One Giant Step For Mankind

Meet your newfound ancestor, a chimplike forest creature that stood up and walked 5.8 million years ago

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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.

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