National Geographic : 2006 Jul
Marvin Lipschutz, a 63-year-old physician who suffered from a herniated, or slipped, vertebral disk, hangs on an inversion table designed to relieve back pain and pressure, a technique dating back to Hippocrates in 400 B.C. Today, roughly 80 per cent of adults experience back problems sometime in their lives-a consequence of our upright posture. Humans have likely suffered this way since our ancestors first stood up, transforming our backbone from a bridge or arch to a col umn that must bear the full weight of the upper body. Especially vulnerable is the lower, or lumbar, region of the column, where pressure on the disks that separate our vertebrae can cause them to bulge or herniate. w O O (. U THE DOWNSIDE OF UPRIGHT 131 Byzantine design? "The human female pelvis is a classic example of evolutionary compromise," Rosenberg answers. Its design reflects a trade off between the demand for a skeletal structure that allows for habitual walking on two feet and one that permits the passage of a baby with a big brain and wide shoulders. Its unique features didn't come about all at once, but at different times in our evolutionary history, in response to different selective pressures. "The result of these different pressures is a jerry rigged, unsatisfactory structure," Rosenberg says. "It works, but only marginally. It's definitely not the type of system you would invent if you were designing it. But evolution is clearly a tinkerer, not an engineer; it has to work with yesterday's model." [ YESTERDAY'S MODEL ] Humans come from a long line of ancestors, from reptile to mammal to ape, whose skeletons were built to carry their weight on all fours. Our ape ancestors probably evolved around 20 million years ago from small primates that carried themselves horizontally. Over the next several million years, some apes grew larger and began to use their arms to hold overhead branches and, perhaps, to reach for fruit. Then, six or seven million years ago, our ancestors stood up and began to move about on their hind legs. By the time the famous Lucy (Australo pithecus afarensis) appeared in East Africa 3.2 million years ago, they had adopted walking as their chief mode of getting around. It was a radical shift. "Bipedalism is a unique and bizarre form of locomotion," says Craig Stanford, an anthropologist at the University of Southern California. "Of more than 250 species of primates, only one goes around on two legs." Stanford and many other scientists consider bipedalism the key defining feature of being human. "Some may think it's our big brain,"