"We have lots of reasons to think that the last common ancestor between chimps and humans was a chimpanzee-like ape," study co-author Pontzer said. (Related: "Chimps, Humans 96 Percent the Same, Gene Study Finds" [August 31, 2005].)
That means chimp locomotion may be a good model to use to understand those early days. But it's not necessarily easy to get chimps to walk upright.
In the wild, chimpanzees prefer to knuckle-walk, though they do walk upright every so often. "So it's not like training a bear to ride a bicycle," Pontzer said.
For the new study, an animal trainer took six months to coax and cajole his five chimp study subjects to walk on treadmills. The trainer also equipped the apes with face masks, which measured how much oxygen the apes were using to fuel their movements.
The study appears in today's online edition of the journal Proceedings of the National Academy of Sciences.
And Then There Were Two
"People have been scratching their heads for a long time for why we began to walk in this peculiar fashion," said Jungers of Stony Brook. "[Upright walking] really has become the calling card of our lineage."
Ever since Darwin wrestled with the idea 150 years ago, scientists have painted various scenarios—"wild and woolly stuff," as Jungers describes it—of the evolution of bipedalism. Walking on two legs likely began five to six million years ago, according to fossil records.
Among the wooliest is the "flasher theory," which says sexual signals were better transmitted by bipeds. Another theory suggests humans first learned to walk in trees like gibbons. (Related: "Upright Walking Started in Trees, Ape Study Suggests" [May 31, 2007].)
"We still don't have a compelling explanation for that first step of bipedalism," Jungers said.
Daniel Schmitt is a biological anthropologist from Duke University in Durham who was not involved in the study.
He believes Pontzer's use of biomechanics to study the evolution of walking is a big step in the right direction.
Pontzer and his team have generated some testable hypotheses, Schmitt said, and "such tests await more studies like this and more fossils."
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