Find after find of hominid remains (Bonanza time for Bonzo – August 2002) undoubtedly forces physical anthropologists to reflect on what their still tiny collections of fossils might signify about the descent of humans. There are two ways of looking at that; as a “tidy” tree and one that is essentially “untidy”. The first seeks a means of connecting the earliest remains to later ones by the simplest possible connections – a touch of Occam’s Razor. However, more diversity and ever increasing ranges of ages and localities for the remains inevitably challenges this kind of palaeontological “good housekeeping”. Bernard Wood of George Washington University has long regarded evolution as untidy, and the finds of Sahelanthropus tchadensis and Orrorin tugenensis, around 6 to 7 Ma old, reinforce his trenchant views (Wood, B. 2002. Who are we? New Scientist, 26 October 2002, p. 44-47).
Because the genetic similarity between humans and their nearest relatives, chimpanzees, seems to suggest that the two clades diverged between 5 and 10 Ma ago, Sahelanthropus and Orrorin may be pretty close in age to that division. But what were they? Wood’s view is interesting, and a worry to the advocates of a parsimonious set of connections. Connectivity in proposed clades rests, for obvious reasons, on purely physical characteristics. There are many examples from the fossil record of animals whose outwardly similar characters, for example those shared by sharks and dolphins, do not signify inheritance from common ancestry. This is homoplasy, and raises the awkward possibility that special characters, regarded as essentially human, need not have arisen only the once and been carried by linear descendants. The often quoted “golden characters” of big brains and upright gait, that confer an opportunity to develop consciousness through freeing of the hands, may well have arisen more than once. The truly odd thing about Sahelanthropus is just how “modern” its face looks. Beetling brows, thick jaw and un-apelike canine teeth would put it on a sort of par with fossils of species of Homo that arose 4 to 5 million years later. Yet none of the fossils in between have this combination.; in the “tidy” scheme of things they are more “primitive”, and “therefore” cannot be our ancestors. Quite a muddle! Faces, the most sought after bits of bone, isolated in time and place could well have led many up the proverbial garden path. Why, suggests Wood, shouldn’t early hominids have been dead ends morphologically, with “primitive” characters making repeated comebacks? Why, too, shouldn’t they have been ancestral chimps, or even neither chimp nor human? The dearth of late-Miocene and Pliocene non-hominid fossils of primates leaves all this as possible. He reckons the search for “missing links” has always been a non-starter. Whatever, by expanding enormously the area of potentially fruitful ground from the narrow confines of the East African Rift, the Sahelanthropus find in Chad may yet lead to a big increase in the number of hominid and other primate fossils over which physical anthropologists can ponder.