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998

Fig. 8. A: Percentage of arboreal pollen in layers 1–62 at Combe Grenal (France), 140 to 50 ka, and the presence of hominins indicated by lithic industries in the same layers. B: Changes in the relative percentage (minimum number of individuals) of red deer and reindeer in layers 55 to 10 at Combe Grenal (approximately 115 to 55 ka) and associated lithic industries. Based on Bordes et al. (1966) and Chase (1986). (Reproduced from Mellars, 1996, with permission of the publisher.)

down, fauna, artifacts, and pollen repre- there a setting comparable to the glacial senting this period are absent. Correla- periods of layers 62 to 56 and Wu¨ rm II. tion with the warm, humid conditions of 4. A dramatic environmental shift back to the last interglacial, lasting for several full glacial conditions, with extremely thousand years, is thus based on the low levels of arboreal pollen (nearly total altered character of the sediments. disappearance of species other than pine), 3. Layers 55 to 38 formed during a succes- is recorded from layers 37 through 1. In sion of four climatic oscillations, includ- the continuous pollen records of eastern ing two severe cold episodes and two France, Grande Pile and Les Echets (Fig. longer warm intervals. These oscillations 5), the transition to glacial conditions is are well portrayed in the pollen record, marked by one or two very rapid oscilla- while the ungulate fauna shows a stable tions (i.e., within 1,000–2,000 years), to preponderance of red deer throughout. warm-humid, then cold, and finally cool- Arboreal pollen reached 60–70% during wet conditions. These fluctuations may the warm times and included deciduous be reflected at Combe Grenal in the warm, species like elm, lime, and alder, which wetter episode of layer 38 and a weather- require warmth, in addition to cool- ing horizon between layers 36 and 35. loving pine and juniper. Although warm Following layer 35, dry, cold steppe pre- and cold phases were dramatically differ- vailed—a markedly rigorous setting for ent from one another, at no time was humans inhabiting the region. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 127 Although data are lacking for the last climate and vegetation varied over time. interglacial (hiatus), it appears that Nean- The possible strategies for accomplishing derthal populations occupied the cave and this might have involved shifting to sea- its surroundings essentially during the en- sonal (e.g., summer) use of the site during tire sequence, leaving late Acheulean and cold phases (isotope stages 5a and 5c), which Mousterian artifacts in all settings. might have enabled continued access to red deer herds, or long-distance tracking of red Discussion. The environmental condi- deer (and transport of parts back to Combe tions of Mousterian toolmakers in western Grenal) as its range shifted in response to Europe ranged from glacial steppe/tundra to climatic oscillation. interglacial temperate forests. The last inter- 18 Both patterns—initial stability and later glacial period (␦ O stage 5e) represented an change in faunal exploitation—suggest the especially warm period that contrasted considerable adaptability of Mousterian tool- starkly with intense, long-lasting glacial makers at Combe Grenal. The same conclu- conditions immediately before. In the initial sion is warranted by the nearly continuous 5 kyr of the last interglacial, global mean presence of toolmakers through a period of annual temperature increased 10–15°C (as vast environmental alteration. As we have did local temperature) (Fig. 5), and sea level seen in other places and times, this body of rose 5–6 m above that of the present (com- evidence best fits the VS hypothesis (e.g., pared with 100 m lower during isotope stage Table 3, expectation 6). The humans of 6). Interglacial conditions (and interstadial periods during the last glacial) established a Combe Grenal certainly were not tied to a temperate climate that included cold win- single type of habitat. Nor did they appear to ters, to which trees of the European decidu- be more successful as the environment ous forests were adapted. Hominin popula- changed in one particular direction com- tions in the vicinity of Combe Grenal, pared with another. Moreover, thousands to therefore, lived in either seasonally or peren- tens of thousands of years passed between nially colder surroundings than populations major remodelings of the adaptive setting. in most other parts of the world. Long-term shifting of vegetation and cli- Western European Neanderthals were ob- mate far exceeded seasonal ranges of varia- viously equipped to deal with large changes tion. This observation conflicts with any in vegetation, fauna, and climate, such as attempt to explain the continuous presence those documented at Combe Grenal. These of hominins in this region over time in terms hominins produced a more complex stone of adaptation to seasonality. technology and diverse series of tool kits Yet on the basis of limb-bone proportions than their predecessors, which may have (Trinkaus, 1981; Holliday and Falsetti, 1995) served a wider variety of special tasks. The and nasal and facial morphology (Wolpoff, pattern of faunal remains at Combe Grenal 1980; Coon, 1962), the skeletal structure of (Fig. 8B), moreover, is informative about European Neanderthals appears to have foraging behavior. The shift from red deer to been adapted specifically to colder environ- reindeer matches the shift from more tem- ments. This body of evidence implies that perate habitat (layers 55 to 37) to cold, dry Neanderthal (or ancestral) populations un- conditions (layers 35 to 10). This suggests derwent directional selection adapting them that the foragers of Combe Grenal were able to habitats ranging from cool to glacial cold. to adjust to major alterations by changing This view is, of course, supported by the their focus from a climatically tolerant yet environmental evidence from western Eu- mainly temperate species (red deer) to a cold rope. From an adaptive perspective, then, steppe species (reindeer). The tremendous the Neanderthals (Mousterian toolmakers stability of red deer representation in layers of the early Wu¨ rm) were an intriguing com- 55 to 37 (approximately 115 to 75 ka), how- posite. They evidently had behavioral strat- ever, further implies that Mousterian tool- egies that permitted them to persist through makers in this area also stabilized their wide changes in habitat. Yet the one consis- access to certain focal resources even as tency was a relatively cold to cool temperate 128 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 (i.e., seasonally cold) climate; their bodies Although the recurrent opportunities and manifested adaptations to this regularity. challenges of savannas, woodlands, and ice- Despite evidence of behavioral adaptabil- age habitats all had an effect on adaptation, ity, there is also evidence that Mousterian the analysis here adds a different dimension toolmakers were somewhat conservative in to the picture of hominin evolution. Direct their responsiveness to changing circum- study of past strata highlights the inconsis- stances. As Chase (1986) points out, for tency of adaptive settings over long time example, faunal exploitation patterns at frames and indicates that certain key homi- Combe Grenal did change with climate but nin adaptations emerged during periods of were slow to do so. Even though the humans intense habitat instability. at this site ultimately switched to hunting The results of this analysis can be summa- reindeer during the period beginning around rized as answers to three questions: layer 37, they did not do so quickly (Fig. 8B). 