sign in sign up
<<
Home , Archaic humans, Australopithecine, Australopithecus, Human, Human evolution, Oreopithecus

© Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. II.18 John

OUTLINE diversity of genera, but most are now included within . 1. Origin of the hominins Hominins. Modern and extinct more 2. Early closely related to humans than to or 3. and the origin of modern humans . 4. Recent . The earliest stone , which emerged about 2.6 million ago and persisted until about Living humans are the sole living representatives of a 1.6 million years ago, when it was replaced by the lineage, the hominins, which diverged from other living more sophisticated . 5 to 7 million years ago. Hominins remained limited Orthograde. An upright posture associated with a bi- to for two-thirds of their . With chimpan- pedal gait, such as occurs in modern humans. zee-sized bodies and brains, early hominins diversified Pronograde. The posture of holding the body parallel to into several lineages with different dietary strategies. the ground, such as is typical of most quadrupedal One of these found a path toward technology, shar- vertebrates. ing, and and gathering, giving rise to our , Homo, approximately 2 million years ago. As popula- While Darwin avoided discussion of the evolution of tions of Homo spread throughout the world, they gave humans in , he soon tackled the rise to regional with their own anatomical issue in The Descent of , which defined the starting and genetic distinctiveness. Within the last 100,000 point for modern evolutionary . In the years, a massive dispersal of humans from Africa ab- nineteenth and early twentieth centuries, the main theme sorbed and replaced these preexisting populations. In the of anthropology was a perceived lack of progeni- time since this latest emergence from Africa, humans tors, prompting a much-hyped search for a “missing have continued to disperse, interact, and evolve. The rise link.” Gradually this concern diminished as paleoan- of agricultural subsistence shifted , - thropologists, especially over the last half century, suc- ing evolution. ceeded in uncovering thousands of fossil specimens, rep- resenting diverse human and collateral rel- atives. While questions still remain, these fossil data GLOSSARY provide a rich history of the origin of many of humanity’s Acheulean. A style of manufacture asso- distinctive physical traits. Furthermore, archaeological ciated with early humans () during finds have provided information on the of the Lower era across Africa and . hominins during the last half of human evolution, giving Acheulean technology is derived from the older, details about and . Today, ge- Oldowan technology and is a progenitor of the neticists can add evidence from whole- com- more complex stone that characterize the parisons of living humans, other , and some . ancient hominins. Through all these lines of evidence a . Members of the hominin with remarkably clear picture of human evolution is now the bipedal gait and of modern humans, but emerging. lacking the enlarged brains of the genus Homo. We can roughly consider human evolution in three species have been assigned to a parts. The first, from 7 million up to around © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. 184 and the History of

4 million years ago, saw the origination of the hominin identifying early hominins, whose identity remains lineage and the initial appearance of our bipedal pattern subject to debate. of locomotion. The second, from 4 million up to around The earliest candidate for hominins 1.8 million years ago, was the age of the austra- share a suite of dental resemblances with later members lopithecines. This group of species had a stable set of of our lineage, including small canine teeth, low- in body and locomotion, while showing crowned teeth, and thick molar enamel. Some substantial dietary and geographic diversity. Our own paleoanthropologists suggest that such dental traits are genus, Homo, arose about 1.8 million years ago from the shared much more broadly with other lineages, australopithecines. The spread of Homo throughout the and may not indicate hominin affinities. Skeletal adap- world, along with many later dispersals and popula- tations to upright posture and bipedal stance provide tion expansions, laid the foundation for today’s human strong evidence that later fossils, after 4.2 million years populations. ago, are human relatives. For earlier fossils, evidence of posture and stance is more equivocal. tchadensis from north central Africa is the earliest 1. ORIGIN OF THE HOMININS known, at around 7 million years ago. Represented by a Chimpanzees and are our closest relatives nearly complete and jaw, it shows an orthograde among living primates. Whole genome comparisons placement of the skull atop the spinal column. suggest that our common ancestors with these apes lived tugenensis, from western dating to 6 million years between 4 million and 7 