Palaeoanthropology: Facing up to complexity.

Abstract

The task of palaeoanthropologists is to reconstruct the evolutionary history of the period between our species, Homo sapiens, and the ancestral species we share exclusively with chimpanzees and bonobos. There must have been a ladder-like sequence of species connecting us with that common ancestor; but it is unclear whether our section of the ‘tree of life’ is restricted to this ancestor– descendant sequence, or whether it includes other, now extinct, lineages. Might there have been multiple lineages early in the history of our own genus, Homo? In this issue, Meave Leakey et al. (page 201) describe fossils recovered from Koobi Fora in northern Kenya that provide compelling evidence for at least two Homo lineages as early as 2 million years ago. For the first half of the last century, conventional wisdom was that the most primitive member of the genus Homo was Homo erectus, best known from fossils found at sites in China and southeast Asia. These creatures had small heads with prominent brows, and what little is known of their limbs suggests that they stood upright and walked on two feet in much the same way as we do. Just over 50 years ago, a series of discoveries at Olduvai Gorge in Tanzania prompted palaeontologist Louis Leakey and his colleagues to add an even more primitive species at the base of the genus Homo (Fig. 1), which they called Homo habilis to reflect the assumption that members of the species had fashioned the simple stone tools found in the same sediment layers at Olduvai. The brain of H. habilis was smaller than that of H. erectus, and the rest of its skeleton was more ape-like. Researchers at Koobi Fora subsequently found further evidence of H. habilis, but they also found other early Homo cranial fossils that did not so obviously resemble those from the Olduvai site. In the late 1970s, I was given the task of trying to make sense of the collection of early Homo fossils from Koobi Fora, which included braincases, faces, lower jaws and teeth. Although a strong case had been made that the variation among the H. habilis fossils from Olduvai could be subsumed within a single species, the question remained: did the two forms of early Homo found at Koobi Fora come from males and females of a single species, or did they represent two species? It was not easy to accommodate the variation among the Koobi Fora early Homo fossils within one species. There were two distinct facial morphologies, and although the larger faces could have belonged to males, and the smaller ones to females, of the same species, the differences were unlike those we see between the males and females of modern humans and the living apes. This discrepancy led researchers to recognize a second early Homo species, Homo rudolfensis. However, the existence of a second early Homo species remained controversial, as there was no single H. rudolfensis fossil specimen that contained both the face and the lower jaw. It was also unfortunate that, although the type specimen (the specimen that is linked with that species name) of H. habilis (called OH 7) had a lower jaw that contained teeth, it lacked a face, whereas the type specimen of H. rudolfensis (KNM-ER 1470) had a face, but neither tooth crowns nor a lower jaw. I made the assumption that the big-boned face of H. rudolfensis would be matched by a robust jaw and large chewing teeth, and ascribed lower jaws with these attributes (such as KNM-ER 1802 from Koobi Fora) to H. rudolfensis. Leakey and colleagues’ three new specimens test these taxonomic hypotheses. The fossils are a well-preserved face (KNMER 62000), a well-preserved lower jaw (KNM-ER 60000) and a fragmentary lower jaw (KNM-ER 62003). None of the three specimens is as old as KNM-ER 1470, which is approximately 2 million years (Myr) old; the face and the fragmentary lower jaw are between 1.95 and 1.9 Myr old, and the betterpreserved lower jaw is younger still, at around 1.83 Myr old. In a nutshell, the anatomy of the specimens supports the hypothesis of multiple early Homo species, but refutes the hypothesis that lower jaws like KNM-ER 1802 went with the type of face — KNM-ER 1470 — that belongs to H. rudolfensis. The KNM-ER 62000 face, which belongs to a late-juvenile individual, is smaller than that of KNM-ER 1470, yet it has the same shape — in both, the cheek bones join the palate quite far forward. Unlike KNM-ER 1470, the KNMER 62000 face does contain a few teeth, and the authors describe its chewing teeth as moderately sized and mesiodistally long (referring to the size and shape of the tooth crowns). Its palate is distinctively short, and the first of the two premolar teeth is at the corner of the almost right angle that is formed between the flat anterior tooth row and the straight, short, posterior tooth row (see Fig. 3a of the paper). By contrast, in H. habilis the two tooth rows are more curved, and it is the canine that is at their more rounded intersection. The new lower jaws are a much better match to the distinctively shaped KNM-ER 1470 and KNM-ER 62000 palates than are mandibles such as KNM-ER 1802. So where do we go from here? More work needs to be done using the faces and lower jaws of modern humans and great apes to check how different the shapes of the palate and lower jaw can be among individuals in living species. We also need to find a way to formally estimate the likelihood that the OH 7 lower jaw came from the same species as either KNM-ER 60000 or KNM-ER 1802. If the latter can be accommodated within H. habilis, then all well and good, but if not (which I think is more likely), then could KNM-ER 1802 and its ilk represent a third species? Finally, some researchers have suggested that evidence from the face and jaws of H. habilis and H. rudolfensis, plus what little fossil evidence we have of these species’ other body regions, stretches the definition of the genus Homo too far. Perhaps these two taxa belonged to a different lineage from that from which H. sapiens arose? My prediction is that by 2064, 100 years after Leakey and colleagues’ description of H. habilis, researchers will view our current hypotheses about this phase of human evolution as remarkably simplistic. ■ 3 2 1 0 H. sapiens

DOI: 10.1038/488162a

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Cite this paper

@article{Wood2012PalaeoanthropologyFU, title={Palaeoanthropology: Facing up to complexity.}, author={Bernard A Wood}, journal={Nature}, year={2012}, volume={488 7410}, pages={162-3} }