A Remarkable New Fossil Hominid from Chad, Central Africa
The distinction of being designated “the earliest hominid” is one that draws worldwide media coverage, but it is a notoriously fickle kind of fame. In the early 1990s, the earliest hominid was dated between 3 and 4 million years old. By 1998, based on some Ethiopian fossils, this had been pushed back to 4.4 million years. In 2000 (based on Kenyan material), the date was further extended to close to 6 million years, and in 2002 a new and startling find from Chad has suggested the current bearer of the “earliest” title is 7 million years old. This brief history is not intended to force unsuspecting students to memorize exactly what was found when. Our point is to illustrate how rapidly discoveries have taken place and that, as a result, the known range of hominid existence has in just in the last ten years almost been doubled! The most stunning of these new finds is also the oldest. A nearly complete cranium was discovered in 2001 at the Toros-Menalla site in northern Chad (Brunet et al., 2002). Provisionally dating using faunal correlation suggests a date of nearly 7 m.y.a. (Vignaud et al., 2002). This very early suggested age of the fossil has proved a major surprise, dating almost one million years earlier than any of the other proposed early hominids (and close to three million years older than the oldest well-established hominid discoveries). The location of the find was also quite unexpected, as it is a full 1500 miles west of the well-known and extremely productive areas in East Africa -- long thought to be the cradle of human origins. The morphology of the fossil is unusual with a combination of characteristics unlike that found in other early hominids. The braincase is small, estimated at no larger than a modern chimpanzee (preliminary estimate in the range 320-380 cm3), but is massively built -- with huge browridges in front, a crest on top, and large muscle attachments in the rear (see photos). Yet combined with these ape-like features, is a smallish vertical face containing front teeth very unlike apes. In fact, the lower face, being more tucked in under the brain vault (and not protruding as in most other early hominids) is more of a derived feature more commonly expressed in much later hominids (especially members of genus Homo). Moreover, also unlike apes (and some early hominids), the upper canine is reduced and is worn down from the tip (rather than shearing along its side against the first lower premolar). Other dental remains have also been found at the site, represented by two lower jaw fragments and three isolated teeth. Along with the dentition found in the nearly complete cranium, these teeth further indicate a non-ape-like pattern of wear of the canines and only moderate thickness of the molar enamel caps (unlike the more derived thick molar caps found in many later hominids). In recognition of this unique combination of characteristics, the lead researcher, Michael Brunet (of the University of Poitiers in France) has placed the Toros-Menalla remains into a new genus and species of hominid, Sahelanthropus tchadensis (Sahel being the region of the southern Sahara in North Africa). These new finds from Chad are very exciting and have forced immediate and significant reassessment of early hominid evolution. Two cautionary comments, however, are in order. First, the dating is only approximate, based, as it is, on biostratigraphic correlation with sites in Kenya (1500 miles to the east). The faunal sequences, nevertheless, seem to be clearly bracketed by two very well-dated sequences in Kenya. Second, and perhaps more serious, is the hominid status of the Chad fossils. Given the facial structure and dentition, it is difficult to see how Sahelanthropus could be anything but a hominid. Still, as we discussed above, the best defining anatomical characteristic of hominids relate to bipedal locomotion. Unfortunately, of the available evidence, there are no postcranial elements preserved. Consequently, at present, we do not know the locomotory behavior of Sahelanthropus -- and this raises even more fundamental questions: What if further finds show this form not to be bipedal? Should we still consider it a hominid? What, then, are the defining characteristics of our family?
Interpretations of the Earliest Hominid Finds With so many new discoveries, it is a very exciting time in paleoanthropology. It is also a time of considerable confusion, as we try to come to grips with the complex nature of hominid evolution. In fact, it may prove impossible to work out clear patterns of relationships among the early hominids. Despite our best attempts, the number and complexity of hominid groups (more technically called taxa; which seem to increase monthly) may frustrate all such attempts at simplification. One particularly difficult obstacle is the varied combination of anatomical characteristics, seen especially in the earliest suggested members of the hominid family. For example, Sahelanthropus has a very primitive looking braincase (especially in the back) combined with a fairly advanced-hominid-looking face and canine teeth. Similarly, Orrorin combines what some claim are very chimpanzee-looking teeth with a highly efficient bipedal gait. Lastly, Ardipithecus combines some aspects of primitive-looking teeth with other components of a suggested bipedal gait. Still, to many of those familiar with the Ardipithecus finds, the manner of locomotion is likely to have been very different from any other hominid (suggesting it might have been a very derived hominid -- and thus an unlikely ancestor of later forms). Beyond all these admittedly confusing combinations of characteristics is the suspicion by some researchers (e.g., Bernard Wood, 2002) that roughly similar characteristics (like some form of bipedality or reduction of the canine teeth) could have evolved more than once -- that is separately in different lineages of hominids (or possibly even in other hominoids closely related to hominids). This evolutionary process results from what we have called homoplasy. If, indeed, this inherently evolutionary messy factor was widespread among our late Miocene and Pliocene relatives, it may prove almost impossible to discern which forms are related to others and which ones are more closely related to us. Worse yet, it may prove extremely tricky to be able even to identify some of these forms as hominids at all. * Table. Key Very Early Fossil Hominid Discoveries (prior to 4 m.y.a.)
References Brunet, Michel,
Frank Guy, David Pilbeam, et al. Vignaud, Patrick,
Phillipe Duringer, Hasssabe Taisso Mackaye, et al. Wood, Bernard Also See: Wolpoff, Milford
H., Bridgette Senut, Martin Pickord, and John Hawks
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