1. Did the origin of major hominin adapta- Compared with modern humans who popu- tions usually coincide with the appearance of lated the same region later, Mousterian a particular type of habitat or with a span of hunters did not specialize nearly as much by rising environmental variability? exploiting different animals when climate On the basis of oxygen isotope data, the changed. In many Upper Paleolithic sites of past 6 million years were a time of rising the last glacial period, for example, reindeer environmental variability. The range of cli- represented up to 95–99% of the faunal matic fluctuation increased in successive remains, suggesting that hunting was more periods. Variability continued to rise to the distinctly tuned to climatic conditions than point that, by the middle Pleistocene, the evidenced at Combe Grenal. This is just one variation in single isotopic oscillations (cov- of several lines of evidence indicating that ering 100 kyr) usually equaled or exceeded later occupants of western Europe were the average change in ␦18O over the past 6 more responsive to environmental dynamics million years. Variability in global climate than the Neanderthals (Mellars, 1996). therefore swamped the overall trend toward cooling and drying. There is no doubt that ENVIRONMENTAL PERSPECTIVES environments did become colder and more ON ADAPTIVE CHANGE arid, but reversals in the trend had arguably a more significant impact on Earth’s cli- From this review, there appears to be no matic history since the late Miocene. The single, all-encompassing environmental ex- emergence of hominins coincided with the planation of hominin evolution. Diverse se- oldest intervals of rising variability. Com- lective processes operated on early human plex adaptations unique to humans origi- populations in different lineages, times, and nated as climatic variability became magni- places. In the anatomy of Homo ergaster and fied. Furthermore, virtually all other proxies Neanderthals, for example, the results of of past environments—continental dust, pol- adaptation to specific conditions are mani- len, and loess sequences—confirm that habi- fested: in H. ergaster, heat stress in an open tat variability increased over time in terres- tropical African habitat; in Neanderthals, trial settings. adaptation to perennially and seasonally 2. How were early humans and environ- cold habitats in ice-age Europe and Asia. ments correlated over the course of hominin Seasonality, moreover, was a consistent part evolution? In regions occupied by early hu- of the contexts in which hominin evolution mans, were their remains consistently associ- occurred. Seasonal variations are critical to ated with a particular type of habitat, a maintaining the savanna mosaic of Africa stable mosaic of settings, or a highly fluctuat- and the temperate habitats of Eurasia (e.g., ing range of environments? deciduous forests and steppe). For this rea- All four time spans—and limited number son, it is often impossible to distinguish of hominin localities—explored here mani- between evolutionary responses to specific fest a high degree of habitat remodeling over types of habitat and to seasonal aridity or time. In long stratigraphic sequences, the cold. amount of variation greatly exceeded sea- Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 129 sonal types of variation; the tempo and kind scenario of human evolution. In each case, of environmental alteration, moreover, dif- these features first appeared and became fered qualitatively from seasonal oscillation. elaborated within very dynamic environmen- Evidence of hominins occurred through es- tal sequences. sentially the full diversity of paleoenviron- Obviously, ground bipedality makes sense ments that ensued over time, as reflected in in a landscape with few trees. On this logic, lengthy stratigraphic sections. This sug- bipedal striding evolved as an adaptation to gests that hominin populations actually en- increasingly open settings, in line with a countered numerous alterations of the land- habitat-specific view. By studying the paleo- scape, climate, and biota in the localities we environmental records of hominin sites, we have examined. These findings violate the find, however, that bipedal behavior emerged expectations of habitat-specific hypotheses— and was maintained over a long time in a expectations 1–3 (Table 3)—and the season- wide variety of contexts—in relatively closed, ality hypothesis. But they are not usually in open, mixed/mosaic, and (where sequences disagreement with expectations 4 to 6 (Table have sufficient time depth) fluctuating envi- 3), implied by the VS hypothesis. ronments. This finding seems to resolve the The data available for most hominin locali- debate about bipedal vs. arboreal compe- ties, though, are still inadequate to fully test tence in A. afarensis: namely, this and other these three latter expectations. It is unclear, early australopiths possessed a locomotor/ for example, whether African hominins of positional system that enabled the com- the early Pliocene encountered more or less bined competence of both. A single anatomi- habitat fluctuation than middle Pleistocene cal complex oriented toward versatility hominins. Whether a wider degree of habi- (climbing and walking) evolved as land- tat change was represented by 500 kyr at scapes were remodeled over many thou- Hadar or by an equivalent time span at sands of years. If this interpretation is cor- Olorgesailie cannot yet be answered. Thus, rect, it means that the locomotor system of the available data cannot distinguish be- the earliest hominins was adapted to the tween expectations 4 and 5 (Table 3) relative instability of late Miocene and early Pliocene to the VS hypothesis. Very few studies have habitats. Although very few records exist in attempted to address the