FIGURE 2.1 Highly simplified summary of hominin evolution over the past 8 Mathe numerous terminating twigs schematically illustrate evolutionary dead-ends.

FIGURE 2.1 Highly simplified summary of hominin evolution over the past 8 Ma—the numerous terminating “twigs” schematically illustrate evolutionary “dead-ends.”

panzees, humans—is firmly established, the dates of these branching splits are less certain (Kumar et al., 2005). The earliest fossils of the human lineage, after the split from the common ancestor of the chimpanzees (Figure 2.1), are fragmentary and the dates of some remain imprecise. A distorted cranium from Chad, Sahelanthropus tchadensis, has a reduced snout compared with apes, and skull characteristics that are sometimes taken to indicate bipedality (Brunet et al., 2002; Zollikofer et al., 2005). The site from which this specimen comes (Koro Toro on Figure 2.2) has recently been dated to between 6.8 and 7.2 Ma (Lebatard et al., 2008), and this estimate is consistent with the faunal evidence. Other early fossils from Kenya (Orrorin tugenensis; Senut et al., 2001) consist of fragmentary jaws and limb bones with dates of 5.7 to 6.0 Ma. Although there is debate about the exact relationship between O. tugenensis and later hominins, recent anatomical analyses of the skeleton (Richmond and Jungers, 2008) indicate that this species was habitually bipedal—a uniquely hominin trait. So by 6 Ma, our earliest



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