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Yet the advantages of walking upright were somehow so great that the behavior endured through thousands of generations. Indeed, the anatomy of our ancestors underwent all sorts of basic changes to accommodate this new way of moving. Many of the changes help the body stay balanced by stabilizing the weight-bearing leg and keeping the upper torso centered over the feet. Lovejoy, who studies the anatomy and biomechanics of locomotion, thinks the changes may have improved coordination as well. "To walk upright in a habitual way, you have to do so in synchrony," he says. "If the ligaments and muscles are out of synch, that leads to injuries. And then you'd be cheetah meat."  

By far the most crucial changes, according to Lovejoy, were those in the spine. The distance between chest and pelvis is longer in humans than in apes, allowing the lower spine to curve, which locates the upper body over the pelvis for balance. The pelvis grew broader, meanwhile, and humans developed a hip joint and associated muscles that stabilize the pelvis. Explains Lovejoy: "That's why a chimp sways from side to side as it walks upright and humans don't."

Changes also had to take place in the femur, or thighbone. For example, the femoral neck-the bent portion at the top of the bone-is broader in humans than it is in apes, which improves balance. The human knee is specialized for walking upright too: to compensate for the thighbone's being at an angle, there's a lump, or groove, at the end of the femur that prevents the patella from sliding off the joint. "A chimp doesn't have this groove because there is no angulation between the hip and the knee," Lovejoy says. "This change says you're a biped."

Finally, there's the foot. "What's important here is the arch," Lovejoy says. "It's a really important shock absorber. It's like wearing a good pair of running shoes." In order to create that arch, the chimp's opposable great toe became aligned with the others, and the toe's muscles and ligaments, which had been used for grasping and climbing, were repositioned under the foot. "The shape of the big toe is indicative of this. You can see it in Lucy's species," Lovejoy says, but not in the bone Haile-Selassie found, because it's from a different toe. "What we can see [in the new discovery's foot] is that the base of the bone adjacent to the knuckle has a distinct angle, showing that the creature walked step after step after step with its heel off the ground, using the front of its foot as a platform."

That's how it walked. Why it walked is tougher to understand, since motivation leaves behind no physical remains. But armed with knowledge about our ancestors' physical attributes and the environment that surrounded them, scientists have come up with several theories. Anthropologist Henry McHenry, of the University of California, Davis, for example, champions the idea that climate variation was part of the picture after all. When Africa dried out, say McHenry and his colleague Peter Rodman, the change left patches of forest widely spaced between open savannah. The first hominids lived mostly in these forest refuges but couldn't find enough food in any one place. Learning to walk on two legs helped them travel long distances over ground to the next woodsy patch, and thus to more food.

Meave Leakey, head of paleontology at the National Museums of Kenya and a member of the world's most famous fossil-hunting family, suspects the change in climate rewarded bipedalism for a different reason. Yes, the dryer climate made for more grassland, but our early ancestors, she argues, spent much of their time not in dense forest or on the savannah but in an environment with some trees, dense shrubbery and a bit of grass. "And if you're moving into more open country with grasslands and bushes and things like this, and eating a lot of fruits and berries coming off low bushes, there is a hell of an advantage to be able to reach higher. That's why the gerenuk [a type of antelope] evolved its long neck and stands on its hind legs, and why the giraffe evolved its long neck. There's strong pressure to be able to reach a wider range of levels."

But for Kent State's Lovejoy, the real answer is sex. Males who were best at walking upright would get more of it, leading to more offspring who were good on two legs, who in turn got more sex. His reasoning, first proposed nearly two decades ago, goes like this: like many modern Americans, monkeys and apes of both genders work outside the home-in the latter case, searching for food. Early humans, though, discovered the Leave It to Beaver strategy: if males handled the breadwinning, females could stay closer to home and devote more time to rearing the children, thus giving them a better shot at growing up strong and healthy.

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 , 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.

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 , 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