million years ago. Our common ago, also has a femur consistent with bipedal weight ancestors with gorillas lived a bit earlier, within the last bearing. kadabba, 5.5 million years old 10 million years, and with even earlier, be- from , combines the aforementioned hominin fore 12 million years ago. Hence it is during the period dental features with a , suggesting that the toe between 10 million and 4 million years ago that pa- generated force during bipedal , as occurs in leontologists look for the immediate precursors of the modern humans. It remains unclear whether these fossil hominin lineage. taxa lived before or after the divergence of the human A rich record of fossil apes has been recovered from and lineages, and if after, whether they are the Miocene geological epoch, which lasted from 23 on the human or chimpanzee side of this evolutionary million to 5.2 million years ago. Before 15 million years split. ago, all known apes lived in Afro-Arabia. Early in the Ardipithecus ramidus, dating to 4.4 million years ago , some apes dispersed into Asia and from Ethiopia, comprises a large fossil sample including Europe, including the Asian ancestors of orangutans. one nearly complete skeleton. From its propor- Miocene apes ranged extensively in body size and tions, grasping feet, and apelike , Ardipithecus was adaptive niche, and evolved a diversity of locomotor a habitual quadruped that also had good climbing abil- strategies. Many were pronograde quadrupeds, essen- ities. But several of its features are similar to those of tially like living monkeys such as hominins, including a shortened and an upright and baboons. A few had shoulders and vertebral col- posture. The teeth and jaws of A. ramidus, like those of umns, indicating an or climbing, but earlier A. kadabba, are among its most hominin-like no early apes are known to have had the long arms and features. It is often interpreted as the earliest - below-branch suspensory capability of today’s great documented member of our lineage. However, the data apes. Vertical, orthograde posture was once to do not rule out the possibility that it is an early member be an ancestral of all apes (including humans); of the chimpanzee or lineages. however, some now believe that this suspensory evolved convergently in the Afri- Australopithecines can and Asian apes. Living humans are obligate bipeds, with pelvis, foot, The first fossils to show clear evidence of a commitment and vertebral adaptations that impede effective quad- to terrestrial bipedal locomotion are assigned to Aus- rupedal gait and climbing. The origin of hominins is tralopithecus anamensis. Between 4.2 and 3.9 million entangled with this unique , but the earliest years ago, this species existed in . After this members of our lineage surely did not have the full time, the same was inhabited by Australopithecus package of bipedal adaptations found in later hominins. afarensis, which is present in more than a dozen fossil- All living apes can move bipedally, and some Miocene bearing localities representing hundreds of known spec- apes such as may have specialized in ter- imens, all dated between 3.9 and 2.9 million years ago. _ restrial bipedality. Recognizing the beginnings of the The teeth of these two closely similar species show sev- hominin adaptation to bipedality has been central to eral temporal trends, toward larger postcanine teeth and © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. Human Evolution 185 functional changes in the canine- cutting anat- remained important to A. afarensis and A. africanus, omy. Because of these trends, most paleoanthropologists even as these hominins moved into more open grassland regard A. anamensis and A. afarensis as successive mem- settings. Still, with hands and legs ill suited for suspen- bers of a single evolving lineage. sion or above-branch quadrupedal walking, early Other lineages of hominins may have been present at hominins must have climbed in a manner analogous to the same time, including platyops from recent humans. With masses around 35 kg and Kenya and Australopithecus bahrelghazali from Chad, males up to 50 kg, they approximated living chimpan- both between 3.5 and 3.3 million years ago. These are zees in body size. The most complete skeletal in- possibly distinct from A. afarensis because of cranial and dividuals, such as the “” skeleton of A. afarensis, dental peculiarities, but in each case the single specimen had statures of 100 to 140 cm, much shorter than the is fragmentary. Likewise, a partial foot skeleton from average of any recent human . Aus- Woranso-Mille, Ethiopia, may represent yet another tralopithecines had small brains, approximately 450 ml lineage with a distinct locomotor strategy, possibly a on average, which contrasts with the 1350 ml brains of direct descendant of earlier Ardipithecus. living humans. A diversity of contemporaneous forms is much clearer among the hominins near the Plio- 2. EARLY HOMO boundary. From