THERE is a popular image of human evolution that you’ll find all over the place, from the backs of cereal packets to advertisements for expensive scientific equipment. On the left of the picture there’s an ape – stocky, jutting jaw, hunched in the knuckle-walking position. On the right, a man – graceful, high forehead, striding purposefully into the future. Between the two is a succession of figures that become ever more like humans, as the shoulders start to pull back, the torso slims down, the arms retract,the legs extend, the cranium expands and the chin recedes. Our progress from ape to human looks so smooth, so tidy. It’s such a beguiling image that even the experts are loath to let it go. But it is an illusion.
Cut to another picture of human evolution. This July, a cracked and twisted face appeared on the front pages of most of the world’s major newspapers. The skull, unearthed in the Djurab Desert in Chad, central Africa, is dated at between 6 and 7 million years old. At the time it was hailed as our oldest ancestor. In recent weeks, that claim has been bitterly disputed. Academic reputations may be at stake, but behind all the ballyhoo, the true significance of this find is emerging. It is forcing us to rethink the idea of human evolution as a smooth progression without blind alleys or dead ends. It can’t possibly be so tidy, as within this framework the Chad fossil makes no sense: this truly ancient specimen has the brain case of a chimp together with a face that looks uncannily like our ancestors living less than a million years ago.
My own research on fossils from East Africa convinced me long ago that our evolutionary history is much more complex than we’d like to think. Other palaeoanthropologists see a single line of descent that you can follow like the trunk of a tree from its apex back down to the roots, but I have long argued that our ancestry is more like a bush with multiple tangled stems. Try following our own lineage back to its origins and you soon get lost in the thicket. The Chad fossil is most likely among the tangled stems at the base of the bush. But is it our oldest known ancestor – the “missing link” between humans and chimps? Indeed, is it one of us at all? If the new find has taught us anything it is that, paradoxically, the more we discover about our origins, the less we know.
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One of the many things molecular biology has done is confirm that humans are merely a rather peculiar African ape. Geneticists estimate that our common ancestor lived between 4 and 10 million years ago, based on the rate at which genetic mutations occur and the measurement that we differ from chimps by 1 to 2 per cent of our DNA. With the new finds from Chad – and other 6-million-year-old remains discovered at a Kenyan Rift Valley site called Lukeino (see New Scientist, 16 December 2000, p 5) – it looks increasingly likely that the split happened earlier rather than later. So some time between say 7 and 10 million years ago genetic variation within the population of our common ancestor polarised into two lineages, or “clades”: one that includes us, and the other containing chimps and bonobos.
Of course, we cannot look at the genes of our earliest ancestors. Instead, we use fossil anatomy as a proxy for the genome, and treat morphological similarities as if they were genetic ones. A competent anatomist will be able to tell the difference between most components of the modern human and chimp skeleton, but we palaeoanthropologists face a much tougher and more subtle task. We must trace these skeletal differences back in time, and work out what they looked like when the chimp and human lineages diverged. This is made more difficult because we have little idea of when the structures and behaviours peculiar to chimps arose. Although chimps have had their own independent evolutionary history going back as far as ours, we have absolutely no fossil record of that evolution.
Nevertheless, the tidy interpretation of the human fossil record suggests that the “gap” between modern humans and our common ancestor with chimps has been filled. In this model you can trace the origins of our own genus, Homo, back in time through Homo erectus to Homo habilis and then to Australopithecus afarensis. The gap to the common ancestor is bridged by Australopithecus anamensis and finally Ardipithecus ramidus. Just one blip spoils the conventional ladder-like succession: the “robust” australopiths, or Paranthropus, who are generally interpreted as an extinct side branch of human evolution. And the place of one other fossil species, Australopithecus africanus from southern Africa, is uncertain.
The tidy model also purports to give clues about the detailed differences between the ancestral humans (hominins) and ancestral chimps (panins) close to the time when they split. For example, studies of teeth from Ar. ramidus suggest that one of the earliest changes was in the size, shape and wear of the canines. Another assumption is that early panins were adapted for life in the trees, holding their torsos horizontally as they walked on all fours, whereas the original hominin was probably a “facultative” biped, who retained some tree-climbing abilities but was partially adapted for walking on two legs. A. afarensis is a facultative biped, A. anamensis is likely to be one also, but we don’t yet know enough about the skeleton of Ar. ramidus to tell how it moved. The creatures down near the base of the human tree might also have had a slightly larger brain than ancestral chimps.
Not long ago most researchers were quite sure that these so-called “golden characters” would help us sort out the hominins from panins. But the fuller the human fossil record becomes, the shakier is the assumption that human characteristics such as manual dexterity, bipedalism and large brains are so special that they evolved only once. If this was the case, then the branching pattern, or cladogram, showing the relationship between fossil specimens should be clear and unambiguous. What’s more, any piece of anatomy, or “character”, we use to reconstruct a cladogram should show the same branching pattern as any other. But the reality is very different: character cladograms conflict. Clearly, species with shared morphology do not always inherit it from a common ancestor. The technical term for this is homoplasy.
