Discussion and Conclusions

A8CDEFGHJ KL

Shungura Members

Fig. 15.7 Relative abundance of suid genera in the Shungura Formation. Note the steady decline of the Nyanzachoerus-Notochoerus lineage and the increasing dominance of Kolpochoerus and Metridiochoerus.

A8CDEFGHJ KL

Shungura Members

Fig. 15.7 Relative abundance of suid genera in the Shungura Formation. Note the steady decline of the Nyanzachoerus-Notochoerus lineage and the increasing dominance of Kolpochoerus and Metridiochoerus.

Proportion of Equus o.o ♦ »r —i— —I— —»

Fig. 15.8 Relative abundance of the genus Equus as a proportion of all specimens of the family Equidae. Although Equus becomes more abundant over time, it continues to coexist with the equid genus Eurygnathohippus during the early Pleistocene.

This contribution addresses two key questions: (1) when does Homo first appear in the Omo-Turkana Basin? and (2) what does the mammalian fauna tell us about the ecological conditions at the time of the first appearance of Homo? We have highlighted quantitatively a fact long-known to field paleontologists: namely that hominins were rare elements of Pliocene-Pleistocene faunas. The consistently collected samples from the Shungura Formation suggest that hominins made up at most 0.6% of the fossils representing large mammals on the surface of the exposures. This percentage does not translate directly into the actual relative abundance of hominins in the Pliocene and Pleistocene ecosystems of eastern Africa, but it does indicate that hominins were relatively rare mammals. Clearly, if we were to consider the entire mammalian fauna from the Plio-Pleistocene, including small mammals that are rarely found in fossil collections, the relative abundance of hominins would be significantly lower than 0.6%. Nevertheless, this estimate indicates that the first and last appearances of hominin species may have more to do with sampling issues than with actual origination or migration events. Here we have placed 95% confidence intervals on the earliest records of Homo in the Omo-Turkana Basin, and have emphasized that we need to understand not just the point in time of these earliest records, but also a span of time preceding them. In this regard, it is noteworthy that earliest Homo and lithic artifacts first appear at about the same time in both the Shungura and Nachukui Formations. Another event of significance in human evolution is the appearance of Homo erectus at about 1.9-1.8 Ma. Homo erectus shows a novel adaptive complex relative to earlier species of the genus (Wood and Collard, 1999; Wood, 2009), but it is also clear that much remains to be discovered about the paleobiology of Plio-Pleistocene Homo.

The faunal evidence reviewed here indicates that the Omo-Turkana Basin consisted of complex and diverse habitats during the late Pliocene and early Pleistocene. These included varying proportions of forests, woodlands, grasslands, and bush. However, multiple lines of evidence indicate that grasslands became more prominent in the landscape during the time interval when Homo first appeared in the fossil record (2.5-2.4 Ma). The appearance of Homo and lithic artifacts in the Omo-Turkana Basin at about 2.4 Ma, which corresponds to an increase in grassland fauna at 2.5-2.4 Ma, suggests that a mix of grasslands and woodlands provided an opportunity for Homo to migrate into the region (Figs. 15.4-15.6).

Although mosaic habitats characterized the landscapes associated with the earliest appearance of Homo, the evidence shows that from about 2 Ma, the Omo-Turkana Basin underwent pronounced faunal changes that likely reflected a significant increase in the extent and persistence of grasslands. This pulse of grassland expansion may have allowed Homo erectus to migrate to the Omo-Turkana Basin from a source area still unknown (Rightmire and Lordkipanidze, 2009). The bovids Connochaetes gentryi, Megalotragus isaaci, Beatragus antiquus, Pelorovis oldowayensis, P. turkanensis, and Tragelaphus strepsiceros are likely immigrants into the region at a time when Homo erectus also makes its first appearance (Bobe and Behrensmeyer, 2004). These bovids, with the exception of T. strepsiceros, represent a community of highly hypsodont grazers.

Both the archeological and fossil records indicate that early Homo migrated to the Omo-Turkana region at about 2.4 Ma, and that the genus likely originated elsewhere at an earlier time. The Omo fauna, which seems to track broader environmental changes rather well (Fig. 15.5), indicates that the latest Pliocene was a time of increasing aridity and environmental instability, and that grasslands were becoming more important components of the vegetation. Further faunal changes near the Pliocene/Pleistocene boundary likely reflect even greater expansion of grassland ecosystems. Homo erectus appeared in the Turkana Basin at the same time as several highly hypsodont bovids and suids that likely were exploiting these grasses. The specific adaptations of Homo species to these ecological conditions remain to be fully discerned. Nevertheless, the emergence of Homo and the spread of Homo erectus can be seen as processes within the larger context of environmental change toward increased areas of grassland and the evolution of more grassland-adapted mammals in eastern Africa in the late Pliocene and earliest Pleistocene.

Acknowledgments We would like to thank the volume editors, Fred Grine, John Fleagle and Richard Leakey for inviting us to participate in this discussion of the first humans. The editors and the two anonymous reviewers provided valuable feedback and suggestions that greatly improved the manuscript. The Turkana Basin Database has been compiled with support from the National Science Foundation (NSF BCS 0137235).

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