Chapter 9 - Early Hominins

Abstract

Three genera of possible human ancestors (Sahelanthropus, Orrorin, and Ardipithecus) have been dated to 7–4.4 mya. Where the same skeletal elements have been recovered the morphology of the three genera is similar. When compared to chimpanzees and bonobos all the three exhibit reduced incisors, smaller canines, and less sectorial lower first premolars. A single fragmentary pelvis displays a mixture of chimpanzee- and hominin-like features. Interpretation of these fossils is confounded by the many australopith-like features in Ouranopithecus, the purported common ancestor of African apes and humans. Ouranopithecus shares with Australopiths (but not these three possible hominins) a robust, prognathic face, tall zygomatics, large incisors, large cheek teeth, and thick molar enamel. Ouranopithecus shares with australopiths and the three possible hominins small canines and a shallow supratoral sulcus. The robusticity of Ouranopithecus makes it less likely that Sahelanthropus, Orrorin, and Ardipithecus are in the hominin lineage, since such a lineage would require an evolutionary progression that starts robust, becomes nonrobust, and then reacquires robusticity. Some paleontologists, however, are skeptical of the place of Ouranopithecus in the human lineage. They find compelling indirect evidence that the common ancestor of African apes and humans was quite chimpanzee-like, which would exclude robust species such as Ouranopithecus from the human lineage. If the common ancestor was chimpanzee-like, then the argument that Sahelanthropus, Orrorin, and Ardipithecus are part of the human ancestry is stronger.