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Aridity and Hominin Environments

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Aridity and Hominin Environments Aridity and hominin environments Scott A. Blumenthala,1, Naomi E. Levinb, Francis H. Brownc, Jean-Philip Brugald, Kendra L. Chritze, John M. Harrisf, Glynis E. Jehlec, and Thure E. Cerlingc aResearch Laboratory for Archaeology, University of Oxford, Oxford OX1 3QY, United Kingdom; bDepartment of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109; cDepartment of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112; dAix-Marseille University, CNRS, UMR 7269, Laboratoire Méditerranéen de Préhistoire Europe Afrique, 13094 Aix-en-Provence Cedex 2, France; eNational Museum of Natural History, Smithsonian Institution, Washington, DC 20013; and fNatural History Museum of Los Angeles County, Los Angeles, CA 90007 Edited by James O’Connell, University of Utah, Salt Lake City, UT, and approved May 25, 2017 (received for review January 11, 2017) Aridification is often considered a major driver of long-term ecolog- but drivers of environmental change might not be equivalent at ical change and hominin evolution in eastern Africa during the Plio- short vs. long time scales and also may vary over time. Pleistocene; however, this hypothesis remains inadequately tested Uncertainties in the relationships between climate and hominin owing to difficulties in reconstructing terrestrial paleoclimate. We environments stem in part from difficulties in reconstructing present a revised aridity index for quantifying water deficit (WD) in terrestrial aridity. Terrestrial climate indicators commonly used in terrestrial environments using tooth enamel δ18O values, and use this eastern Africa, including the isotopic composition of pedogenic approach to address paleoaridity over the past 4.4 million years in carbonates (21, 27, 30), mammal taxonomy (31–33), and mor- eastern Africa. We find no long-term trend in WD, consistent with phology (34), provide valuable insight into past environments, but other terrestrial climate indicators in the Omo-Turkana Basin, and no are sensitive to multiple environmental and evolutionary changes, relationship between paleoaridity and herbivore paleodiet structure making it difficult to identify the specific role of aridity. In ad- among fossil collections meeting the criteria for WD estimation. Thus, dition, existing faunal records (31–34) typically combine fossils we suggest that changes in the abundance of C4 grass and grazing from multiple sites and may integrate relatively long (but varying) herbivores in eastern Africa during the Pliocene and Pleistocene may time periods. Other climate proxies, such as the deuterium iso- have been decoupled from aridity. As in modern African ecosystems, tope composition of leaf wax biomarkers (17) and fossil leaf other factors, such as rainfall seasonality or ecological interactions morphology (35), have not been widely applied in Pliocene- among plants and mammals, may be important for understanding Pleistocene sequences in Africa. ANTHROPOLOGY the evolution of C4 grass- and grazer-dominated biomes. In the present study, we address paleoaridity using oxygen iso- tope ratios (δ18O) in herbivore tooth enamel. Our goal is to in- oxygen isotopes | terrestrial paleoclimate | human evolution | mammals | vestigate the role of climate in shaping hominin environments over Africa the past 4.4 million years, concentrating on individual stratigraphic horizons associated with hominin fossil and archaeological mate- central challenge of human evolutionary studies is un- rial. We focus on the Omo-Turkana Basin, where sediments pre- Aderstanding the role of climatic change in shaping early serve abundant evidence of early hominin evolution and associated hominin environments and selective pressures (1, 2). Aridity influ- environments throughout the Pliocene and Pleistocene. The envi- EARTH, ATMOSPHERIC, ences the distribution and abundance of vegetation in African en- ronmental history of this basin is not necessarily representative of AND PLANETARY SCIENCES vironments (3), and changes in aridity over both long and short time eastern Africa (1), but nonetheless provides a useful study system scales have been suggested to drive changes in hominin environ- for investigating interactions between climate and ecology. A major ments leading to adaptation, dispersal, speciation, and extinction (2, benefit of analyzing herbivore tooth enamel is the possibility of 4, 5). The notion that aridity may have driven certain adaptations comparing paired oxygen and carbon isotope records from the has been fundamental to discussions of hominin evolution since same fossil collections in which hominin specimens or stone tools 1925 (6), and continues to feature prominently in studies addressing have been