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Figure 4. Homo species femurs, 1.9-million-year-old and modernClick to Enlarge Image

Bone is a remarkably dynamic tissue. Even after we reach adult stature, our skeletons continue to change in response to metabolic needs and mechanical stresses. Our author, a functional anatomist, views human evolution through the lens of a growing body of knowledge on how these needs and stresses shaped the structures of bones in the fossil record. Ruff explains that even 5,000 years ago (before much of the technology that insulates us from the physical world), the bones of human beings were only half as strong as those of human ancestors from several million years ago. Despite this trend, our bones retain their ancient capacity to grow strong, such that a long-time tennis player will have a playing-arm bone on average 40 percent stronger than the bone in his non-playing arm. This research sheds light on the causes and cures for osteoporosis, and on the effects of weightlessness on astronauts.

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