Aridity and hominin environments

Significance

Oxygen isotopes in modern and fossil mammals can provide information on climate. In this study, we provide a new record of aridity experienced by early hominins in Africa. We show that past climates were similar to the climate in eastern Africa today, and that early hominins experienced highly variable climates over time. Unexpectedly, our findings suggest that the long-term expansion of grasses and grazing herbivores since the Pliocene, a major ecological transformation thought to drive aspects of hominin evolution, was not coincident with aridification in northern Kenya. This finding raises the possibility that some aspects of hominin environmental variability might have been uncoupled from aridity, and may instead be related to other factors, such as rainfall seasonality or ecological interactions among plants and mammals.

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.

A central challenge of human evolutionary studies is understanding the role of climatic change in shaping early hominin environments and selective pressures (1, 2). Aridity influences the distribution and abundance of vegetation in African environments (3), and changes in aridity over both long and short time scales have been suggested to drive changes in hominin environments leading to adaptation, dispersal, speciation, and extinction (2, 4, 5). The notion that aridity may have driven certain adaptations has been fundamental to discussions of hominin evolution since 1925 (6), and continues to feature prominently in studies addressing changes in hominin locomotion, body proportions, thermoregulation, food acquisition, tool use, and social organization (7–10).

Changes in African climate are driven principally by changes in Earth’s orbital geometry, which has been documented in the geologic past using marine and continental sedimentary records (4, 11–15). Marine core records of dust, leaf wax biomarkers, pollen, and sapropels indicate long-term aridification across Africa since the late Miocene (4, 12, 14, 16–18), which has been linked to global cooling (19), changes in ocean circulation and temperature gradients (20), high-latitude glaciation (4), low-latitude atmospheric circulation (14), and tectonic uplift (21). Increasing aridity has been thought to drive the origin and subsequent expansion of C₄ plants (grasses and sedges) (22). The long-term increase in the abundance of C₄ plants throughout the Pliocene and Pleistocene has been well documented in eastern Africa using carbon isotope ratios in pedogenic carbonates and leaf wax biomarkers (23, 24) and coincides with an increasing reliance on C₄-based resources among mammals, including hominins and other primates (25, 26). Variation in the timing of vegetation change across basins indicates that existing continental- and regional-scale climate records are not sufficient to understand the drivers of basin- and local-scale ecological change, and do not reflect local hominin environments (23, 27). Evidence for vegetation changes with precession-scale timing suggests direct climate forcing of such changes over thousands of years (28, 29), but drivers of environmental change might not be equivalent at short vs. long time scales and also may vary over time.

Uncertainties in the relationships between climate and hominin environments stem in part from difficulties in reconstructing terrestrial aridity. Terrestrial climate indicators commonly used in eastern Africa, including the isotopic composition of pedogenic carbonates (21, 27, 30), mammal taxonomy (31–33), and morphology (34), provide valuable insight into past environments, but are sensitive to multiple environmental and evolutionary changes, making it difficult to identify the specific role of aridity. In addition, existing faunal records (31–34) typically combine fossils from multiple sites and may integrate relatively long (but varying) time periods. Other climate proxies, such as the deuterium isotope composition of leaf wax biomarkers (17) and fossil leaf morphology (35), have not been widely applied in Pliocene-Pleistocene sequences in Africa.

In the present study, we address paleoaridity using oxygen isotope ratios (δ¹⁸O) in herbivore tooth enamel. Our goal is to investigate the role of climate in shaping hominin environments over the past 4.4 million years, concentrating on individual stratigraphic horizons associated with hominin fossil and archaeological material. We focus on the Omo-Turkana Basin, where sediments preserve abundant evidence of early hominin evolution and associated environments throughout the Pliocene and Pleistocene. The environmental history of this basin is not necessarily representative of eastern Africa (1), but nonetheless provides a useful study system for investigating interactions between climate and ecology. A major benefit of analyzing herbivore tooth enamel is the possibility of comparing paired oxygen and carbon isotope records from the same fossil collections in which hominin specimens or stone tools have been found, providing indicators of climate and ecology at spatial scales directly relevant to hominin environments. We cannot address short-term orbital scale environmental variability, however, owing to discontinuous sedimentation associated with terrestrial vertebrate fossil collections.

