Sahelanthropus or 'Sahelpithecus'?

Abstract

Researchers have long hoped to discover a hominid (humanlike) ancestor from the late Miocene (5-10 million years ago). Fossil remains of a Miocene primate designated Sahelanthropus tchadensis were recently identified that exhibit an intriguing mix of early human (Homo erectus) and apelike characteristics. Although some researchers are hailing the new species as the earliest hominid known, others are far more skeptical. The canine teeth are small, as in humans, yet the breadth of canines is more similar to that of the chimpanzee and gorilla. Although the post-canine teeth are larger than those of chimpanzees, they are similar to those of known Miocene apes. The specimen has a large supraorbital torus (the bone structure above the eye socket), a feature found in Homo species. However, because this feature is typical of the later Homo erectus, it does not provide much phylogenetic (evolutionary progression) information. The intermediate thickness of the specimen’s post-canine tooth enamel is presented as evidence of hominid ancestry, although, this adaptation may reflect a dietrequiring powerful mastication rather than an evolutionary advance. The foramen magnum (the point at which the spine is joined to the skull) is assigned an anterior position by the researchers, who believe the species is a hominid ancestor because this orientation would indicate a more upright posture. The anterior edge of the foramen, however, is much farther from the back of S. tchadensis’ third molar than in known hominids, but similar in placement to that of chimpanzees and gorillas. Many other features of S. tchadensis seem more akin to chimpanzees and gorillas than to hominids, especially the long, flat nuchal (backof the neck) plane and the angle of its orientation.