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We humans gave up stability and speed. We gave up the foot as a grasping tool. We gained spongy bones and fragile joints and vulnerable spines and difficult, risky births that led to the deaths of countless babies and mothers. Given the trade-offs, the aches and pains and severe drawbacks associated with bipedalism, why get upright in the first place?
A couple of chimps named Jack and Louie may offer some insights. The chimps are part of an experiment by a team of scientists to explore the origin of bipedalism in our hominin ancestors.
Theories about why we got upright have run the gamut from freeing the arms of our ancestors to carry babies and food to reaching hitherto inaccessible fruits. "But," says Mike Sockol of theUniversityofCalifornia,Davis, "one factor had to play a part in every scenario: the amount of energy required to move from point to point. If you can save energy while gathering your food supply, that energy can go into growth and reproduction."
Paleogeographical studies suggest that at the time our ancestors first stood upright, perhaps six to eight million years ago, their food supplies were becoming more widely dispersed. "Rainfall in equatorial East Africa was declining," Sockol says, "and the forest was changing from dense and closed to more open, with more distance between food resources. If our ape ancestors had to roam farther to find adequate food, and doing so on two legs saved energy, then those individuals who moved across the ground more economically gained an advantage."
To test the theory that the shift to two feet among our ancestors may have been spurred by energy savings, Sockol and his colleagues are looking at the energy cost of locomotion in the chimp. The chimp is a good model, Sockol says, not just because it's similar to us in body size and skeletal features and can walk both bipedally and quadrupedally, but also because the majority of evidence suggests that the last common ancestor of chimps and humans who first stood upright was chimplike. By understanding how a chimp moves, and whether it expends more or less energy in walking upright or on all fours (knuckle-walking), the scientists hope to gain insight into our ancestors' radical change in posture. Jack and Louie and several other young adult chimps have been trained by skillful professional handlers to walk and run on a treadmill, both on two legs and on four. One morning, Jack sits patiently in his trainer's lap while Sockol's collaborators, Dave Raichlen and Herman Pontzer of Harvard University, paint small white patches on his joints—the equivalent of those silver balls I wore on Dan Lieberman's treadmill. Only occasionally does Jack steal a surreptitious lick of the sweet white stuff. Once he's marked, he jumps on the treadmill and runs along on two legs for a few minutes, then drops to four. Every so often, his trainer hands him a fruit snack, which Jack balances on his lower lip, 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 connected 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 ancestors who became bipedal. That's the dream."
Scientists are the first to admit that much work needs to be done before we fully understand the origins of bipedalism. But whatever drove human ancestors to get upright in the first place—reaching for fruit or traveling farther in search of it, scanning the horizon for predators or transporting food to family—the habit stuck. They eventually evolved the ability 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 technology, and consider the roots of its own quirky and imperfect upright being.
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