C₄ Diet of P. boisei.

The carbon isotope composition of P. boisei is fundamentally different from that of all known living and fossil hominoids, which vary from nearly pure C₃ consumers like gorillas (16) and chimpanzees (17, 18) to variable C₃ consumers like A. africanus and P. robustus (12, 13). Indeed, the only known haplorrhine primate with a similar carbon isotope composition is the extinct grass-eating baboon Theropithecus oswaldi (x = -2.3‰, SD = 1.5‰, n = 12) (see Table S2 and references therein) from the same general time period. Despite a once broad distribution, the genus Theropithecus is now limited to a single species living in the highlands of Ethiopia. These observations suggest that the organisms with which P. boisei most likely competed for resources were not contemporaneous Homo, Papio (savanna baboons), or other frugivorous taxa, but probably C₄ biomass consumers including equids, suids, hippos, Theropithecus, and some bovids.

The vast majority of C₄ biomass is grass, although there can be large quantities of C₄ sedges, especially in wetland environments such as the Okavango Delta today (19). A much smaller percentage of C₄ dicots and crassulacean acid metabolism plants with similar carbon isotope compositions exists, although the typically low abundance of the former and low abundance and frequent toxicity of the latter make them unlikely to be regular targets for early hominins (20). Thus, it is almost certain that P. boisei had a diet focused on grasses, sedges, or both.

It has recently been suggested that sedges were an important hominin resource because they are often found in the riverine woodlands favored by many savanna primates and because their tubers are a potentially high energy resource for which tool-wielding hominins would have had little competition (14, 21). Nevertheless, sedges often utilize C₃ photosynthesis, are not widely distributed in many habitats, and might have been of dubious nutritional value without cooking (22, 23). Moreover, there is no record of any large mammal feeding on sedges to such an extent. Regardless, if P. boisei was a wetland sedge specialist, it would suggest an extremely limited distribution in ancient landscapes with important implications for our understanding of early hominin biology and biogeography.

Grasses, in contrast, are widely abundant and use the C₄ photosynthetic pathway in most African savanna environments; they are utilized extensively by taxonomically diverse mammals including the primate Theropithecus. A reason for thinking that grass blades were not consumed by P. boisei is that the low occlusal relief of its cheek teeth is the opposite of what might be expected for a consumer of leaves. Folivores tend to have great occlusal relief, whereas flat teeth are usually found in hard-object consumers among frugivorous primates (1). Nevertheless, the dental microwear complexity profiles of P. boisei and Theropithecus are similar, suggesting a diet with comparable mechanical properties (24). Although the high anisotropy (directionality) of Theropithecus molar microwear is very different from that of P. boisei, it has been suggested that this results from different constraints posed by their dentognathic morphology rather than diet (24). In other words, it is possible that they were utilizing similar foods but chewing them in different ways.

P. boisei cheek teeth display notable gradients of gross wear, resulting in large, deeply excavated dentine exposures, and in this regard, they are similar to other australopith species (e.g., A. afarensis and A. africanus) that also possess low tooth cusps with thick enamel. Thus, like other australopiths, P. boisei undoubtedly had a diet that consisted of foods with abrasive qualities—the gross wear is as likely due to repetitive loading of phytolith-rich tough foods as exogenous grit. Thus, either grass or sedge consumption and/or exogenous grit might well have contributed to P. boisei’s notable wear gradient.

Of perhaps greater moment than its potential specific similarities, the microwear of P. boisei molars, which shows remarkable uniformity over time from about 2.3 Ma to about < 1.4 Ma (9, 24), stands in stark contrast to the wear fabrics exhibited by primate hard-object consumers. Indeed, there is no evidence beyond the anecdotal [e.g., the broken left first permanent molar crown in the KNM-ER 729 P. boisei mandible (8) and the observation that a couple of P. boisei molars show antemortem enamel chipping (25)] that these food items were hard.

Paleoenvironment of P. boisei.

