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Awash study area, Ethiopia. The main differences between Ar. kadabba and Ar. ramidus are that the apical crests of the upper canine crown of the former taxon are longer and the P3 crown outline of Ar. kadabba is more asymmetrical than that of Ar. ramidus. Haile-Selassie et al. (2004) suggest that there is a morphocline in upper canine morphology, with Ar. kadabba exhibiting the most ape-like morphology and Ar. ramidus and Au. afarensis interpreted as becoming progressively more like the lower and more asymmetric crowns of later hominins. The proximal foot phalanx (AME-VP-1/71) combines an ape-like curvature with a proximal joint surface that is like that of Au. afarensis (Haile-Selassie, 2001). These four taxa could be primitive hominins, but they could also belong to separate clades of apes that share homoplasies with the human clade.

CHALLENGES

Differences Between an Early-Hominin Taxon and a Taxon in a Closely Related Clade

The differences between the skeletons of living modern humans and their closest living relatives, common chimpanzees and bonobos, are particularly marked in the brain case, dentition, face and base of the cranium, and in the hand, pelvis, knee, and the foot. But the differences between the first, or stem, hominins and the first, or stem, panins were likely to have been much more subtle. In what ways would the earliest hominins have differed from the last common ancestor (LCA) of chimps/bonobos and modern humans, and from the earliest panins? Compared with panins they would most likely have had smaller canine teeth, larger chewing teeth, and thicker lower jaws. There would also have been some changes in the skull, axial skeleton, and the limbs linked with more time spent upright and with a greater dependence on the hind limbs for bipedal locomotion. These changes would have included, among other things, a forward shift in the foramen magnum, adjustments to the pelvis, habitually more extended knees, and a more stable foot.

But all this assumes there is no homoplasy (see below) and that the only options for a 8–5-Ma African higher primate are being the LCA of modern humans and chimps/bonobos, a primitive hominin, or a primitive panin. It is, however, also possible that such a creature may belong to an extinct clade (e.g., a sister taxon of the LCA of modern humans and chimps/bonobos, or the sister taxon of the earliest hominins or panins).



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