Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

The woeful inefficiency of knuckle walking is rarely a problem for chimpanzees deep in the forest, but it must have been a serious challenge for our missing-link ancestors about seven to nine million years ago. During this period of rapid climate change, the rain forest that covered much of Africa shrank and split into thousands of fragments interspersed with drier, open woodlands. For apes living in the depth of the rain forests, life went on as usual, but those at the margins of the forest must have faced a crisis. As woodlands replaced the forest, the fruits that dominated their diet became less abundant and more dispersed. They had to travel farther to get the same amount of food. Because life is fundamentally about acquiring and using scarce energy to make more life, those better able to conserve energy would have had a reproductive advantage. However, because these apes still benefited from using their long arms, fingers, and toes to climb trees, natural selection evidently favored those who walked efficiently without compromising their ability to still climb effectively.