habitat specificity Africa from the period prior to the earliest of hominins within narrow stratigraphic in- accepted australopiths, the global ocean re- tervals and across time (Table 3, expectation cord (e.g., Fig. 2) suggests that environmen- 6) (Potts et al., in press). The oxygen isotope tal instability was in fact on the rise during record gives a hint that environmental diver- the latest Miocene. sity, although already magnified in the pe- A similar conclusion applies to other ma- riod between 6 and 3 Ma, was further exag- jor adaptive features when analyzed in an gerated in successive intervals over the span environmental context. Enlargement of the of hominin evolution (Fig. 2B). It remains to brain relative to body size during the middle be seen whether the sum of environmental Pleistocene is a case in point. Our analysis records from hominin sites during this span reveals that habitats of this period under- will show the same pattern. went large revisions and that populations 3. What were the functions of key adapta- were subject to strong inconsistencies in tions? Were they designed to best meet the selective environment. Considering the ac- regular, consistent challenges of one main tual evidence, the view often propounded by type of environment or directional change or evolutionary psychologists, who have made to best respond to large-scale habitat incon- a large impact on the study of human cogni- sistency over the long term? tive evolution, needs to be reassessed. It is We have briefly assessed the functions of patently incorrect to characterize the hu- certain hominin adaptations (or adaptive man ancestral environment as a set of spe- trends), including bipedality, encephaliza- cific repetitive elements, statistical regulari- tion, stone tool transport, and growing cogni- ties, or uniform problems which the cognitive tive sophistication. These fundamental char- mechanisms unique to humans are designed acteristics are included in virtually every to solve (e.g., Tooby and Cosmides, 1989, 130 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 1990, 1993; Pinker, 1994). This portrait of mental variability in hominin localities was the Pleistocene environment should be dis- not both rapid and extreme (i.e., unlike the carded and with it the view that the human Greenland ice cap). Rather, variability in mind is composed mainly of innate special- landscape structure, vegetation, and re- purpose devices or algorithms tied to a par- sources became greatly extended over spans ticular array of past adaptive possibilities of 104–105 years and involved a substan- (Potts, in press a). tially different type and range of environ- A variability-selection perspective, by con- mental remodeling than exhibited on a sea- trast, implies that the cognitive mecha- sonal or lifetime scale. Adaptation to short- nisms evolved in Pleistocene humans fos- term patterns of variation does not therefore tered the input, analysis, and mental result in adaptation to the longer time frame representation of highly varied external in- of habitat remodeling. formation and the output of versatile, novel As a final point, it is important to note response. The archeological record of the that the episodic revision of landscapes in- past 700 kyr strongly suggests that brain habited by hominins would also have altered functions and actual behavior were designed the social context in which individuals com- to accommodate a diverse series of adaptive peted for resources. Resource restructuring settings, also evidenced in the archives of and uncertainty, according to the VS hypoth- Pleistocene environmental history. esis, fostered novel strategies of competition It is unlikely that the emergence and and social cooperation. Given the environ- spread of modern H. sapiens, in its ecological mental dynamics we have discovered, there diversity, was preceded by a long process of is little basis for dismissing ecological fac- habitat-specific adaptation. The apparent tors while embracing social competition as replacement of Neanderthals in western Eu- an explanation of cognitive evolution and rope by populations more tropically adapted behavioral complexity (contra Humphrey, (Trinkaus, 1981; Holliday and Falsetti, 1995) 1976). The two surely went hand in hand. is especially significant, for it implies prior Indeed, the case can be made that a strongly adaptation to environmental diversity on a cooperative dynamic within social groups grand scale, which yielded populations fit to proved highly beneficial over the long term accommodate to the hot tropics, the cold as a means of accommodating to unexpected regions where Neanderthals had lived, and ecological variability. even more extreme environments. The so- CONCLUSIONS cial adaptations of these late Pleistocene humans included sophisticated means of Three main kinds of environmental expla- buffering environmental uncertainty and nation can be considered in scenarios of thriving in novel settings (Potts, in press b). hominin adaptive evolution. My goal has To summarize, there is very little basis for been to present these ideas as testable hy- thinking that adaptive settings were consis- potheses. Habitat-specific hypotheses focus tent over many generations or changed in a on the adaptive opportunities or challenges consistent direction during the course of of open savanna, densely wooded, or ice-age hominin evolution. Habitat-specific sce- (cool temperate and periglacial) environ- narios, which have prevailed in paleoanthro- ments. The seasonality hypothesis empha- pology, furnish a poor framework for explain- sizes the impact of short-term environmen- ing the origin of most complex human tal variation on the adaptive process. The adaptations. Short-term habitat variation variability selection (VS) hypothesis postu- also offers a limited basis on which to ac- lates that wide variability in adaptive set- count for key adaptive changes. This hypoth- tings over time ultimately favored and esis, as we noted earlier, relies on the claim helped build complex adaptations which that short-term variation, such as seasonal- were responsive to novel conditions. Based ity, was sufficiently large to have led to the on preliminary tests of environmental re- adaptive flexibility observed in humans. The cords from various hominin sites, the main environmental records examined here con- findings of this review can be summarized as tradict this claim. The time frame of environ- follows. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 131 1. Although usually ignored in explana- properties than to the consistent proper- tions of human evolution, dramatic envi- ties of environments since the late ronmental fluctuation became magnified Miocene. over long periods of time and had substan- 4. It is important to note that most organ- tial impact on adaptive evolution in homi- isms are tied to a narrow range of adap- nins. According to the VS hypothesis, the tive conditions—that is, they are habitat functions of key adaptations and adap- specialists. In the face of environmental tive trends—Pliocene bipedality, stone fluctuations, these species were appar- tool transport and use, and Pleistocene ently sufficiently mobile or dispersed to brain enlargement—primarily served the enable geographic tracking of requisite purposes of behavioral versatility. These habitats and resources. Other lineages traits enabled ways of adapting to envi- became extinct, a frequent occurrence in ronmental novelty—that is, to the dy- most clades of Pleistocene large mam- namic properties of environments, thus mals. In still other cases, populations or including settings that had not been pre- species lineages became adapted over viously encountered. The selective pro- time to a growing diversity of ecological cess that brought about such traits must settings. These lineages underwent a pro- have operated over broad time scales of cess of adaptive change that decoupled habitat disparity. them from any one specific habitat. This 2. Environment-specific adaptations did process seems to have occurred repeat- characterize certain lineages of homi- edly in hominins over the past 5 million nins. These features, however, may have years (Potts, in press a). largely involved anatomical structures 5. Further understanding of the adaptive and related physiological functions rather history of hominins requires well-cali- than complex behavioral, cognitive, and brated data on paleoenvironments and social functions, which typically are cen- their exact association with hominins. tral to accounts of human evolution. Habi- Tests of environmental hypotheses rest tat-specific scenarios do not adequately heavily on correlation. The strength of explain the key adaptations they were these tests depends on improved strati- originally meant to explain. The savanna graphic resolution of the fossil, artifac- hypothesis, for example, does not seem to tual, and environmental records at homi- offer a sufficient account of the locomotor nin localities. Raising the density of versatility that arose in early Pliocene paleoenvironmental data will prove espe- hominins. cially valuable. Future progress cannot 3. Short-term environmental variation was rely on data solely from individual locali- ubiquitous in the regions occupied by ties. Instead, comparative study of early human populations, though season- samples of hominin localities from par- ality or generational-scale variation usu- ticular time spans is required. This type ally left no mark in the geologic record. A of research therefore leans heavily on much more impressive archive of environ- integrated work by teams of geologists, mental fluctuation was left over spans of paleoanthropologists, environmental sci- many thousands of years. Stratigraphic entists, and geochronologists. Without records at hominin sites show that rela- this integrated approach, paleoanthropol- tively stable environmental periods were ogy is relegated to sketching the phyloge- repeatedly interrupted by large shifts in netic and technological outline of human climate, basin hydrology, and landscape evolution while missing the substance of structure. The origin of important hu- the story, the contexts and processes of man characteristics emerged as hominin adaptive change. populations confronted a widely dispar- ACKNOWLEDGMENTS ate range of selective environments. In other words, the general trends of human Funding for field research (Olorgesailie, adaptive evolution are better portrayed Bose, Olduvai) was provided by the Smithso- as a response to the dynamic (unstable) nian’s National Museum of Natural History, 132 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998 the Scholarly Studies Program, the National British Archaeological Reports International Series 283. Science Foundation, and the Boise Fund. Blumenschine RJ (1987) Characteristics of an early For discussions on variability selection and hominid scavenging niche. Curr. Anthrop. 28:383– adaptation, I especially thank Alison Brooks, 407. Blumenschine RJ (1995) Percussion marks, tooth marks, John Fleagle, Bernard Wood, and Kay Be- and experimental determinations of the timing of hrensmeyer. The manuscript benefited from hominid and carnivore access to long bones at FLK comments by Peter deMenocal and Chris Zinjanthropus, Olduvai Gorge, Tanzania. J. Hum. Evol. 29:21–52. Ruff. I am grateful to Jennifer Clark for Bock WJ (1980) The definition and recognition of biologi- preparing the figures and to Brian Rich- cal adaptation. Amer. Zool. 20:217–227. Boesch-Achermann H, and Boesch C (1994) Hominiza- mond for corrections. This is a publication of tion in the rainforest: The chimpanzee’s piece of the the Smithsonian’s Human Origins Program. puzzle. Evol. Anthropol. 3:9–16. Bond G, Heinrich H, et al. (1992) Evidence for massive LITERATURE CITED discharges of icebergs into the North Atlantic ocean during the last glacial period. Nature 360:245–261. Aiello LC, and Wheeler P (1995) The expensive-tissue Bonnefille R (1984) Cenozoic vegetation and environ- hypothesis: The brain and the digestive system in ments of early hominids in East Africa. In RO Whyte human and primate evolution. Curr. Anthrop. 36:199– (ed.): The Evolution of the East Asian Environment, 221. Vol. 2. Hong Kong: Centre of Asian Studies, pp. Ambrose SH, and Sikes NE (1991) Soil organic carbon 597–612. isotopic evidence for vegetation change in the Kenya Bonnefille R (1995) A reassessment of the Plio-Pleis- Rift Valley. Science 253:1402–1405. tocene pollen record of East Africa. In ES Vrba, GH Amundson R (1996) Historical development of the con- Denton, TC Partridge, and LH Burckle (eds.): Paleocli- cept of adaptation. In MR Rose and GV Lauder (eds.): mate and Evolution, With Emphasis on Human Ori- Adaptation. San Diego: Academic Press, pp. 11–53. gins. New Haven: Yale University Press, pp. 299–310. Andrews P (1989) Paleoecology of Laetoli. J. Hum. Evol. Bonnefille R, and DeChamps R (1983) Data on fossil 18:173–181. flora. In J de Heinzelin (ed.): The Omo Group: Ar- Andrews P (1992) Evolution and environment in Miocene chives of the International Omo Research Expedition. hominoids. Nature 360:641–646. Tervuren: Muse´e Royal de l’Afrique Centrale, pp. Andrews P, and Walker A (1976) The primate and other 191–207. fauna from Fort Ternan, Kenya. In GL Isaac and ER Bonnefille R, and Letouzey R (1976) Fruits fossiles McCown (eds.): Human Origins: Perspectives on Hu- d’Antrocaryon dans la valle´e de l’Omo (Ethiopie). man Evolution, Vol. 3. Menlo Park, CA: WA Benjamin, Adansonia 16:65–82. pp. 279–304. Bonnefille R, and Riollet G (1980) Palynologie, ve´ge´ta- Andrews P, Harrison T, Delson E, Bernor RL, and tion et climat de Bed I et Bed II a` Olduvai, Tanzanie. Martin L (1996) Distribution and biochronology of In Proceedings of the Eighth Pan-African Congress of European and Southwest Asian Miocene catarrhines. Prehistory and Quaternary Studies. Nairobi. Nairobi: In RL Bernor, V Fahlbusch, and H-W Mittmann (eds.): TILLMIAP, pp. 123–127. The Evolution of Western Eurasian Neogene Mannal Bonnefille R, and Riollet G (1987) Palynological spectra Faunas. New York: Columbia University Press, pp. from the Upper Laetolil Beds. In MD Leakey and JM 168–207. Harris (eds.): Laetoli: A Pliocene Site in Northern Bartholomew GA, and Birdsell JB (1953) Ecology and Tanzania. Oxford: Clarendon, pp. 52–61. the protohominids. Am. Anthropol. 