Our geochemical approach for quantifying aridity in tropical African ecosystems relies on differing oxygen isotopic effects among taxa that are evaporation-sensitive (ES) or evaporation-insensitive (EI) (36, 37). This method, which builds on earlier work that focused on oxygen isotope variation among individual taxa (38–42), relies on a comparison of multiple taxa and simultaneously accounts for isotopic variation related to changes in both climate and environmental water. This proxy has advantages over previously used paleoaridity indicators because it is largely insensitive to changes in (i) vegetation, which control mammal taxonomic abundances, diet, and carbon isotopic records from tooth enamel, soil carbonates, and leaf wax biomarkers; (ii) moisture source, soil temperature, and elevation, which influence oxygen isotopic records reflecting meteoric water, such as soil carbonates and leaf wax biomarkers; and (iii) mammal physiology and behavior, which affect oxygen isotopic records of individual species. Previous applications of this approach to the African fossil record have been hampered by uncertainties in the selection of appropriate taxa and the unavailability of appropriate fossil collections.

Aridity is expressed as water deficit (WD), which describes the annual difference (in mm/y) between water loss (evaporation and transpiration) and water gain (precipitation) and is a useful indicator of water availability in African ecosystems (43, 44). δ¹⁸O values in large mammalian herbivore tooth enamel are in equilibrium with body water, which reflects oxygen inputs from food, drinking water, and air, and ultimately relate to meteoric (precipitation-derived) water (45). In the tropics, the oxygen isotopic composition of meteoric water is related to rainfall amount, elevation, and moisture source (46). Evaporation enriches the remaining water in the heavy isotope ¹⁸O relative to source water, such that aridity can be quantified by comparing one isotopic record that tracks meteoric water with another that tracks evaporative enrichment (36, 37).

The aridity index (36) is based on regressions between the WD and the oxygen isotopic enrichment between tooth enamel and local meteoric water (ε_enamel-mw). Mammalian herbivore taxa for which ε_enamel-mw increases with WD are classified as ES, and taxa for which ε_enamel-mw does not change with WD are classified as EI. Meteoric water cannot be measured directly in the fossil record; therefore, these relationships can be extended to the fossil record to predict WD by using δ¹⁸O values of EI taxa to represent meteoric water, because ε_ES-EI and ε_ES-mw both track aridity (36). Applying the aridity index to the fossil record requires the assessment of appropriate taxa, geological context, diagenetic alteration, and sample size (SI Appendix).

To revise the aridity index, we present a compilation of new and previously published δ¹⁸O values (n = 1,224 in 57 species) measured on tooth enamel from modern mammalian herbivores from 37 locations in eastern and central Africa (Fig. 1), along with climate data and WD estimates for each location and δ¹⁸O values in meteoric water (n = 161) from 33 of these locations (SI Appendix, Table S1 and Datasets S1 and S2). Our compilation significantly expands on a previously published dataset (36) and includes δ¹⁸O data from many more locations and taxa, and also expands the WD scale owing to the correction of a mathematical error in calculating potential evapotranspiration (SI Appendix, Fig. S1). To address paleoaridity in eastern Africa, we present a compilation of new and previously published mammalian herbivore δ¹⁸O_enamel values (n = 273) from 26 fossil collections (Fig. 1) ranging in age from ∼4.4–0.01 Ma, chosen based on their potential for addressing paleoaridity and their association with hominin fossil and archaeological material (SI Appendix, Dataset S3). We use a subset of δ¹⁸O_enamel values (n = 160) from 11 fossil collections in the Omo-Turkana Basin, including specimens from the Kanapoi, Koobi Fora, Nachukui, and Kibish Formations (Fig. 1 and SI Appendix, Fig. S3 and Tables S2 and S3), that meet the criteria for application of the aridity index to evaluate long-term changes in paleoaridity in this basin. We also estimate paleoaridity using previously published δ¹⁸O_enamel values from two eastern African fossil collections outside the Turkana Basin that meet the criteria for applying this method, including Aramis, Ethiopia (4.4 Ma), and Kanjera South, Kenya (2.0 Ma). Finally, we investigate the relationship between aridity and ecosystem structure in eastern Africa using a compilation of previously published modern mammalian herbivore tissue δ¹³C values (n > 1,600) (25) and a compilation of new and previously published fossil tooth enamel δ¹³C values (n = 658) from fossil collections with paleoaridity estimates.

