Sahelanthropus : introduction

In 2002, French paleoanthropologist Michel Brunet and his team announced the discovery of a fossil skull and unassociated jaws and teeth from Chad, in North-Central Africa, which may be the oldest hominid yet found (Brunet 2002). At around 7 million years, the remains from Toros-Menalla are not only older than any East African hominid fossil, they are twice the age of any other hominids known from outside East Africa. The team called the discovery Sahelanthropus tchadensis, a name that emphasizes the hypothesis that the remains represent the earliest known hominids.

Several features of the skull are similar to early hominids. The canine teeth of the skull and mandibles are relatively small, and may have wear on their tips that indicates that they were not used as cutting teeth, as they are in apes. Also, the foramen magnum is placed more anteriorly than in most great ape skulls, which may reflect a habitual upright posture with the spine positioned below the head. Facial characteristics, including a thick supraorbital torus and a relatively vertical face, especially below the nose, are similar to later hominids and are not found in living apes. And the teeth have relatively thickened enamel, more similar to early hominids than to living chimpanzees and gorillas.

Some scientists do not accept these features as strong evidence that Sahelanthropus is a hominid. Pointing out that small canines are sometimes observed in Miocene apes, especially female remains, Milford Wolpoff and colleagues (Wolpoff et al. 2002) argue that this and other features provide no evidence for a close relationship with hominids. The foramen magnum position of the specimen is not outside the range of chimpanzee variation, while the long and flat nuchal plane is inclined to a degree seen only in gorillas. And while the facial features somewhat resemble much later hominids, they do not resemble other early hominids, so they apparently are parallel developments rather than homologies, and thus do not show a close relationship between Sahelanthropus and early hominids.

Of course, even if the Toros-Menalla remains are not hominids, they are immensely important nonetheless. At around 7 million years , they are the only known ape fossils from the critical time period when hominids, chimpanzees, and gorillas all began to differentiate from each other. If they are not hominids themselves, they may well represent the gorilla lineage, a line with no fossil record at all. Even if they belong to an extinct Miocene ape lineage, the remains provide essential evidence for Late Miocene ape diversity in Africa, and may be the nearest fossil relative to the chimpanzee-human-gorilla common ancestor. The fossils therefore potentially tell us much about the ancestral condition for hominids, and help to clarify the confusion caused by the contrast between the chimpanzee-like Aramis dental remains and the dental morphology preserved at other early hominid sites, discussed in this chapter. And perhaps most importantly, the fossils focus attention on a broader range of geographic locations and ecological conditions within Africa as possible contexts of early hominid evolution. Like much later remains from Chad, these fossils remind paleoanthropologists that East Africa is not the only place where early hominid evolution occurred, and may not even be an especially important place, despite the great preservation of hominid fossils in this area.