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And if you're going to bring home the bacon, or the Miocene equivalent, it helps to have your hands free to carry it. Over time, female apes would choose to mate only with those males who brought them food--presumably the ones who were best adapted for upright walking. Is that the way it actually happened? Maybe, but we may never know for sure. Leakey, for one, is unconvinced. "There are all sorts of hypotheses," she says, "and they are all fairy tales really because you can't prove anything."
If paleontologists argue about why bipedalism evolved, they're even more contentious over the organization of the human family tree. According to Haile-Selassie and his colleagues, the picture looks pretty straightforward from about 5.8 million years ago to the present. First comes Ardipithecus ramidus kadabba, the newest find. Then, more than a million years later, its descendant, the newly renamed Ardipithecus ramidus ramidus, appears. After that comes a new genus, called Australopithecus (where Lucy belongs), and finally, about 2 million years ago, the first members of the human genus Homo.
But not everyone buys the story. Indeed, the French and Kenyan team that presented a 6 million-year-old fossil last December insists that theirs, known as Orrorin tugenensis (or, more familiarly, Millennium Man because it was announced in 2000), is the true human ancestor and that Ardipithecus is nothing more than a monkey's uncle--or a chimp's great-great-grandfather, anyway. They even dismiss Lucy and her close kin, about as firmly entrenched in the human lineage as you can get, as evolutionary dead ends that left no living descendants.
No one disputes that this competing ancestor is 6 million years old and thus more ancient than Ardipithecus. What's still to be proved is that it's a hominid. Says Leakey: "If you read their paper, almost everything they say about the teeth suggests it's more apelike." And when they get to the femur, she says, they present no evidence disproving that it walked on all fours. Haile-Selassie makes precisely the same point. But Brigitte Senut of the National Museum of Natural History in Paris and Martin Pickford, chairman of paleoanthropology and prehistory at the College de France, co-leaders of the team that found Orrorin, dismiss the criticisms. Additional fossils found just last March, they say, along with the more detailed analysis they now have in hand of the earlier bones, will prove their case. "We are absolutely delighted about it," says Senut. "We had the possibility to show the evidence to some colleagues in South Africa recently, and just looking at the cast they said, 'Fantastic, it's a biped! And a better biped than Lucy.'"
Even if they're right, though, establishing the precise path of human descent might be very hard. For most of the past 6 million years, multiple hominid species roamed the earth at the same time--including a mere 30,000 years ago, when modern humans and Neanderthals still coexisted. We still can't figure out exactly how Neanderthals relate to the human family; it's all the more difficult to know where these newly discovered species, with far fewer fossil remains to study, belong.