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).

Results and Discussion

Terrestrial Aridity Proxy.

Across modern localities, WD increases nonlinearly with decreasing mean annual precipitation (logarithmic regression, R² = 0.7546, P < 0.0001) and increasing mean annual temperature (quadratic polynomial regression, R² = 0.6829, P < 0.0001) (SI Appendix, Fig. S1). ε_enamel-mw values for Hippopotamidae, Elephantidae, and Rhinocerotidae do not vary with WD, and these taxa are classified as EI (Fig. 2A). ε_enamel-mw values for Giraffidae, Hippotragini, and Tragelaphini increase with WD, and these taxa are classified as ES (Fig. 2B). The slopes of WD-ε_ES-mw regressions vary significantly from one another (P < 0.05, F test), except for Giraffidae and Hippotragini (P > 0.05, F test). Despite preliminary observations to the contrary (36), the addition of more locations in this dataset reveals that Antilopini, Bovini, and Neotragini should be excluded from the ES category (Fig. 2C). Other sampled bovids, suids, and equids (Fig. 2C) have no significant relationship with WD (P > 0.05), except Cephalophini (P < 0.05), which are not considered further owing to ε_enamel-mw values that are more variable and/or span a restricted WD range. Additional data are needed to address the variability in ε_enamel-mw values across bovid genera, although many bovid fossils are identifiable only to tribe.

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.

We use a body water model to identify possible physiological and behavioral mechanisms driving the relationship between ε_enamel-mw and WD among EI and ES taxa (SI Appendix). A static oxygen budget, in which body water is influenced solely by changes in the isotopic composition of oxygen influxes rather than by changes in the balance of influxes, is inconsistent with the ε_enamel-mw values of either ES or EI taxa (SI Appendix, Fig. S2 C and D); therefore, ¹⁸O enrichment of leaf water in arid environments is insufficient to explain the relationship between ε_enamel-mw values and WD among ES taxa. Instead, sensitivity to evaporation is likely related to differences in drinking behavior and associated changes in the balance of oxygen influxes as the environment varies. Predicted ε_enamel-mw values suggest that EI taxa reflect meteoric water as aridity increases owing to a balance between increasing drinking water and decreasing food water (SI Appendix, Fig. S2C), and ES taxa track increasing aridity owing to a balance between decreasing drinking water and increasing intake of food water and O₂, both of which are sensitive to evaporation (SI Appendix, Fig. S2D).

The aridity index reflects the relationship between WD and the enrichment between ES and EI taxa (ε_ES-EI). Significant WD-ε_ES-EI regressions (P < 0.05) that can be used to estimate paleoaridity include ε_{Giraffid-Hippopotamidae,} ε_{Tragelaphini-Hippopotamidae}, ε_{Hippotragini-Hippopotamidae}, ε_{Tragelaphini-Elephantidae}, and ε_{Tragelaphini-Rhinocerotidae} (Fig. 3 and SI Appendix, Table S4). The SEs of these regression models are relatively low (±193 to ±478.1 mm/y) and coefficients of determination are high (R² = 0.81–0.92), and thus these models have sufficient predictive power to estimate paleoaridity in the fossil record. Slopes are different among WD-ε_ES-EI regressions (P < 0.05, F test); therefore, a pooled or common slope, as suggested previously (36), is not appropriate. We use the mean of WD values calculated with WD-ε_ES-EI regressions for all available ES-EI pairs from each fossil collection. Uncertainty in WD estimates (∼800 mm/y) corresponds to ∼20% of the WD range in modern eastern African environments, sufficient to detect long-term trends in Turkana (SI Appendix). Other WD-ε_ES-EI regressions are not significant (P > 0.05) and should not be used to estimate paleoaridity.

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.

Paleoaridity.