55:481–498. Bordes F (1961) Mousterian cultures in France. Science Begun DR (1992) Miocene fossil hominids and the 134:803–810. chimp-human clade. Science 257:1929–1933. Bordes F, Laville H, and Paquereau MM (1966) Observa- Behrensmeyer AK, Todd NE, Potts R, and McBrinn GE tions sur le Ple´istoce`ne supe´rior du gisement de (1997) Late Pliocene faunal turnover in the Turkana Combe-Grenal (Dordogne). Actes de la Socie´te´ Lin- Basin, Kenya and Ethiopia. Science 278:1589–1594. ne´enne de Bordeaux 103:3–19. Berger LR, and Tobias PV (1996) A chimpanzee-like Brain CK (1981) The evolution of man in Africa: Was it a tibia from Sterkfontein, South Africa and its implica- consequence of Cainozoic cooling? Annex. Transv. Geol. tions for the interpretation of bipedalism in Australo- Soc. S. Afr. 84:1–19. pithecus africanus. J. Hum. Evol. 30:343–348. Brandon RN (1990) Adaptation and Environment. Prin- Binford LR, and Ho CK (1985) Taphonomy at a distance: ceton: Princeton University Press. Zhoukoudian, ‘‘the cave home of Beijing Man’’? Curr. Broecker WS, and Denton GH (1990) The role of ocean- Anthrop. 26:413–442. atmosphere reorganisations in glacial cycles. Quatern. Bishop WW (1976) Pliocene problems relating to human Sci. Rev. 9:305–343. evolution. In GL Isaac and ER McCown (eds.): Human Brown FH (1981) Environments in the Lower Omo Origins: Perspectives on Human Evolution, Vol. 3. Basin from one to four million years ago. In G Rapp Jr Menlo Park, CA: WA Benjamin, pp. 139–153. and CF Vondra (eds.): Hominid Sites: Their Geologic Bishop WW (1978) Geological Background to Fossil Settings. AAAS Symposium 63. Boulder: Westview, Man. Edinburgh: Scottish Academic Press. pp. 149–164. Bishop WW, and Clark JD (1967) Background to Evolu- Brown FH, and Feibel CS (1991) Stratigraphy, deposi- tion in Africa. Chicago: University of Chicago Press. tional environments and palaeogeography of the Koobi Black D, Teilhard de Chardin P, Young CC, and Pei WC Fora Formation. In JM Harris (ed.): Koobi Fora (1933) Fossil Man in China. Geological Memoirs Se- Research Project, Vol. 3. Oxford: Clarendon, pp. 1–30. ries A, Number 11. Peiping: Geological Survey of Bunn HT, and Kroll EM (1986) Systematic butchery by China. Plio-Pleistocene hominids at Olduvai Gorge, Tanza- Blumenschine RJ (1986) Early Hominid Scavenging nia. Curr. Anthrop. 27:431–452. Opportunities: Implications of Carcass Availability in Butler PM, and Greenwood M (1973) The early Pleis- the Serengeti and Ngorongoro Ecosystems. Oxford: tocene hedgehog from Olduvai Gorge, Tanzania. In L Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 133

Leakey, R Savage, and S Coryndon (eds.): Fossil Delson E, and Tattersall I (1997) Primates. In: Encyclo- Vertebrates of Africa, Vol. 3. London: Academic Press, pedia of Human Biology, Vol. 7. New York: Academic pp. 7–42. Press, pp. 93–104. Butler PM, and Greenwood M (1976) Elephant-shrews deMenocal PB (1995) Plio-Pleistocene African climate. (Macroscelididae) from Olduvai and Makapansgat. In Science 270:53–59. R Savage and S Coryndon (eds.): Fossil Vertebrates of deMenocal PB, Ruddiman WF, and Pokras EM (1993) Africa, Vol. 4. London: Academic Press, pp. 1–56. Influences of high- and low-latitude processes on Butzer KW (1971) Environment and Archeology: An African terrestrial climate. Palaeoceanography 8:209– Ecological Approach to Prehistory. Chicago: Aldine- 242. Atherton. deMenocal PB, and Bloemendal J (1995) Plio-Pleis- Butzer KW, and Isaac GL (1975) After the Australopithe- tocene climatic variability in subtropical Africa and cines. The Hague: Mouton. the paleoenvironment of hominid evolution. In ES Cadman A, and Rayner RJ (1989) Climatic change and Vrba, GH Denton, TC Partridge, and LH Burckle the appearance of Australopithecus africanus in the (eds.): Paleoclimate and Evolution, With Emphasis on Makapansgat sediments. J. Hum. Evol. 18:107–113. Human Origins. New Haven: Yale University Press, Calvin WH (1996) How Brains Think. New York: Basic pp. 262–288. Books. Eldredge N (1989) Macroevolutionary Dynamics. New Cande SC, and Kent DV (1992) A new geomagnetic York: McGraw-Hill. polarity time scale for the Late Cretaceous and Ceno- Elliot Smith GE (1924) The Evolution of Man. London: zoic. J. Geophys. Res. 97:13917–13951. Oxford University Press. Cavallo JA, and Blumenschine RJ (1989) Tree-stored Endler JA (1986) Natural Selection in the Wild. Prince- leopard kills: Expanding the hominid scavenging ton: Princeton University Press. niche. J. Hum. Evol. 18:393–399. Feibel CS (1988) Paleoenvironments of the Koobi Fora Cerling TE (1992) Development of grasslands and savan- Formation, Northern Kenya. Ph.D. dissertation, Uni- nas in East Africa during the Neogene. Palaeogeogr. versity of Utah. Palaeoclimat. Palaeoecol. 97:241–247. Feibel CS, Harris JM, and FH Brown (1991) Palaeoenvi- Cerling TE, and Hay RL (1986) An isotopic study of ronmental context for the late Neogene of the Turkana Basin. In JM Harris (ed.): Koobi Fora Research Project, paleosol carbonates from Olduvai Gorge. Quatern. Vol. 3. Oxford: Clarendon, pp. 321–370. Res. 25:63–78. Fisher DC (1985) Evolutionary morphology: Beyond the Cerling TE, Bowman JR, and O’Neil JR (1988) An analogous, the anecdotal, and the ad hoc. Paleobiology isotopic study of a fluvial-lacustrine sequence: The 11:120–138. Plio-Pleistocene Koobi Fora sequence, East Africa. Foley R (1987) Another Unique Species. Essex: Long- Palaeogeogr. Palaeoclimat. Palaeoecol. 63:335–356. man. Cerling TE, Quade J, Ambrose SH, and Sikes N (1991) Gould SJ (1977) Ontogeny and Phylogeny. Cambridge, Fossil soils, grasses, and carbon isotopes from Fort MA: Harvard University Press. Ternan: Grassland or woodland? J. Hum. Evol. 21:295– Gould SJ, and Vrba ES (1982) Exaptation—a missing 306. term in the science of form. Paleobiology 8:4–15. Cerling TE, Harris JM, MacFadden BJ, Leakey MG, Gray T (1980) Environmental Reconstruction of the Quade J, Eisenmann V, and Ehleringer JR (1997) Hadar Formation (Afar, Ethiopia). Ph.D. dissertation, Global vegetation change through the Miocene/ Case Western Reserve University. Pliocene boundary. Nature 389:153–158. Guiot J, Pons A, de Beaulieu JL, and Reille M (1989) A Chase PG (1986) The Hunters of Combe Grenal. Oxford: 140,000-year continental climate reconstruction from British Archaeological Reports International Series two European climate records. Nature 338:309–313. 