Orrorin Sahelanthropus Ardipithecus which is the first human

Since 2000, the race to find the oldest possible human fossil has been intense. Several new finds have been announced that have pushed the records back from the Pliocene to the late Miocene. These early dates are of course within the range of molecular estimates for the split of humans from chimps (8-5 Myr ago), but they exceed the favoured estimate of 5 Myr ago that was derived from genetic analyses.

There are two ancient contenders, both dated at about 6 Myr old and both announced in rapid succession by rival teams. The first is Orrorin tugenensis, named by Senut et al. (2001) from teeth, jaw fragments and broken limb bones from sediments in Kenya dated at about 6 Myr old. The teeth are rather ape-like, the arm bones indicate some ability to brachiate,but the femora suggest that Orrorin was an upright biped. This report proved to be controversial at once, with claims that Or-rorin was not bipedal, was an ape rather than a human, or at least that the remains were inadequate to be sure (Haile-Selassie, 2001; Cela-Conde and Ayala, 2003).

Sahelanthropus from sediments of similar age in Chad was named by Brunet et al. (2002) on the basis ofa distorted, but nearly complete, cranium (Figure 11.8) and fragmentary lower jaws. The skull shows a mixture of primitive and advanced characters: the brain size, at 320-380 cm3, is comparable to that of chimpanzees, but the canine teeth are small, more like those of a human, and the prominent brow ridges are of the kind seen only in Homo. There has been some dispute about the location of the foramen magnum, whether it lies below the skull (indicating bipedality) or towards the back (ape-like quadrupedality). Sahelanthropus has gen-

Fig. 11.8 The near-complete skull ofSahelanthropus,possibly the oldest human ancestor, from the upper Miocene of Chad. (Photograph courtesy of Michel Brunet.)

erally been accepted, however, as a basal hominid (Cela-Conde and Ayala, 2003), perhaps the closest we will find to the common ancestor of chimps and humans.

Less controversial is Ardipithecus ramidus from Ethiopia, dating from 4.4Myr ago (White et al., 1994) and an older species, A. kadabba,from 5.8 to 5.2 Myr ago (Haile-Selassie, 2001; Haile-Selassie et al., 2004). The younger material consists of an associated set of upper and lower teeth, bones from the forehead and lower region of two skulls, an associated humerus, radius and ulna, and other isolated teeth and bones. Ardipithecus retains relatively large canine teeth, narrow molars, thin enamel and other primitive features, but these teeth are more hominine than in any of the great apes (Figure 11.9(a)). They indicate a diet mainly of fruit and leaves. In addition, Ardipithecus has a forwardly-placed foramen magnum, cited as proof that it was a biped.

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