approximately 2.8 to 2.3 million years ago, was the of Australopithecus By 1.8 million years ago, a very different kind of hominin africanus, also represented by large fossil samples and in had emerged and spread into Eurasia. Homo erectus was most respects similar in cranial and teeth to A. the size and stature of recent human hunter-gatherer afarensis. Additionally, by 2.5 million years ago, the populations, bigger than any known australopithecine. robust australopithecines appeared in East and later in The of H. erectus also contained dispropor- South Africa. Robust refers to the chewing of tionately larger brains than australopithecines, initially these hominins, which combined powerful jaw muscles between 600 and 900 ml, and relatively small teeth. with extraordinarily large molar and premolar teeth. The earliest clear fossil evidence of H. erectus occurs at The robust australopithecines had approximately the in the Republic of and Modjokerto, same body size as other australopithecines but clearly Java, with additional fossil discoveries in East and South had a different diet, featuring many more leaves and Africa. In each of these areas, remains of H. erectus ex- hard seeds. Australopithecus robustus was a South isted along with evidence of stone tool manufacture and African form, apparently descended from A. africanus. of stone. The evidence indicates that H. erectus Australopithecus boisei was the apex of this trend to- relied on a higher-quality diet including , which im- ward plant dietary specialization, and constitutes the posed greater demands on technical abilities and social majority of hominin fossils from East Africa between 2.5 organization, but created opportunities for dispersal and and 1.5 million years ago. range expansion, explaining the species’ extra-African The australopithecines were obligate bipeds, meaning distribution. their skeletal adaptations to bipedality precluded effec- At present, identifying the population that gave rise tive quadrupedal movement. Their feet had a first toe to H. erectus is one of the most engaging problems in the aligned with the other , minimal opposability or study of human evolution. Stone tools are known from grasping ability, and arches similar to the feet of living several sites in Ethiopia and Kenya before 2.5 million . Their were angled to promote effective years ago, and cut marks on bone indicate that weight support in a bipedal stance, and did not rotate to these tools were often used for butchering . Be- facilitate grasping with the feet. In contrast to nonhuman tween these earliest stone tools and the appearance of apes, humans and australopithecines have short hip- clear examples of Homo fossils lie nearly 700,000 years that make a broad, bowl-shaped structure to of time, during which the distinctive features of H. support the viscera when upright. In addition, the erectus must have been evolving, but the identity of the broader hip and shorter ischium enabled effective muscle themselves remains mysterious. Between 1.9 control of the lower limbs during bipedal walking and and 1.5 million years ago, a number of fossil crania and a running. Our bipedal form of locomotion is not as fast as handful of partial skeletons may represent a species chimpanzee or gorilla knuckle walking, but it is highly known as . The crania have larger brains energetically efficient. than typical for australopithecines, ranging from 500 to However, despite their clear bipedality, australo- 750 ml, and their teeth and jaws are smaller than earlier pithecines had relatively long, heavily muscled arms, australopithecines. Still, it remains unclear whether H. curved toes and finger bones, and a long clavicle and habilis was ancestral to H. habilis, and scholars disagree apelike shoulder , all suggesting that climbing about how many species these early specimens represent. © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. 186 Phylogenetics and the

Australopithecus sediba is an exceptionally - by 1 million years ago, where later they may have ing sample, dating to 2 million years ago from Malapa, evolved into a late-surviving isolated dwarf population South Africa. Two very complete skeletons of this spe- called Homo floresiensis. cies combine Homo-like teeth and hands with the body In Africa, the fossil record is sparser but supports the proportions, , and possible arboreal adapta- that Homo increased in variability in the period tions of earlier hominins. Whether this species could be after 1.2 million years ago. The West and South Asian ancestral to H. erectus or H. habilis or both could be archaeological records show that these were also influenced by analysis of a handful of fossil fragments occupied by early human populations, but scant fossils from East Africa. These have, in the past, been assigned remain. Europe was inhabited by 1.2 million years ago, to Homo, but until more is known about their anatomy, but the skeletal record