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That really should not be surprising. Most types of animals monkeys, whales, cats, apes come in multiple varieties. As recently as 30,000 or 40,000 years ago, when Homo sapiens was sufficiently evolved to make jewelry and paint hauntingly evocative drawings on cave walls, we shared the planet with a second hominid species, the Neanderthals. And although it seems natural to us that only one species of hominid lives today, it is in fact an exception to nature's way of doing things.
But if multiple hominid species have been the rule all along, there is no reason to think this wasn't the case right from the beginning. Evolution provides plenty of examples in which new types of animals emerge not just as single species but as collections of similar species that share many but not all physical attributes. The rich diversity of finches that Charles Darwin discovered in the Galapagos Islands is perhaps the most famous example.
According to many anthropologists, Brunet's discovery supports the idea that evolutionary diversity was true for hominids as well. "My guess," Wood offers, "is that Sahelanthropus is the first of what will turn out to be a whole handful of apes and apelike creatures living throughout Africa 6 or 7 million years ago." In this bushy model of evolution, even a remarkably modern face might not guarantee that Brunet's new hominid was a direct ancestor of modern humans. Maybe it was just one of several modern-looking hominids that arose at about the same time.
Or maybe all these learned experts are barking up the wrong evolutionary tree after all. At least one equally eminent paleontologist, Tim White of the University of California, Berkeley, disputes the assertion that Toumai derails the standard evolutionary family tree, let alone plants a bush in its place. The discovery is a tremendous accomplishment, he says. "This fossil is the closest we've got to the common ancestor. But dentally, it's just like Ardipithecus, except for a few minor characteristics." The mix of primitive and more advanced traits leaves him similarly unimpressed, since such mixing has been seen in various species discovered over the past 80 years. In spite of what his colleagues say, White believes that Toumai may well be the direct ancestor of all later hominids, Lucy and ourselves included.
If the new hominid does eventually upend the conventional wisdom, however, it will raise all sorts of questions. For example, if Sahelanthropus had descendant species that gave rise to H. habilis, asks Harvard's Lieberman, where are they? Nobody knows, moreover, what triggered the emergence of the earliest hominids in the first place. Virtually everyone now agrees that walking upright was the key physical adaptation that set the hominid line in motion. But that adaptation had to have some evolutionary advantage for it to persist. What, exactly, was so great about walking on two legs?
A decade ago, the leading theory suggested that climate change had dried Africa out, replacing the forests, where apes thrived, with grasslands. A walking ape would be better suited to this environment, since tree climbing would be useless. Standing would give a better view over the top of the grasses of potential enemies. Also, a vertical position would offer less exposure to the harsh rays of the sun.
That all made sense until field scientists, including White, began finding early hominids who lived in partly wooded places, not pure savanna. As deduced from the sorts of animals Brunet found at Toros-Menalla, that seems to be the kind of environment Sahelanthropus inhabited as well. With the simple climate theory already on the way out, paleontologists have come up with other ideas. They now believe that woods survived in the changed climate but were probably interspersed with patches of savanna precisely the setting in which Brunet found Toumai.
An ape walking on two legs could traverse these open expanses, much as the earlier theory contended, to get to a safe and comfortable habitat in the next forest over. With its free hands, the ape could carry extra food very useful when crossing expanses where fruit might not be available for the plucking. Free hands might also be useful for sex, although not in the way you might think. The best male upright walkers could bring back food for the females of their species, increasing their chances of winning a mate and passing on their genes or so suggests C. Owen Lovejoy of Ohio's Kent State University, the leading proponent of this theory.