Previous interpretations of the environmental conditions of P. boisei are varied and include closed wet habitats (26), scrub woodland to arid shrubland (27), and semiarid savanna associated with woodlands and gallery forest (28). The oxygen isotope composition of P. boisei and contemporaneous mammal tooth enamel provides further information about its water utilization and environment (29). Fig. 2 shows the δ¹⁸O and δ¹³C values for the water-dependent hippopotamus, the water-independent browser Giraffa, and two grazers (equids and suids) from the Kay Behrensmeyer Site (KBS) and Okote members (Table S1); it also shows the δ¹⁸O and δ¹³C values for their modern counterparts in the Tsavo region in Kenya (Table S3). These data suggest that P. boisei was very water dependent based on its δ¹⁸O values, which are more negative than those of coeval suids, equids, and giraffids. The similarity in suggests that the environment in the Turkana Basin in KBS-Okote time had a water deficit (30) similar to that in the Tsavo region today (mean annual temperature = 25 °C; mean annual precipitation = 550 mm per year). Δ47 values from paleosols in the Turkana region indicate soil temperatures between 30 and 37 °C, indicating high mean temperatures and an open habitat in the Koobi Fora and Nachukui regions where these fossils were found (31).

Overall, the stable isotope evidence from paleosols and tooth enamel is compatible with semiarid savanna with riparian woodlands or with woodlands associated with lakes (32). Grasses or sedges would have been highly available in such an environment.

South Africa vs. East Africa: P. robustus vs. P. boisei.

Why the difference in carbon isotope composition between Paranthropus in eastern and southern Africa? If these congeners had similar biology then it could be argued that the difference represents a generalist genus eating different things in disparate regional environments. However, most herbivorous taxa do not show evidence of diet change between the regions (compare ref. 33 with ref. 34), including Theropithecus (Table S2). Moreover, there is little reason to believe that the potential environments of P. boisei in East Africa were homogenous over time and space, or that its habitat was so different from that of P. robustus that the carbon isotope compositions of their diets did not overlap. In fact, a variety of paleoenvironments have been reconstructed for P. boisei that clearly overlap with those of P. robustus (27, 28). Most studies, however, have emphasized open and well-watered habitats for P. boisei [e.g., deltaic environments and/or edaphic grasslands (26, 27, 35)]. P. boisei and P. robustus carbon isotope values could also differ if both were principally sedge consumers and there was a differential distribution of C₃ and C₄ sedges. Indeed, there is some evidence that C₄ sedges are more common in East African habitats today (23).

Given current evidence, however, the simplest explanation is adaptive divergence between the eastern and southern African P.aranthropus populations, with the former focusing on grasses or sedges and the southern population consuming a more traditional hominoid diet that included tree fleshy fruits, as well as variable C₄ resources. In short, P. robustus had an expanded dietary repertoire relative to extant apes that included C₄ resources, whereas P. boisei had completely abandoned the presumed ancestral diet (C₃-based foods) to focus on a resource abundant in savanna and wetland environments.

Implications for Craniodental Adaptations.

These results might also have broader significance for our understanding of australoptith craniodental adaptations. Earlier taxa such as A. anamensis and A. afarensis exhibit, albeit in an incipient state, craniodental features that have been surmised to indicate a diet that consisted of hard objects (36–39) Traditional thinking has been that such masticatory adaptations permitted hominins to colonize increasingly seasonal and open environments (37). However, recent dental microwear studies suggest that the mechanical properties of A. afarensis (and A. anamensis) diets were nearly identical to those of P. boisei (9, 24, 40–42). If this is so, could it be that the australopith masticatory package represents an adaptation to C₄ resources such as grasses or sedges? The similarity in dental microwear fabrics among the eastern African australopiths, all of which lack any evidence for hard-object food consumption (9, 24, 40–42), is consistent with the notion that their craniodental morphology could reflect “repetitive loading” rather than hard-object consumption (7, 8, 43). If this is borne out, it would suggest our understanding of early hominin ecology and biomechanics needs rethinking, but must ultimately wait on stable isotope data from earlier hominins.