found, providing indicators of climate and ecology at changes in hominin locomotion, body proportions, thermoregula- tion, food acquisition, tool use, and social organization (7–10). Significance Changes in African climate are driven principally by changes in Earth’s orbital geometry, which has been documented in the geo- Oxygen isotopes in modern and fossil mammals can provide in- logic past using marine and continental sedimentary records (4, 11– formation on climate. In this study, we provide a new record of 15). Marine core records of dust, leaf wax biomarkers, pollen, and aridity experienced by early hominins in Africa. We show that past sapropels indicate long-term aridification across Africa since the climates were similar to the climate in eastern Africa today, and late Miocene (4, 12, 14, 16–18), which has been linked to global that early hominins experienced highly variable climates over time. cooling (19), changes in ocean circulation and temperature gradi- Unexpectedly, our findings suggest that the long-term expansion ents (20), high-latitude glaciation (4), low-latitude atmospheric of grasses and grazing herbivores since the Pliocene, a major circulation (14), and tectonic uplift (21). Increasing aridity has been ecological transformation thought to drive aspects of hominin evolution, was not coincident with aridification in northern Kenya. thought to drive the origin and subsequent expansion of C4 plants (grasses and sedges) (22). The long-term increase in the abundance This finding raises the possibility that some aspects of hominin environmental variability might have been uncoupled from aridity, of C4 plants throughout the Pliocene and Pleistocene has been well documented in eastern Africa using carbon isotope ratios in ped- and may instead be related to other factors, such as rainfall sea- ogenic carbonates and leaf wax biomarkers (23, 24) and coincides sonality or ecological interactions among plants and mammals. with an increasing reliance on C4-based resources among mam- Author contributions: S.A.B., N.E.L., F.H.B., and T.E.C. designed research; S.A.B., N.E.L., mals, including hominins and other primates (25, 26). Variation in F.H.B., J.-P.B., K.L.C., J.M.H., G.E.J., and T.E.C. performed research; S.A.B. and N.E.L. ana- the timing of vegetation change across basins indicates that existing lyzed data; and S.A.B. and N.E.L. wrote the paper. continental- and regional-scale climate records are not sufficient to The authors declare no conflict of interest. understand the drivers of basin- and local-scale ecological change, This article is a PNAS Direct Submission. and do not reflect local hominin environments (23, 27). Evidence 1To whom correspondence should be addressed. Email: [email protected]. for vegetation changes with precession-scale timing suggests direct This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. climate forcing of such changes over thousands of years (28, 29), 1073/pnas.1700597114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1700597114 PNAS Early Edition | 1of6 Downloaded by guest on September 26, 2021 spatial scales directly relevant to hominin environments. We can- A 5 km BC not address short-term orbital scale environmental variability, however, owing to discontinuous sedimentation associated with KI1 SHUNGURA FM. NC3 KL3&6 NC1 NY4 terrestrial vertebrate fossil collections. KL2&6 NY3 LK KOOBI FORA FM. NY2 KL1 Our geochemical approach for quantifying aridity in tropical KS1 FwJj20 KS2 KK African ecosystems relies on differing oxygen isotopic effects FxJj82 10°N KG1&2 105-1311 LA1 4°N 105-0208 among taxa that are evaporation-sensitive (ES) or evaporation- FxJj1&3 LO3 104 LO8 NACHUKUI FM. insensitive (EI) (36, 37). This method, which builds on earlier work LO6 LO7 Allia Bay LO4 LO1/2 that focused on oxygen isotope variation among individual taxa KU2 0° LO5 (38–42), relies on a comparison of multiple taxa and simulta- LO10 KU1 KU3 3°N neously accounts for isotopic variation related to changes in both LO7 Lothagam climate and environmental water. This proxy has advantages over Kanapoi previously used paleoaridity indicators because it is largely in- 36°E 37°E 20°E 30°E sensitive to changes in (i) vegetation, which control mammal tax- onomic abundances, diet, and carbon isotopic records from tooth Fig. 1. Map of the study area. (A) Detailed map of fossil exposures (red areas) ii and sites (red circles) and drainages associated with the Nachukui Formation, enamel, soil carbonates, and leaf wax biomarkers; ( ) moisture west of Lake Turkana. (B) Fossil collection sites and formations in the Omo- source, soil temperature, and elevation, which influence oxygen Turkana Basin. (C) Map of Africa with sampling locations for modern teeth isotopic records reflecting meteoric water, such as soil
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