These days, most palaeoanthropologists agree that homoplasy occurs in the later stages of hominin evolution – and also in the evolution of extinct apes – so why not at the beginning of human evolution? In addition, there is no reason why rudimentary upright walking and the ability to make crude tools could not have evolved in a creature that was genetically closer to a chimp than to a modern human. This scenario becomes even more complicated if we entertain the possibility that around 6 or 7 million years ago there may have been apes that were neither ancestral chimps nor ancestral humans. Personally, I think that this was most likely the case. If so, the haystack gets larger, and the odds of finding and correctly identifying the ancestral human needle become even longer.
This is the context in which the Chad discoveries should be set. The fossil remains of Sahelanthropus tchadensis – the ape’s correct scientific name – which were unearthed by Michel Brunet from the University of Poitiers and his colleagues, include a lower jaw and teeth as well as the cranium containing the face. The jaw is thicker than a chimp’s, and the canines show the first signs of moving away from an ape-like design – evidence consistent with its being an ape at, or near, the base of the human clade. However, what most definitely clashes with its 6 or 7-million-year-old date is the beetle-browed upper face. We are used to seeing brow ridges on the fossils of much later Homo species, none of which is older than 2 million years. And prominent brow ridges like the Chad ape’s tend to come much later.
If we use anatomy as a proxy for genetic relatedness, then the Chad face connects it with hominins dated at less than a million years old. That clearly can’t be right. Although the Chad fossils were not dated by isotope methods – there are no nearby volcanoes to provide the necessary ash for carbon dating – fossil animals found at the site match others from East Africa that date to between 6 and7 million years old. There is no way this ape is anywhere near as recent as the Homo specimens it resembles. More ridiculous still, under the terms of the tidy model where everything evolves just once, the Chad specimen’s ancient date and modern-looking brow ridge means that any later fossil hominins with more primitive faces can’t possibly be our ancestors. That includes the famous “Lucy” skeleton and even species we include in our own genus.
As a supporter of the untidy model of human evolution, I interpret the Chad face very differently. The face of a skull is one of the skeleton parts most likely to be affected by homoplasy. Despite their physical similarities, the faces of the Chad ape and those of later hominins were not necessarily inherited from a common ancestor. Instead, they may simply have been shaped by similar social or dietary demands. If so, then the Chad remains don’t fit neatly into a smooth progression of human evolution. But my interpretation is surely less radical than throwing out 4 to 5 million years’ worth of the hominin fossil record.
So, here is evidence for homoplasy right down at the base of the human evolutionary tree. The Chad find strengthens my conviction that our evolutionary history is far more complicated that many experts are willing to admit. Accepting this is liberating. It allows us to look afresh at the human fossil record and reclassify some of the oddball specimens that most experts have previously attempted to shoehorn into the tidy family tree. The resulting bush may not appeal to our innate wish to impose order on the world, but it is a more faithful representation of the evidence.
And the implications of the Chad finds stretch even further. I believe these fossils also suggest that the earliest direct ancestors of modern humans and chimps were just two components of a diverse great ape fauna. This evolutionary bonanza, known as an “adaptive radiation”, was most probably a response to the global cooling that was causing the contraction of African tropical forests and the expansion of woodland environments around 8 million years ago. Here, the location of the Chad fossil remains is significant.
Today, the site at Toros-Menalla in the Djurab Desert is little more than sand as far as the eye can see, but 6 or 7 million years ago it was forested, game was abundant and fish were plentiful in nearby lakes. This also reminds us that when the first human ancestors were beginning to appear, apes and ape-friendly environments were almost certainly much more widespread than they are today. More tellingly still, the Djurab Desert is over 1500 kilometres west of the East African Rift Valley, long touted as the home of the original hominins. Some experts have already questioned the conventional wisdom that hominin evolution was somehow triggered by the appearance of savannah grasslands in and around the Rift Valley. The new finds confirm that this scenario must be thrown out.
So where does the Chad ape fit into this bigger, more complex picture? It may well be a creature very close to the base of the human bush, but it is impossible to tell whether it is a direct ancestor of modern humans. Indeed, if we accept that we evolved at a time when many new species of ape were emerging under the pressures of environmental change, then identifying the fragmentary remains of one of these apes as the hominin ancestor is likely to be a tall order. Facultative bipedalism, increased dexterity, and even a bigger brain, might have occurred in more than one member of an adaptive radiation. In fact, we should expect to see novel mixtures of familiar adaptations, and even novel adaptations, in a 7 to 10-million-year-old radiation of African apes.
We may never get a clear picture of the thicket of stems at the base of our evolutionary bush. Certainly, the search for the “missing link” is doomed to failure. But we can increase our understanding of human evolution by recovering new evidence from known sites, and from new sites in hitherto unexplored regions in central and West Africa. We must also search for the remains of extinct panin, which will provide a unique perspective on our own evolution.