286. Guiot J, de Beaulieu JL, Cheddadi R, David F, Ponel P, Clarke RJ and Tobias PV (1995) Sterkfontein Member 2 and Reille M (1993) The climate in western Europe foot bones of the oldest South African hominid. Sci- during the last glacial/interglacial cycle derived from ence 269:521–524. pollen and insect remains. Palaeogeog. Palaeoclimat. Cooke HSB (1978) Faunal evidence for the biotic setting Palaeoecol. 103:73–93. of early African hominids. In CJ Jolly (ed.): Early Harris JM, and Cerling TE (1995) Dietary preferences of Hominids of Africa. New York: St. Martin’s Press, pp. Lothagam mammals (late Miocene, northern Kenya). 267–284. J. Vert. Paleontol. 15(Supplement to no. 3):33A. Coon C (1962) The Origin of Races. New York: Knopf. Hay RL (1976a) Geology of the Olduvai Gorge. Berkeley: Croll J (1875) Climate and Time. New York: Appleton. University of California Press. Crowley TJ, and North GR (1991) Paleoclimatology. Hay RL (1976b) Environmental setting of hominid activi- New York: Oxford University Press. ties in Bed I, Olduvai Gorge. In GL Isaac and ER Dansgaard WS, Johnsen SJ, et al. (1993) Evidence for McCown (eds.): Human Origins: Perspectives on Hu- general instability of past climate from a 250-kyr man Evolution, Vol. 3. Menlo Park, CA: WA Benjamin, icecore. Nature 364:218–220. pp. 209–225. Dart R (1925) Australopithecus africanus: The man-ape Hay RL (1981) Paleoenvironment of the Laetolil Beds, of south Africa. Nature 115:195–199. northern Tanzania. In G Rapp Jr and CF Vondra Dart R (1953) The predatory transition from ape to man. (eds.): Hominid Sites: Their Geologic Settings. AAAS Int. Anthrop. Ling. Rev. 1:201–218. Symposium 63. Boulder: Westview, pp. 7–24. Darwin C (1871) The Descent of Man and Selection in Hay RL (1987) Geology of the Laetoli area. In MD Relation to Sex. London: Murray. Leakey and JM Harris (eds.): Laetoli: A Pliocene Site deGraff G (1960) A preliminary investigation of the in Northern Tanzania. Oxford: Clarendon, pp. 23–47. mammalian microfauna in Pleistocene deposits of Hill A, and Ward S (1988) Origin of the Hominidae: The caves in the Transvaal system. Palaeont. Afr. 7:59– record of African large hominoid evolution between 14 118. my and 4 my. Yrbk. Phys. Anthropol. 31:49–83. Deino A, and Potts R (1990) Single crystal 40Ar/39Ar Holliday TW, and Falsetti AB (1995) Lower limb length dating of the Olorgesailie Formation, southern Kenya of European early modern humans in relation to rift. J. Geophys. Res. 95:8453–8470. mobility and climate. J. Hum. Evol. 29:141–153. 134 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998

Howell FC (1976) An overview of the Pliocene and Latimer B, and Lovejoy CO (1990) Hallucal tarsometa- earlier Pleistocene of the Lower Omo Basin, southern tarsal joint in Australopithecus afarensis. Am. J. Ethiopia. In GL Isaac and ER McCown (eds.): Human Phys. Anthropol. 82:125–133. Origins: Perspectives on Human Evolution, Vol. 3. Laville H, Rigaud J-P, and Sackett J (1980) Rock Shel- Menlo Park, CA: WA Benjamin, pp. 227–268. ters of the Perigord. New York: Academic Press. Howell FC, and Bourlie´re F (1963) African Ecology and Leakey MD (1971) Olduvai Gorge, Vol. 3. London: Human Evolution. New York: Wenner-Gren Founda- Cambridge University Press. tion. Leakey MD, and Harris JM (1987) Laetoli: A Pliocene Humphrey N (1976) The social function of intellect. In Site in Northern Tanzania. Oxford: Clarendon. PPG Bateson and RA Hinde (eds.): Growing Points in Leakey MG, Feibel CS, McDougall I, and Walker AC Ethology. Cambridge: Cambridge University Press, (1995) New four-million-year-old hominid species from pp. 303–317. Kanapoi and Allia Bay, Kenya. Nature 376:565–571. Isaac GL (1976) East Africa as a source of fossil evidence Leakey MG, Feibel CS, Bernor RL, Harris JM, Cerling for human evolution. In GL Isaac and ER McCown TE, Stewart KM, Storrs GW, Walker A, Werdelin L, (eds.): Human Origins: Perspectives on Human Evolu- and Winkler AJ (1996) Lothagam: A record of faunal tion, Vol. 3. Menlo Park, CA: WA Benjamin, pp. change in the late Miocene of East Africa. J. Vert. 121–137. Paleontol. 6:556–570. Isaac GL (1977) Olorgesailie: Archaeological Studies of a Lewontin R (1970) The units of selection. Annu. Rev. Middle Pleistocene Lake Basin in Kenya. Chicago: Ecol. Syst. 1:1–14. University of Chicago Press. Liu T (1988) Loess in China. Beijing: China Ocean Isaac GL (1978) The Olorgesailie Formation: Stratigra- Press. phy, tectonics and the palaeogeographic context of the Liu Z (1983) The palaeoclimatic changes inferred from Middle Plestocene archaeological sites. In WW Bishop Peking Man Cave deposits in comparison with the (ed.): Geological Background to Fossil Man. Edin- climatic sequence of other formations. Acta Anthropo- burgh: Scottish Academic Press, pp. 173–206. logica Sinica 2:172–183. Isaac GL, and McCown ER (1976) Human Origins: Lovejoy CO, Heiple KG, and Burstein AH (1973) The Perspectives on Human Evolution, Vol. 3. Menlo Park, gait of Australopithecus. Am. J. Phys. Anthropol. CA: WA Benjamin. 38:757–780. Jablonski D (1987) Heritability at the species level: McKee JK (1996) Faunal turnover patterns in the Analysis of geographic ranges of Cretaceous molluscs. Pliocene and Pleistocene of southern Africa. S. Afr. J. Science 238:360–363. Sci. 92:111–113. Jaeger JJ (1976) Les rongeurs (Mammalia, Rodentia) du Mellars P (1996) The Neanderthal Legacy. Princeton: Ple´istoce`ne infe´rior d’Olduvai Bed I (Tanzania). In R Princeton University Press. Savage and S Coryndon (eds.): Fossil Vertebrates of Milankovitch MM (1941) Canon of Insolation and the Africa, Vol. 4. London: Academic Press, pp. 58–120. Jia L, and Huang W (1990) The Story of Peking Man. Ice-Age Problem. Belgrade: Koniglich Serbische Acad- Beijing: Foreign Language Press. emie. Johanson DC, Taieb M, and Coppens Y (1982) Pliocene Miller KG, Fairbanks RG, and Mountain GS (1987) hominids from the Hadar Formation, Ethiopia (1973– Tertiary oxygen isotope synthesis, sea level history, 1977). Am. J. Phys. Anthropol. 57:373–402. and continental margin erosion. Paleoceanography Jolly CJ (1970) The seed eaters: A new model of hominid 2:1–19. differentiation based on a baboon analogy. Man 5:5– Miller KG, Mountain GS, et al. (1996) Drilling and 26. dating New Jersey Oligocene-Miocene sequences: Ice Jolly CJ (1978) Early Hominids of Africa. New York: St. volume, global sea level, and Exxon records. Science Martin’s Press. 