represents chiefly the last 800,000 it will be difficult to test hypotheses about their years. relationships. Everywhere they lived, humans used stone tools. The The expansion of brain size from Australopithecus to basics of production involved the procurement of stone Homo is correlated with many aspects of life history and raw material either from rocky outcrops or from behavior. Neural tissue imposes a high metabolic cost, rounded cobbles in streambeds. People were selective which humans met by adopting dietary and behavioral about material, choosing fine-grained stone with pre- strategies that provide high caloric returns. The first dictable fracture dynamics, which they sought and postnatal of development includes a transported over kilometers. Removing a sharp flake by rapid expansion of brain size and concomitant shape itself yields a reliable cutting edge; removing several changes, in contrast to developmental trajectories of flakes from a rock, or “core,” can shape an edge suitable other primates. Neural development in humans extends for chopping or piercing bone. This basic technological across a long childhood, with late sexual maturation and pattern is called Oldowan. After 1.6 million years ago, an adolescent growth spurt. These ontogenetic patterns however, mainly in Africa and later in Europe and West appeared in concert with increasing brain size in Pleis- Asia, people shaped core tools into symmetrical tools tocene humans. An increased dependence in hunting and with long edges, called . The resulting Acheu- meat scavenging compared to other primates yielded a lean stone industry persisted for some 1.3 million years. net increase in diet quality, but imposed several risks, Along with stone, archaeologists know that Pleistocene such as with large , unreliability humans often used fire, wooden and other im- of , and long training necessary for skill develop- plements, and sometimes tools made of bone. ment. Modern humans mitigate these risks by food By 300,000 years ago, brain size had increased the sharing, sexual division of labor, and gathering of plant range of Homo to between 800 and 1300 ml. Most pa- and animal including honey. Hunter- leoanthropologists refer these later remains to species gatherer social groups are relatively egalitarian, with other than H. erectus. In Africa and Europe, they are decision making regulated by a coalition of many group often called , while many scien- members. In this setting, of social rules and tists call them “archaic Homo sapiens.” Whatever they communication about social norms are fundamental are called, these people began to experiment with dif- determinants of survival and . This social ferent technical forms, including a process of stone tool environment is thought to be a major selective driver of manufacture known as a prepared core technique. The larger brains that allowed for more sophisticated com- result was a greater control over the shape of end prod- munication and inferences about the intentions of other ucts, sometimes yielding blades and points that were social actors. Whereas australopithecines had vocal attached (hafted) onto spears as compound tools. These tracts similar in form to those of chimpanzees and goril- stone industries are called Middle Stone Age (MSA). las, early Homo had both vocal and auditory traits that could have supported humanlike sound production and 3. NEANDERTHALS AND THE ORIGIN OF reception. MODERN HUMANS After its origin, Homo diversified into regional pop- ulations with some morphological differences. In East Genetic evidence has greatly clarified our understanding Asia, Homo erectus occupied a range from north China of the human populations of the last 250,000 years. to Java, which was connected to the Asian mainland Archaeology and skeletal remains help to complete the during periods of low sea level. Across this range, pop- story, adding perspective on the causes and timing of the ulations developed regional variation in the shape of key events. This was a time of vast migrations and the browridge and forehead, extent of muscle develop- mixture of distant populations with each other. _ ment of the jaw and , and shape of the teeth. Some of By 250,000 years ago, MSA people had developed re- these people made a deepwater crossing to the island of gional tool industries with little evidence of interregional © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. Human Evolution 187 movement or exchange. A small skeletal sample repre- of the ancestry of living people, but a small multiregional sents these MSA populations from across Africa. These component of ancestry has remained in the of this represent the earliest humans with modern anatomical and subsequent migrations. characteristics, including a high forehead, face tucked beneath the front of the braincase, and a rounded cranial 4. vault. The functional import of these changes is not yet understood, but they seem to reflect a basic shift