Aridity and hominin environments

Abstract

Aridification is often considered a major driver of long-term ecological change and hominin evolution in eastern Africa during the Plio-Pleistocene; however, this hypothesis remains inadequately tested owing to difficulties in reconstructing terrestrial paleoclimate. We present a revised aridity index for quantifying water deficit (WD) in terrestrial environments using tooth enamel δ¹⁸O values, and use this approach to address paleoaridity over the past 4.4 million years in eastern Africa. We find no long-term trend in WD, consistent with other terrestrial climate indicators in the Omo-Turkana Basin, and no relationship between paleoaridity and herbivore paleodiet structure among fossil collections meeting the criteria for WD estimation. Thus, we suggest that changes in the abundance of C₄ grass and grazing herbivores in eastern Africa during the Pliocene and Pleistocene may have been decoupled from aridity. As in modern African ecosystems, other factors, such as rainfall seasonality or ecological interactions among plants and mammals, may be important for understanding the evolution of C₄ grass- and grazer-dominated biomes.

Keywords: Africa; human evolution; mammals; oxygen isotopes; terrestrial paleoclimate.

Fig. 1.

Map of the study area. (A) Detailed map of fossil exposures (red areas) and sites (red circles) and drainages associated with the Nachukui Formation, west of Lake Turkana. (B) Fossil collection sites and formations in the Omo-Turkana Basin. (C) Map of Africa with sampling locations for modern teeth and meteoric water (white circles) and fossil sites (red circles).

Fig. 2.

Isotopic enrichment between enamel and meteoric water (ε_enamel-mw) among eastern and central African herbivores. (A) EI taxa. (B) ES taxa. (C) Other bovids, equids, and suids. Error bars represent propagated SE of ε_enamel-mw values. Data are compiled in SI Appendix, Datasets S1 and S2.

Fig. 3.

Isotopic enrichment (ε_ES-EI) between modern ES and EI taxa. ε_ES-EI was calculated using mean δ¹⁸O_enamel values of sampled ES taxa (rows) and EI taxa (columns) from eastern and central Africa. Dashed lines indicate significant WD-ε_ES-EI regressions (SI Appendix, Table S4). Error bars represent propagated SE of ε_ES-EI. Calculated from values provided in SI Appendix, Dataset S2.

Fig. 4.

Compilation of data indicating aspects of climate and ecology over the past 5 million years in the Omo-Turkana Basin. (A) Paleoaridity estimates, with error bars indicating age uncertainty and propagated SE of mean WD estimates using all available combinations of ES and EI taxa (SI Appendix, Table S3). (B) Deep lake intervals (62). (C) Paleosol carbonate clumped-isotope temperatures (63). (D) Carbon isotope values of pedogenic carbonates (δ¹³C_pc) (64). There is a trend toward increasing δ¹³C values over time (R² = 0.2442, P < 0.0001). (E) Percent C₄ grazers among Artiodactyla-Perissodactyla-Proboscidea (APP). There is a trend toward including the proportion of C₄ grazers over time (R² = 0.7391, P < 0.001). (F) Schematic timeline showing the appearance of major hominin behaviors and taxa in eastern Africa (SI Appendix).

Fig. 5.

WD (mm/y) and the proportion of C₄ grazers, C₃-C₄ mixed feeders, and C₃ browsers calculated using the average δ¹³C value of each taxon. (A) Modern collections in eastern and central Africa. (B) Fossil collections in eastern Africa. Modern data (51, 65) and fossil data are summarized in SI Appendix, Table S5 and compiled in SI Appendix, Dataset S4.