271:1092–1095. Jouzel J, Barkov NI, et al. (1993) Extending the Vostok Monahan CM (1996) New zooarchaeological data from ice-core record for palaeoclimate to the penultimate Bed II, Olduvai Gorge, Tanzania: Implications for glacial period. Nature 364:203–207. hominid behavior in the Early Pleistocene. J. Hum. Kappelman J, Plummer T, Bishop L, Duncan A, and Evol. 31:93–128. Appleton S (1997) Bovids as indicators of Plio- Morwood MJ, O’Sullivan PB, Aziz F, and Raza A (1998) Pleistocene paleoenvironments in East Africa. J. Hum. Fission-track ages of stone tools and fossils on the east Evol. 32:229–256. Indonesian island of Flores. Nature 392:173–176. Kay RF, Cartmill M, and Balow M (1998) The hypoglos- Movius H (1969) Lower Paleolithic archaeology in south- sal canal and the origin of human vocal behavior. Proc. ern Asia and the Far East. In WW Howells (ed.): Early Natl. Acad. Sci. U. S. A. 95:5417–5419. Man in the Far East. Studies in Physical Anthropol- Kibunjia M (1994) Pliocene archaeological occurrences ogy No. 1. New York: Humanities Press, pp. 17–77. in the Lake Turkana basin. J. Hum. Evol. 27:159–172. Ohman JC, Krochta TJ, Lovejoy CO, Mensforth RP, and Kingston JD, Marino BD, and Hill A (1994) Isotopic Latimer B (1997) Cortical bone distribution in the evidence for Neogene hominid paleoenvironments in femoral neck of hominoids: Implications for the locomo- the Kenya Rift Valley. Science 264:955–959. tion of Australopithecus afarensis. Am. J. Phys. Anthro- Kitching JW (1980) On some fossil Arthropoda from the pol. 104:117–131. Limeworks, Makapansgat, Potgietersrus. Palaeont. Owen RB, and Renaut R (1981) Paleoenvironments and Afr. 23:63–68. sedimentology of the Middle Pleistocene Olorgesailie Klein RG (1989) The Human Career: Human Biological Formation, southern Kenya rift valley. In J Coetzee and Cultural Origins. Chicago: University of Chicago and EM van Zinderen Bakker (eds.): Palaeoecology of Press. Africa. Rotterdam: Balkema, pp. 147–174. Kretzoi M, and Dobosi VT (1990) Ve´rtesszo¨lo¨s: Site, Man Pinker S (1994) The Language Instinct. New York: and Culture. Budapest: Akade´miai Kiado´. Morrow. Kukla G, and An Z (1989) Loess stratigraphy in central Plummer TW, and Bishop LC (1994) Hominid paleoecol- China. Palaeogeog. Palaeoclimat. Palaeoecol. 72:203– ogy at Olduvai Gorge, Tanzania as indicated by ante- 225. lope remains. J. Hum. Evol. 27:47–75. Laporte LF, and Zihlman AL (1983) Plates, climate and Potts R (1988) Early Hominid Activities at Olduvai. New hominoid evolution. S. Afr. J. Sci. 79:96–110. York: Aldine de Gruyter. Potts] ENVIRONMENTAL HYPOTHESES OF HOMININ EVOLUTION 135

Potts R (1989) Olorgesailie: New excavations and find- Shackleton RM (1978) A geologic map of the Olorgesailie ings in Early and Middle Pleistocene contexts, south- Formation. In WW Bishop (ed.): Geological Back- ern Kenya rift valley. J. Hum. Evol. 18:477–484. ground to Fossil Man. Edinburgh: Scottish Academic Potts R (1994) Variables vs. models of early Pleistocene Press (in pocket). hominid land use. J. Hum. Evol. 27:7–24. Shipman P, and Harris JM (1988) Habitat preference Potts R (1996a) Humanity’s Descent: The Consequences and paleoecology of Australopithecus boisei in Eastern of Ecological Instability. New York: Morrow. Africa. In FE Grine (ed.): Evolutionary History of the Potts R (1996b) Evolution and climate variability. Sci- ‘‘Robust’’ Australopithecines. New York: Aldine de ence 273:922–923. Gruyter, pp. 343–382. Potts R (in press a) Variability selection in hominid Sikes NE (1994) Early hominid habitat preferences in evolution. Evol. Anthropol. East Africa: Paleosol carbon isotopic evidence. J. Potts R (in press b) Behavioral response to variable Hum. Evol. 27:25–45. Pleistocene landscapes. In A Nowell (ed.): The Archae- Sikes NE, Potts R, and Behrensmeyer AK (1997) Isoto- ology of Intelligence. New York: Cambridge Univer- pic study of Pleistocene paleosols from the Olorge- sity Press. sailie Formation, southern Kenya rift. J. Hum. Evol. Potts R, Ditchfield P, Hicks J, and A Deino (1997) 32:A20–A21. Paleoenvironments of late Miocene and early Pliocene Sikes NE, Potts R, and Behrensmeyer AK (in press) strata of Kanam, western Kenya. Am. J. Phys. Anthro- Early Pleistocene habitat in Member 1 Olorgesailie pol. 24(Suppl.):188–189. based on paleosol stable isotopes. J. Hum. Evol. Potts R, Behrensmeyer AK, and Ditchfield P (in press) Stanley SM (1992) An ecological theory for the origin of Paleolandscape variation in early Pleistocene homi- Homo. Paleobiology 18:237–257. nid activities: Members 1 and 7, Olorgesailie Forma- Stauffer B, Blunier T, Da¨llenbach A, Indermu¨ hle, tion, Kenya. J. Hum. Evol. Schwander J, Stocker TF, Tachumi J, Chappellaz J, Prentice ML, and Denton GH (1988) The deep-sea Raynaud D, Hammer CU, and Clausen HB (1998) oxygen isotope record, the global ice sheet system and Atmospheric CO2 concentration and millennial-scale hominid evolution. In FE Grine (ed.): Evolutionary climate change during the last glacial period. Nature History of the ‘‘Robust’’Australopithecines. New York: 392:59–62. Aldine de Gruyter, pp. 383–403. Stern JT Jr and Susman RL (1983) Locomotor anatomy Raymo ME, Ganley K, Carter S, Oppo DW, and McMa- of Australopithecus afarensi. Am. J. Phys. Anthropol. nus J (1998) Millennial-scale climate instability dur- 60:279–317. ing the early Pleistocene epoch. Nature 392:699–702. Steudel KL (1994) Locomotor energetics and hominid Rayner RJ, Moon BP, and Masters JC (1993) The evolution. Evol. Anthrop. 3:42–48. Makapansgat australopithecine environment. J. Hum. Susman RL, and Stern JT (1991) Locomotor behavior of Evol. 24:219–231. early hominids: Epistemology and fossil evidence. In Reed K (1997) Early hominid evolution and ecological Y Coppens and B Senut (eds.): Origine(s) de la Bipe´die change through the African Plio-Pleistocene. J. Hum. chez les Hominide´s. Paris: Editions du CNRS, pp. Evol. 32:289–322. 121–131. Rightmire PG (1991) The Evolution of Homo erectus. Tauxe L, Deino AD, Behrensmeyer AK, and Potts R Cambridge: Cambridge University Press. (1992) Pinning down the Brunhes/Matuyama and Robinson JT (1954) The genera and species of the upper Jaramillo boundaries: A reconciliation of orbital Australopithecinae. Am. J. Phys. Anthropol. 12:181– and isotopic time scales. Earth Planet. Sci. Lett. 200. 109:561–572. Robinson JT (1961a) The australopithecines and their Taylor KC, Lamorey GW, et al. (1993) The ‘‘flickering bearing on the origin of man and of stone tool-making. switch’’ of late Pleistocene climate change. Nature S. Afr. J. Sci. 57:3–13. 