in de- After modern human populations became established velopmental patterning. throughout the world, evolution continued to shape The later MSA , after 120,000 years ago, be- our . Early human populations in Europe and came regionally differentiated. In both southern Africa northeast Asia likely found themselves poorly suited for and the Maghreb, people collected shells and marked the low temperature and insolation of these regions. The objects, for example, with natural and ostrich tropical regions of Asia had a similar physical eggshells. In Mozambique, people gathered large stores but very different floral and faunal than of wild grains; in Ethiopia, they transported Africa. Watercraft allowed people to colonize Australia, over hundreds of kilometers. Melanesia, and other island regions, and facilitated the Some African population dispersed into West Asia by migration of people from the Bering Land Bridge into the 105,000 years ago, taking with it a subset of the genetic southern parts of the before 14,000 years ago. variation present in Africa. In and Europe Rapid evolution by in all these novel they encountered the Neanderthals, whose remains are environments was inevitable. dated between 200,000 and 300,000 years ago. Beginning As humans dispersed throughout the world, they also from a common anatomical background with modern increased vastly in numbers. At the end of the last gla- humans, Neanderthals evolved a number of traits that ciation, people expanded their dietary breadth to a appeared nowhere else: long, barrel-shaped skulls with a greater number of plant and animal species, a process rearward projection called an “occipital bun,” thick called the Broad Spectrum Revolution. Some experi- curving long bones with large joints, and at least in the mented with planting and keeping seed crops; others European part of their range, body proportions now began managing herd animals more intensively. Over associated with inhabitants of very cold environments. many generations, these processes led to Neanderthals were a small population dispersed over a of former wild species, settlement of many human large , and even more than their contemporaries in groups into villages and , and the rise of political Africa, depended heavily on meat from large prey ani- and economic elites. Pastoralists sustained large popu- mals. The Neanderthals were probably a minor com- lations on formerly less hospitable plains and steppes, ponent of the overall Pleistocene human population, but sometimes migrating over long distances. their skeletal and archaeological remains are numerous, was one result of this agricultural revolution; warfare so we understand their lifeways better than other pop- and serfdom were others. ulations. Additionally, it has proved possible to obtain Human skeletal traits (and by inference ) have a partial genome sequence from Neanderthals, which changed during the last 20,000 years at a rate unmatched has shed great light on the genetic ancestry of modern by earlier periods. Humans became more gracile as cra- humans outside of Africa. nial muscle attachments and structures such as the brow- A sign of the weaknesses of the skeletal record is the ridge became lighter. After the introduction of agri- Denisova genome, from the Altai of southern , smaller teeth and jaws became common, and a . This genome represents a population living at higher proportion of failed to develop third the same time, but to the east of the Neanderthals, but molars, or “ teeth,” entirely. Along with such substantially distinct from the known Neanderthals ge- evolutionary changes, skeletal samples document the ca- netic sample. Living people in Australia and New Guin- tastrophic health effects resulting from and ea derive around 5 percent of their ancestry from a village life. population similar to the Denisova . Nean- Pathogens have been among the most obvious derthals themselves contributed between 2 and 4 percent causes of recent human evolution. For example, more of the ancestry of present populations throughout the than 20 different that protect to some extent world (including Australasia), except within Africa it- from falciparum are known from different self. These genetic results may help to explain morpho- human populations, many of which have arisen within logical features that imply some degree of regional the last few thousand years. Diet is another important continuity of human populations in Europe, East Asia, cause of recent evolutionary changes, as some human and Australasia; however, the spread of Africans within groups have specific genetic adaptations to starchy the last 100,000 years accounts for more than 90 percent grains and consumption. The physical environment © Copyright, Princeton University Press. No part of this book may be distributed, posted, or reproduced in any form by digital or mechanical means without prior written permission of the publisher. 188 Phylogenetics and the History of Life

