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In the case of Ardipithecus, says Donald Johanson, professor of anthropology and director of the Institute of Human Origins at Arizona State University (and the man who discovered Lucy back in 1974), "when you put 5.5 million-year-old fossils together with 4.4 million-year-old ones as members of the same species, you're not taking into consideration that these could be twigs on a tree. Everything's been forced into a straight line." Beyond that, he's dubious about categorizing the 5.2 million-year-old toe bone with the rest of the fossils: not only is it separated in time by several hundred thousand years, but it was also found some 10 miles away from the rest.
If Orrorin turns out to be a hominid, the same skepticism will apply to any claims about its pivotal position on the family tree. According to University of Tokyo paleontologist Gen Suwa, a co-discoverer of the 4.4. million-year-old Ardipithecus ramidus ramidus, Orrorin could well be ancestral to the new Ardipithecus remains, rather than the other way around."There is nothing in the fossils," he says, "that would preclude such a position. But which side of the chimp-hominid split Orrorin occupies can be determined only by further analyses and new finds." Indeed, suggests Haile-Selassie, while Orrorin may be one of the earliest chimps or an ape that became extinct, it could also turn out to be the last common ancestor of humans and chimps--a creature paleontologists have been dreaming of finding for decades.
One of the most intriguing questions the new discoveries raise, says Bernard Wood, a professor of human origins at George Washington University, is whether bipedalism should still be considered the defining characteristic of being human. After all, all birds have wings, but not all creatures with wings are birds. It's already clear that eastern Africa was bubbling with evolutionary experiments 6 million years ago. Maybe two-legged walking evolved independently in several branches of the primate family. Says Wood: "This might be the first example of a creature it's not possible to label as hominid ancestor or chimp ancestor. But that doesn't make it the last common ancestor of both. I think it's going to be very hard to pin the tail on that donkey."
In the end, that may be the most exciting thing about these latest discoveries from the human race's birthing ground. Not that long ago, paleontologists were pretty certain we started on the road to becoming human by standing upright on the grassy savannah. Now that science is actually bringing in hard evidence, the story is getting more complicated--and more interesting. Clearly, there are still plenty of questions to ask, and plenty of surprises left to uncover, in the ancient sediments of eastern Africa.
--With reporting by Simon Robinson/Nairobi