361:432–436. Robinson JT (1961b) Australopithecines and the origin Teilhard de Chardin P, Young CC, Pei WC, and Chang of man. Annu. Rept. Smiths. Instit., pp. 479–500. HC (1935) On the Cenozoic formations of Kwangsi Rogers MJ, Feibel CS, and Harris JWK (1994) Changing and Kwangtung. Bull. Geolog. Soc. China 14:179–205. patterns of land use by Plio-Pleistocene hominids in Thieme H (1997) Lower Palaeolithic hunting spears the Lake Turkana Basin. J. Hum. Evol. 27:139–158. from Germany. Nature 385:307–310. Ruff CB (1991) Climate, body size and body shape in Tooby J, and Cosmides L (1989) Evolutionary psychol- hominid evolution. J. Hum. Evol. 21:81–105. ogy and the generation of culture, part I. Ethol. Ruff CB, and Walker A (1993) Body size and body shape. Sociobiol. 10:29–49. In Walker A and Leakey RE (eds.): The Nariokotome Tooby J, and Cosmides L (1990) The past explains the Homo erectus Skeleton. Cambridge: Harvard Univer- present: Emotional adaptations and the structure of sity Press, pp. 234–265. ancestral environments. Ethol. Sociobiol. 11:375–424. Ruff CB, Trinkaus E, Walker A, and Larsen CS (1993) Tooby J, and Cosmides L (1993) The psychological Postcranial robusticity in Homo. I: Temporal trends foundations of culture. In JH Barkow, L Cosmides, and mechanical interpretation. Am. J. Phys. Anthro- and J Tooby (eds.): The Adapted Mind. New York: pol. 91:21–53. Oxford Press, pp. 19–136. Ruff CB, Trinkaus E, and Holliday TW (1997) Body mass Trinkaus E (1981) Neanderthal limb proportions and and encephalization in Pleistocene Homo. Nature cold adaptation. In CB Stringer (ed.): Aspects of 387:173–176. Human Evolution. London: Taylor and Francis, pp. Sandweiss DH, Richardson JB III, Reitz EJ, Rollins HB, 187–224. and Maasch KA (1996) Geoarchaeological evidence Van Couvering JAH, and Van Couvering JA (1976) Early from Peru for 5000 years B.P. onset of El Nin˜ o. Science Miocene mammal fossil from East Africa. In GL Isaac 273:1531–1533. and ER McCown (eds.): Human Origins: Perspectives Shackleton NJ (1995) New data on the evolution of on Human Evolution, Vol. 3. Menlo Park, CA: WA Pliocene climatic variability. In ES Vrba, GH Denton, Benjamin, pp. 155–207. TC Partridge, and LH Burckle (eds.): Paleoclimate Vrba ES (1975) Some evidence of chronology and pal- and Evolution, With Emphasis on Human Origins. aeoecology of Sterkfontein, Swartkrans, and Krom- New Haven: Yale University Press, pp. 242–248. draai from the fossil Bovidae. Nature 254:301–304. 136 YEARBOOK OF PHYSICAL ANTHROPOLOGY [Vol. 41, 1998

Vrba ES (1980) Evolution, species and fossils: How does Washburn SL, and McCown ER (1978) Human Evolu- life evolve? S. Afr. J. Sci. 76:61–84. tion: Biosocial Perspectives. Perspectives on Human Vrba ES (1985) Environment and evolution: Alternative Evolution, Vol. 4. Menlo Park, CA: Benjamin/Cum- causes of the temporal distribution of evolutionary mings. events. S. Afr. J. Sci. 81:229–236. Waters MR, Forman SL, and Pierson JM (1997) Diring Vrba ES (1988) Late Pliocene climatic events and homi- Yuriakh: A Lower Paleolithic site in central Siberia. nid evolution. In FE Grine (ed.): Evolutionary History Science 275:1281–1284. of the ‘‘Robust’’ Australopithecines. New York: Aldine Wesselman HB (1984) The Omo Micromammals: System- de Gruyter, pp. 405–426. atics and Paleoecology of Early Man Sites from Ethio- Vrba ES (1989) Levels of selection and sorting with pia. Basel: S. Karger. special reference to the species level. In PH Harvey Wesselman HB (1995) Of mice and almost-men: Re- and L Partridge (eds.): Oxford Surveys in Evolution- gional paleoecology and human evolution in the Tur- ary Biology, Vol. 6. Oxford: Oxford University Press, kana Basin. In ES Vrba, GH Denton, TC Partridge, pp. 111–168. and LH Burckle (eds.): Paleoclimate and Evolution, Vrba ES (1992) Mammals as a key to evolutionary With Emphasis on Human Origins. New Haven: Yale theory. J. Mammalogy 73:1–28. University Press, pp. 356–368. Vrba ES (1995a) On the connection between paleocli- White TD (1995) African omnivores: Global climatic mate and evolution. In ES Vrba, GH Denton, TC change and Plio-Pleistocene hominids and suids. In Partridge, and LH Burckle (eds.): Paleoclimate and ES Vrba, GH Denton, TC Partridge, and LH Burckle Evolution, With Emphasis on Human Origins. New (eds.): Paleoclimate and Evolution, With Emphasis on Haven: Yale University Press, pp. 24–45. Human Origins. New Haven: Yale University Press, Vrba ES (1995b) The fossil record of African antelopes pp. 369–384. (Mammalia, Bovidae) in relation to human evolution and paleoclimate. In ES Vrba, GH Denton, TC Par- Williamson PG (1985) Evidence for an early Plio- tridge, and LH Burckle (eds.): Paleoclimate and Evolu- Pleistocene rainforest expansion in East Africa. Na- tion, With Emphasis on Human Origins. New Haven: ture 315:487–489. Yale University Press, pp. 385–424. Winkler AJ (1995) Small mammals from the early Vrba ES, Denton GH, and Prentice ML (1989) Climatic Pliocene at Kanapoi, West Turkana, northern Kenya. influences on early hominid behavior. Ossa 14:127– J. Vert. Paleontol. 15(Supplement to no. 3):60A. 156. Woillard GM (1978) Grande Pile peat bog: A continuous Vrba ES, Denton GH, Partridge TC, and Burckle LH pollen record for the last 140,000 years. Quatern. Res. (1995) Paleoclimate and Evolution, With Emphasis on 9:1–21. Human Origins. New Haven: Yale University Press. WoldeGabriel G, White TD, Suwa G, Renne P, de Walter RC, Manega PC, Hay RL, Drake RE, and Curtis Heinzelin J, Hart WK, and Helken G (1994) Ecological GH (1991) Laser-fusion 40Ar/39Ar dating of Bed I, and temporal placement of early Pliocene hominids at Olduvai Gorge, Tanzania. Nature 354:145–149. Aramis, Ethiopia. Nature 371:330–333. Wang K, Zhang Y, and Jian H (1991) Migration of Wolpoff MH (1980) Paleoanthropology. New York: Knopf. paleovegetational zone and paleoclimatic changes of Wood BA (1991) Koobi Fora Research Project, Vol. 4. Quaternary in the coastal region of South China. Oxford: Clarendon. Chinese Science Bulletin 36:1721–1724. Woodruff F, Savin SM, and Douglas RG (1981) Miocene Washburn SL (1960) Tools and human evolution. Sci. stable isotope record: A detailed deep Pacific Ocean Am. 203(3):63–75. study and its paleclimatic implications. Science 212: Washburn SL (1961) Social Life of Early Man. Chicago: 665–668. Aldine. Wright JD, and Miller KG (1992) Miocene stable isotope Washburn SL (1963) Classification and Human Evolu- stratigraphy, Site 747, Kerguelen Plateau. Proceed- tion. Chicago: Aldine. ings of the Ocean Drilling Program, Scientific Results Washburn SL, and Dolhinow P (1972) Perspectives on 120:855–866. Human Evolution, Vol. 2. New York: Holt, Rinehart Xu Q, and Ouyang L (1982) Climatic changes during and Winston. Peking Man’s time. Acta Anthropologica Sinica 1: Washburn SL, and Jay PC (1968) Perspectives on Hu- 80–90. man Evolution, Vol. 1. New York: Holt, Rinehart and Zhu Z, Wang J, et al. (1995) Red soil, loess and global Winston. change. Quatern. Sci. 3:267–277.