has exerted its own selection on populations at high al- J. Hawks, E. T. Wang, G. Cochran, H. C. Harpending, and R. titude, with selection affecting oxygen transport, and at K. Moyzis. 2007. Recent acceleration of human adaptive high latitude, with recent strong selection on genes as- evolution. Proceedings of the National Academy of Sci- sociated with pigmentation. ences USA 104: 20753–20758. Available from dx.doi.org/ Industrial populations of the last 200 years have un- 10.1073/pnas.0707650104. An analysis showing that contrary to common , humans have experi- dergone further radical changes in longevity, residence enced recent, rapid adaptive evolution that is likely on- patterns, and size. Nevertheless, selection and going and accelerating. evolution of modern human populations is ongoing, Leakey, M. G., F. Spoor, F. H. Brown, P. N. Gathogo, C. with documented selection on quantitative traits of Kiarie, L. N. Leakey, and I. McDougall. 2001. New medical and biometric interest. The future direction of hominin genus from eastern Africa shows diverse middle human evolution cannot be predicted from our past lineages. 410: 433–440. Description of history (see chapter VIII.12), but the pace of recent Kenyanthropus and discussion of its relationship to evolution suggests that our species may have many more Australopithecus. changes ahead. McBrearty, S., and A. S. Brooks. 2000. The revolution that wasn’t: A new interpretation of the origin of modern . Journal of Human Evolution 39: 453– FURTHER READING 563. A discussion of the origins of many modern human traits, such as sophisticated Middle Stone Age tools and Aiello, L. C., and J.C.K. . 2002. Energetics and the other artifacts; argues that these traits emerged gradually evolution of the genus Homo. Annual Review of An- within Africa but then rapidly spread to the rest of the thropology 31: 323–338. A summary of the impacts of a world 40,000–50,000 years ago. transition to an energy-dense (high-meat) diet and its McHenry, H. M., and K. Coffing. 2000. Australopithecus to significance to some of the main differences between Homo: Transformations in body and . Annual Re- Homo and its progenitors. view of Anthropology 29: 125–146. A review providing Anto´n, S. C., W. R. Leonard, and M. L. Robertson. 2002. An useful perspectives on the origin of the genus Homo. ecomorphological model of the initial dispersal Patterson, N., D. J. Richter, S. Gnerre, E. S. Lander, and D. from Africa. Journal of Human Evolution 43: 773–785. Reich. 2006. Genetic evidence for complex of Analysis and discussion of the dispersal dynamics of Homo. humans and chimpanzees. Nature 441: 1103–1108. A Boehm, C. 1993. Egalitarian behavior and reverse dominance genomic analysis that dates the human-chimpanzee di- . Current Anthropology 34: 227–254. Available vergence and argues for flow between the two from www.jstor.org/stable/2743665. An interesting dis- lineages after their initial split. cussion of the conditions under which egalitarian human Reich, D., R. E. Green, M. Kircher, J. Krause, N. Patterson, E. are expected to arise, with implications for the Y. Durand, B. Viola, et al. 2010. Genetic history of an social structures in . archaic hominin group from Denisova in Siberia. Brumm, A., G. M. Jensen, G. D. van den Bergh, M. J. Mor- Nature 468: 1053–1060. Available from dx.doi.org/ wood, I. Kurniawan, F. Aziz, and M. Storey. 2010. 10.1038/nature09710. Description of the genome se- Hominins on Flores, , by one million years ago. quencing of archaic human remains that were con- Nature 464: 748–752. Available from dx.doi.org/ temporaneous with Neanderthals, but geographically 10.1038/nature08844. Description of a diminutive fossil separated. hominin from the Indonesian island of Flores. White, T. D., B. Asfaw, Y. Beyene, Y. Haile-Selassie, O. C. Dunbar, R.I.M. 2003. The social brain: Mind, , and Lovejoy, G. Suwa, and G. Wolde. 2009. Ardipithecus in evolutionary perspective. Annual Review of ramidus and the of early hominids. Anthropology 32: 163–181. A review of the hypothesis 326: 75–86. Available from dx.doi.org/10.1126/science. that the unusually large brains of Homo evolved in re- 1175802. Description of Ardipithecus ramidus and dis- sponse to the cognitive demands of living in social groups cussion of its significance for understanding of human with complex bonds. origins. Green, R. E., J. Krause, A. W. Briggs, T. Maricic, U. Stenzel, Wood, B., and T. Harrison. 2011. The evolutionary context M. Kircher, N. Patterson, et al. 2010. A draft sequence of of the first hominins. Nature 470: 347–352. Available the Neandertal genome. Science 328: 710–722. Available from dx.doi.org/10.1038/nature09709. A review of the from dx.doi.org/10.1126/science.1188021. Description of challenge of identifying fossil hominins and determining the genome sequence of Neanderthals and a comparison whether they belong to the lineage leading to modern with modern human variation. humans.

_