Our oxygen isotope analyses of fossil tooth enamel for paleoaridity estimation were restricted to fossil collections with well-defined stratigraphic and sedimentological context and the preservation of appropriate ES and EI taxa. Among all fossil collections used to evaluate paleoaridity, we find highly variable conditions, including both mesic (WD < 0) and arid (WD > 0) climates that fall within a WD range (∼−550–1,700 mm/y) encompassing ∼61% of the modern range (Fig. 4 and SI Appendix, Fig. S4). The mean WD estimated by fossil tooth enamel is 471 mm/y, similar to the present-day mean WD in eastern and central Africa (251 mm/y; P > 0.05) (SI Appendix, Figs. S1 and S4 and Tables S1 and S3). Among fossil collections from the Omo-Turkana Basin, we detect no long-term trend in WD between ∼4.2 and 0.01 Ma (P > 0.05) (Fig. 4).

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).

The paleoaridity record (Fig. 4 and SI Appendix, Fig. S4 and Tables S2 and S3) begins in the Pliocene with a highly uncertain estimate of arid conditions associated with Ardipithecus ramidus fossils from the Lower Aramis Member, Sangatole Formation (∼4.4 Ma) (47). In the Omo-Turkana Basin, we find arid conditions at Kanapoi (∼4.16 Ma) and a highly uncertain estimate of mesic conditions at Allia Bay (∼4.0 Ma), both associated with Australopithecus anamensis (48), and pedogenic carbonate δ¹³C values indicative of vegetation ranging from woodland/bushland/shrubland to wooded grassland (49). Mid- to late-Pliocene (∼3.5–2.8 Ma) fossil collections in Turkana indicate variable conditions that include arid (Kangatukuseo KU1) and arid to mesic (Lomekwi LO4/5) climates, associated with pedogenic carbonate δ¹³C values indicating woody cover, including woodland/bushland/shrubland (Fig. 4) (49). This time interval in Turkana includes fossils of the hominin genera Kenyanthropus and Paranthropus. The early Pleistocene (∼2.5–1.5 Ma) is represented in the Turkana Basin by fossil assemblages in the upper Burgi Member of the Koobi Fora Formation as well as the Kaito Member of the Nachukui Formation, which indicate arid (FwJj20 and Kalochoro KL3/6, Naiyena Engol NY2/3), nearly balanced (Kokiselei KS2 and Kangatukuseo KU2/3), and mesic (Kokiselei KS1) conditions. Pedogenic carbonate δ¹³C values indicate that relatively open vegetation, including wooded grasslands, became more prevalent during this time interval in Turkana (49), which includes an abundant fossil record of Homo and Paranthropus. Mesic conditions prevailed at Kanjera South KS-2 in southwestern Kenya, associated with an open grassland ecosystem (50). Archaeological occurrences at Kanjera South, as well as in the Nachukui and Koobi Fora Formations in the Turkana Basin, demonstrate that Oldowan tool-making hominins inhabited mesic and arid environments. The late Middle Pleistocene to Holocene (∼0.2–0.01 Ma) is represented in the Turkana Basin by fossil assemblages in the Kibish Formation, which indicate arid conditions in Member 4 and arid to mesic conditions in Member 1. Fossils identified as Homo sapiens (Omo I and Omo II) are from Member 1, and other human specimens are derived from either Member 3 or Member 4. WD estimates in Members 1 and 4 are substantially lower than those in Turkana today (modern WD = 2,386 mm/y; SI Appendix, Table S1), consistent with deposition during relatively humid periods associated with high lake levels and sapropel formation intervals (51).

Relationships Between Climate and Ecology.

