National Geographic : 2006 Jul
An infant's bowed legs and first toddling steps look more like those of its ape ancestors than its human parents. But once we're up on two feet, stresses on the thighbone promote more growth on one side of the bone than the other, forming the carrying angle that distinguishes our species. By age six or seven, we possess the stable leg-swinging gait that allows our kind to carry tools, meat, and babies. We gained efficient upright loco motion, but it came with a plague of problems in hips, knees, and feet. 0) a) * 0) to (n THE DOWNSIDE OF UPRIGHT 145 thrust out as far as it will go, before rolling the fruit forward and flicking it into his mouth. For a set time, Jack breathes into a small mask con nected to equipment that gathers information on how much oxygen he consumes-a measure of energy expenditure-while the movements of his limbs (marked by those white dots) are monitored with cameras to help the scientists understand how the energy is being used. Once the scientists have refined their model for how things work in the chimp-for what limb movements are used in the two types of locomotion and how each consumes energy they hope to apply this model to the fossils of our ancestors. "We use the biomechanical data to determine the types of anatomical changes that would have reduced energy expenditure," Raichlen explains. "Then we look at the fossil record and ask, Do we see these changes? If we do, that's a pretty good clue that we're looking at selection for reduced energy costs in our an cestors who became bipedal. That's the dream." Scientists are the first to admit that much work needs to be done before we fully under stand the origins of bipedalism. But whatever drove human ancestors to get upright in the first place-reaching for fruit or traveling far ther in search of it, scanning the horizon for predators or transporting food to family-the habit stuck. They eventually evolved the abil ity to walk and run long distances. They learned to hunt and scavenge meat. They created and manipulated a diverse array of tools. These were all essential steps in evolving a big brain and a human intelligence, one that could make poetry and music and mathematics, assist in difficult childbirth, develop sophisticated tech nology, and consider the roots of its own quirky and imperfect upright being. O Of Chimps and Humans Watch video of photog rapher Cary Wolinsky's session with a chimpanzee, and find Web-exclusive images at ngm.com/0607.