Bovids in the context of Hominid Origins

The oldest hominids known are Sahelanthropus tchadensis from Chad (ca.7Ma, Lebatard et al. 2008), Ardipithecus kadabba from Ethiopia (5.7Ma, WoldeGabriel et al. 2001), and Orrorin tugenensis from Kenya (ca.6Ma, Pickford and Senut 2001). Though these may be the three most popular actors on the late Miocene stage, it is on the basis of the remainder of the fossil fauna that the contextual picture (climatic, vegetational, and biogeographic) is drawn. A major impediment to increasing the resolution of the paleontological 'big picture' in the late Miocene is the limited nature of the African fossil record prior to 7Ma. Even with such limitations, Bovidae, often the most abundantly represented mammalian group in these fossil contexts, have the greatest potential to inform the biogeographic and climatic contexts of early hominid origins. For example, the absence of tragelaphines from the late Miocene and Pliocene Chadian fossil record (Geraads et al. 2001; Vignaud et al. 2002) hints at deep biomic discontinuities between the Lake Chad basin and the entire East African Rift Valley throughout this time. The abundance of Tragelaphus at the 4.4Ma Aramis hominid site provides a key indication that Ardipithecus ramidus inhabited a wooded environment (WoldeGabriel et al. 1994). Changing proportions of alcelaphines and antilopines at late Pliocene hominid sites provide a main support for the hypothesis that Homo and Paranthropus originated during a major drying trend between 2.8–2.0Ma (Vrba 1985; Bobe and Eck 2001).

The environmental and ecological dynamics responsible for the evolution and distribution of early hominid taxa are rarely to be reconstructed from the hominids themselves. That bovids are ecologically diverse and present in abundance at hominid sites means this clade provides some of the strongest evidence with which to propose and test hypotheses relating human evolution to environmental changes. To those interested in the context of early hominid evolution, these are interesting times as multiple independent fieldwork efforts, in Africa and elsewhere, target the period between 5 and 10Ma. This span of time is much better known from Europe and will soon be better understood from southern and central Asia with work currently in progress. The grand task of threading together the inter-continental picture of evolutionary, biogeographic, and climatic changes will prove to be a major objective of those working on bovid paleontology and systematics in the years to come.