Why do we humans walk on two legs?
Because it's easier than walking on four.
That's the basic finding of a study conducted by a team of researchers that included University of Arizona anthropologist David Raichlen. Its findings appear today in the Proceedings of the National Academy of Sciences.
The scientists studied the energy used by four humans walking upright on a treadmill and compared it with that expended by five chimpanzees walking both upright and in the four-limbed alignment known as quadrupedal knuckle-walking.
On average, the chimpanzees used four times the amount of energy expended by humans in both alignments, but there were wide differences in energy used by individual chimpanzees. The chimpanzee with the anatomy that allowed the longest stride used the least energy walking upright.
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That was the exciting part of the findings, Raichlen said, because it allowed the team to hypothesize that such variations occurred in the common ancestors from which both humans and chimpanzees evolved.
Scientists believe that bipedalism developed in Africa about 6 million years ago when a drying climate forced our ancestors to forage more widely for food in open woodlands.
Upright walkers could use their arms for carrying and feeding. Those who walked most easily may have evolved more successfully because they had energy left over for foraging and, perhaps more importantly, for sex.
"The name of the game in biology is to minimize energy in all that you do except reproduction," said Raichlen, an assistant professor of anthropology at the UA.
The study was conducted by Raichlen; Herman Pontzer, of Washington University in St. Louis; and lead author Michael Sockol, with the University of California-Davis.
Sockol said the hypothesis that energy-saving was a factor in the evolution of upright walking among hominids has "been around for a very long time," but it never was tested fully because of the difficulty of working with adult chimpanzees.
Sockol found a trainer at a wildlife sanctuary in California who had a good rapport with five adult chimpanzees, which he trained to walk upright on a treadmill while wearing masks that captured their exhalations, allowing Sockol to measure the energy they used in walking.
Three of the chimps used more energy walking upright, and one used the same amount walking upright or on four limbs.
But one female chimpanzee, who was better adapted anatomically to upright walking, used considerably less energy, Sockol said.
"We expected to find bipedalism was more costly for all of them," Sockol said. "We couldn't have asked for a better outcome."
Raichlen and Pontzer, meanwhile, recorded the chimpanzees' biomechanical processes, transforming camera images into computer models of limb and muscle movement, and measuring the force of the animals' stride.
The information can be used to predict what anthropologists might find in the fossil record, Sockol said, as they search for evidence of a common ancestor to humans and chimpanzees believed to have existed 7 million to 9 million years ago.
