Our understanding of the origin of the genus Homo has been hampered by a limited fossil record in eastern Africa between 2.0 and 3.0 million years ago (Ma). Here we report the discovery of a partial hominin mandible with teeth from the Ledi-Geraru research area, Afar Regional State, Ethiopia, that establishes the presence of Homo at 2.80 to 2.75 Ma. This specimen combines primitive traits seen in early Australopithecus with derived morphology observed in later Homo, confirming that dentognathic departures from the australopith pattern occurred early in the Homo lineage. The Ledi-Geraru discovery has implications for hypotheses about the timing and place of origin of the genus Homo.

Chewing efficiency has been associated with fitness in mammals, yet little is known about the behavioral, ecological, and morphological factors that influence chewing efficiency in wild animals. Although research has established that dental wear and food material properties independently affect chewing efficiency, few studies have addressed the interaction among these factors. We examined chewing efficiency, measured as mean fecal particle size, as a function of seasonal shifts in diet (and corresponding changes in food fracture toughness) in a single breeding population of a grazing primate, the gelada monkey, at Guassa, Ethiopia. We also measured dental topographic traits (slope, angularity, and relief index) and relative two- and three-dimensional shearing crest lengths in a cross-sectional wear series of gelada molars. Chewing efficiency decreased during the dry season, a pattern corresponding to the consumption of foods with higher fracture toughness. Older individuals experienced the most pronounced decreases in chewing efficiency between seasons, implicating dental wear as a causal factor. This pattern is consistent with our finding that dental topographic metrics and three-dimensional relative shearing crest lengths were lowest at the last stage of wear. Integrating these lines of behavioral, ecological, and morphological evidence provides some of the first empirical support for the hypothesis that food fracture toughness and dental wear together contribute to chewing efficiency. Geladas have the highest chewing efficiencies measured thus far in primates, and may be analogous to equids in their emphasis on dental design as a means of particle size reduction in the absence of highly specialized digestive physiology.

The rise and spread of tropical grasslands was a signal event in the Cenozoic, causing many ungulates to evolve adaptations to a diet of graminoid tissues, or graminivory. In parallel, a lineage of monkeys (Theropithecus) is distinguished among primates for its large size and commitment to graminivory, a trait expressed by species throughout the Plio-Pleistocene fossil record and T. gelada, the sole surviving species today. An open question concerns the mechanics of how species handled graminoid tissues. They might have exhibited preference, selecting tissues within a tuft, or they might have practiced indiscriminate bulk-feeding in a manner similar to large grazing ungulates.. Variation in this behavior explains a significant amount of variation in body mass through time, and we describe these covarying traits, which peaked during the Pleistocene, as evolutionary traps. To support this characterization, we report evidence of temporal increases in strontium isotope variability among North African theropiths, a result that suggests greater lifetime travel and energetic costs in response to diminishing food resources, a probable factor in the extinction of T. oswaldi, the largest monkey that ever lived.

Hawks et al. argue that our analysis of Australopithecus sediba mandibles is flawed and that specimen LD 350-1 cannot be distinguished from this, or any other, Australopithecus species. Our reexamination of the evidence confirms that LD 350-1 falls outside of the pattern that A. sediba shares with Australopithecus and thus is reasonably assigned to the genus Homo.H awks et al.(1) claim that we misinterpreted the mandibular anatomy of the Malapa hominins and, as a result, failed to satisfactorily distinguish the LD 350-1 jaw from Australopithecus. We stand by our assessments of the mandibular and dental anatomy of A. sediba. On the right side of the MH2 mandibular corpus, the inferior border is damaged posterior to mid-M 2 , but any reasonable estimate of "minimum corpus depth" [as defined in (2); Hawks et al. do not specify their method] below M 2 is 2.5 to 3.5 mm less than the depth below P 3 . The left side of the MH2 corpus [specimen UW88-55 (3)] is also preserved, however, and is undamaged from the M 1 level to posterior to M 3 . The inferior margin of the damaged right corpus (4) can be inferred by photographically superimposing the left side onto the right (Fig. 1), which reveals the primitive (australopith-like) height relationships of the corpus, deeper anteriorly and considerably less so posteriorly. On a cast of MH2, we measure minimum lingual corpus height at left M 2 as 26.6 mm and at right P 3 as 30.3 mm.With regard to the juvenile MH1 mandible (M 3 unerupted), our data on chimpanzees and A. afarensis show that, although corpus depth does indeed change with emergence of the final molar, the relative anterior to posterior depth (P 3 :M 2 ) is essentially stable from the time of M 2 emergence. Therefore, in MH1, the clear discrepancy in corpus depth [see figure 1 in (3)] is unlikely to have changed substantially with the attainment of adulthood, confirming our assessment of a posteriorly shallowing corpus in the Malapa hominins, as commonly seen in Australopithecus.Hawks et al. argue that the orientation of the mental foramen in MH2 is lateral, rather than anterior, as we described. Without prejudicing our assessment of MH2, we note that a lateral orientation of the foramen fits the description of A. africanus [as noted in (2)], whereas a clear posterior opening of the foramen into a short groove on the corpus, as in LD 350-1, is typically seen only in Homo among nonrobust early hominins. With regard to the root of the ascending ramus, our definition [as noted in the legend of figure S7 in (2)] is the point at which the anterior ramus margin becomes independent of the corpus, which on MH2 clearly occurs at M 2 /M 3 , as we described (Fig. 1). It is the concave vertical segment of the anterior margin that leaves the M 3 visible in lateral view, not an extreme posterior origin of the margin as in LD 350-1.The flaring buccal face of the molars in MH1, the standard condition in Australopithecus, may or not have been present in MH2. Heavy occlusal wear, to the point of substantially reducing crown height a...

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