To understand the significance of aridity in shaping hominin environments in eastern Africa, we further consider the relationship between climate and ecology in modern African ecosystems. Vegetation in Africa is shaped by complex interactions between multiple abiotic (e.g., rainfall amount and seasonality, fire, atmospheric pCO₂) and biotic (e.g., herbivory) factors, and the relative importance of these factors is contingent on the ecological history of each area (52–54). Although woody cover is constrained by aridity (55), vegetation does not respond in a direct or continuous manner to changes in annual rainfall, and each biome (e.g., forest, savanna, grassland) is distributed over a wide rainfall range (1,000–3,000 mm/y) (52, 56, 57). We find that among modern eastern and central African ecosystems, the proportion of C₄ grazers increases with WD (R² = 0.262, P = 0.00536), and the proportion of C₃ browsers decreases with WD (R² = 0.1884, P = 0.021) (Fig. 5). These correlations are weak, however, and during the Pliocene-Pleistocene forests were rare in the Turkana Basin (25, 27, 49, 58) and elsewhere in eastern Africa (23). After excluding forests, we find no relationship between WD and the proportional abundance of each diet guild (Fig. 5). Thus, although the abundances of C₄ plants and C₄ grazing herbivores are often used as an indicators of aridity (21, 30), variation in C₄ biomass among nonforest biomes can be decoupled from aridity.

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.

Paleoaridity records from δ¹⁸O of tooth enamel provide a means to investigate links between climate and ecology in hominin environments, but also are highly discontinuous owing to the incompleteness of the terrestrial fossil record, compounded in this case by the scarcity of fossil assemblages meeting the criteria for applying the aridity index. We address this problem in three ways. First, we examine the fidelity of long-term environmental records derived from the fossil collections used for paleoaridity analysis. The long-term increase in the proportion of C₄ grazers among Artiodactyla-Perissodactyla-Proboscidea over the Pliocene-Pleistocene in the Omo-Turkana Basin is similar (P > 0.05, F test) when calculated using tooth enamel δ¹³C values from paleoaridity fossil collections (R² = 0.3868, P = 0.02324) or from a larger fossil tooth enamel δ¹³C dataset divided into long time bins (>100 ka) (R² = 0.7391, P = 0.0006911) (SI Appendix, Fig. S5). Therefore, it is possible to recover first-order environmental trends using fossils from these discontinuous depositional intervals.

Second, we compare our WD record with previously published geological and faunal-based reconstructions of terrestrial paleoclimate in Turkana with varying time representation and analytical biases. There are no trends in paleoclimate based on paleosol calcic depth, mammal hypsodonty and lophedness (k-nearest-neighbor model), or bovid tribe abundance (SI Appendix, Fig. S6). Estimated precipitation decreases over time based on mammal community structure (R² = 0.2295, P = 0.03258) and mammal hypsodonty and lophedness (linear regression model) (R² = 0.0583, P = 0.004477), but these trends are weak and based on proxies influenced by evolutionary and dietary changes, respectively, that might not be related to aridity (SI Appendix, Fig. S6). Taken together, evidence for marked long-term aridification in the Turkana Basin is weak.

Third, we examine the relationship between WD and ecology in the fossil record irrespective of time. There are no relationships (P > 0.05) between WD and the proportion of C₄ grazers, C₃-C₄ mixed feeders, or C₃ browsers (Fig. 5 and SI Appendix, Table S4). Owing to the lack of suitable fossil collections for the application of our tooth enamel aridity proxy, we do not address climate before ∼4 Ma. The Late Miocene appears to have been more humid in Turkana and elsewhere (34), although aridification before ∼4 Ma predates the long-term increase in C₄ vegetation and C₄ grazing mammalian herbivore that continued throughout the Pliocene and Pleistocene (25, 27).

Despite the coarse time resolution associated with the tooth enamel δ¹⁸O WD calculations, we suggest that the Pliocene-Pleistocene expansion in C₄ plants and C₄ grazing herbivores appears to not be coincident with significant long-term aridification in the Omo-Turkana Basin (Fig. 4 and SI Appendix, Fig. S6). The possibility of a smaller long-term increase in aridity, undetected owing to uncertainty in WD estimates, cannot be discounted, but would not necessarily have been a major environmental driver, given that ecological feedback in African biomes inhibits vegetation responses to climate change (52–54). Thus, the cause of the major long-term expansion of C₄ biomass within Turkana and elsewhere remains unclear, but may be related to climatic and ecological dynamics that are unrelated to annual WD and need not be equivalent across basins or regions (1, 23, 25). Our results do not preclude the possibility of climate-driven change in hominin environments generally, but highlight the need to address possible variability in the determinants of environmental change in different areas, because basins do not necessarily respond in a straightforward way to continental- and regional-scale aridification. Similarly, previous paleosol and leaf wax biomarker paleovegetation studies demonstrate that the timing and magnitude of the expansion of C₄ plants is not uniform across eastern and northeastern Africa (23, 27, 59). Thus, climatic and ecological dynamics appear to vary across basins, and regional-scale climate proxies must be contextualized by terrestrial, basin-scale environmental records most relevant to hominin evolution.

