According to Evolutionary Anthropology Department chairman Daniel Schmitt, something special happened in humans' family tree about 2 million years ago. Long before that, our line already had become the only mammals to walk fully erect. But fossilized bones from our ancestor Homo erectus suggest that by 2 million years ago we were also walking in the modern way.
Even toddlers do that spontaneously, Schmitt said in a Jan. 14 seminar on global health at Duke's John Hope Franklin Center, as the image of an 18-month-old example flashed on a screen. Sporting a fashionable blue hat, the shoeless little girl dug in one sandy heel while pivoting forward on her other foot's toes as she strode purposefully down a beach.
The result is a straight-legged, pivoting walk that converts potential into kinetic energy at an exceptional 70 to 75 percent efficiency said Schmitt, who as an anthropologist and Medical Center lecturer is interested in the gaits of various animals as well as human arthritis, bone disease and obesity.
While our walk is efficient, all that heel pounding transmits a force about equal to our body's weight along our legs and knees. To compensate, the human line has evolved bone sockets and joints that grew considerably larger as our walking style changed, he said. But over time, our knees and other pressure points may still pay a price in bone and joint diseases, especially if we're overweight.
Our closest living primate relatives never have walked like we do, Schmitt said. When Great Apes choose to walk erect, they adopt a flat-footed gait that lets their knees and hips to bend in a style he labeled "compliant walking."
Fossil evidence suggests human forebearers older than homo erectus were compliant walkers too. And so was a famous 20th century comic, he said, as a cartoon caricature flashed on the screen. Needing no introduction to adults of a certain age, there was Groucho Marx stooping towards his jutting cigar as he ambled flat footed across the stage.
"Compliant walking can reduce loadings," Schmitt said. And studies have shown it can lower forces on ankles and joints, he added. So perhaps we can learn something from our own evolutionary history, he suggested. For some people, walking like Groucho might be therapeutic.