Our aridity record is consistent with the notion that climate instability may be an important driver of hominin evolution (2, 30). Arid conditions were prevalent during two large lake intervals ∼4.0 and 2.0 Ma (Fig. 4), consistent with climate variability including periods of increased aridity occurring within generally humid periods characterized by widespread lake formation (30). Orbital-scale environmental change has been demonstrated using leaf wax biomarkers from Early Pleistocene lake sediments at Olduvai Gorge, suggesting a direct link between rainfall and changes in the balance of woody and grassy vegetation (28). This case, and other episodes of climate-driven vegetation change (29, 60), are entirely consistent with ecological dynamics in cases where woody cover is not constrained by other factors, or when drastic changes in precipitation, particularly during periods of heightened climatic variability, exceed thresholds for stable biome states otherwise maintained by herbivory, fire, or other factors (44, 53).

Aridity and Human Evolution.

Our paleoaridity record demonstrates that hominins were able to accommodate variable environments throughout the Pliocene-Pleistocene in eastern Africa, and is consistent with the notion that biological and behavioral changes in hominins, including upright posture, hair loss, sweating, and long-distance scavenging or running, may be related to thermophysiological challenges associated with surviving periodically arid conditions and high heat loads (58). The relative abundance of C₃ woody vegetation during the Pliocene (Fig. 4) is consistent with the notion that early bipedal hominins could have relied on areas with shade-providing plants that may have reduced water and heat stress. Archaeological occurrences in Turkana during the early Pleistocene (∼2.4–1.4 Ma) are preferentially associated with lower δ¹³C values of paleosol carbonate compared with those from nonarchaeological deposits, indicating that hominins concentrated their activities in more wooded areas (61). In contrast, archaeological occurrences at Kanjera South in southwestern Kenya (2.0 Ma) demonstrate hominins repeatedly using an open grassland (50) when aridity was low (SI Appendix, Fig. S4). Thus, early hominin land use patterns were likely structured by the interplay between aridity and vegetation, such that the exploitation of increasingly open C₄-dominated ecosystems may have been limited during periods of high aridity owing to constraints on the availability of water and shade.

Conclusion

Our findings demonstrate how mammal tooth enamel δ¹⁸O values can be used to quantify paleoaridity directly associated with the hominin fossil and archaeological record. WD values estimated from fossil tooth enamel δ¹⁸O values suggest that early hominins experienced highly variable climatic conditions within the range of present-day environments in the region, and could accommodate arid conditions as early as ∼4.2 Ma. The modern hyperarid climate in Turkana is not a useful analog for paleoaridity in the basin. The lack of evidence for marked, long-term aridification, along with the absence of any relationship between aridity and herbivore diet structure, suggest that other abiotic or biotic determinants may have driven long-term ecological restructuring in the Omo-Turkana Basin. The complex interplay of ecology and behavior suggests that disentangling the influence of climate on the evolution of humans and other mammals remains a significant challenge. Future interbasinal and intrabasinal studies are needed to investigate relationships among changing basinal geometry, biogeography, climate, depositional setting, ecology, and evolution.

Materials and Methods

Additional details on isotopic and statistical methods, WD calculations, and models of leaf water, leaf cellulose, and body water δ¹⁸O, along with an expanded discussion on criteria for the application of the aridity index, are provided in SI Appendix. Modern meteoric water samples were compiled from the literature (SI Appendix, Dataset S1). Modern and fossil samples of mammalian tooth enamel were analyzed for δ¹⁸O using standard methods or were compiled from the literature (SI Appendix, Datasets S2 and S3). Information on the geological context of fossil specimens is provided in SI Appendix, Dataset S3.