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Humans Beat Chimps at Walking Efficiently

Anne Casselman
for National Geographic News
July 16, 2007
 
Groucho Marx's chimplike stage walk might be good for laughs, but it's sure not great for energy efficiency.

The bent-over gait of the chimpanzee drains much more energy than our upright walk, a new study reports.

A team of U.S. anthropologists compared the biomechanics and energy expenditure of both the two-legged and four-legged chimpanzee gaits with a human walk to get a glimpse of what drove the evolution of our bipedal stride.

"The first thing we [saw] off the bat is that humans are really efficient walkers relative to apes," said study co-author Herman Pontzer, a biological anthropologist at Washington University in St. Louis, Missouri.

When it comes to walking, humans use about one-fourth of the energy chimpanzees do, he added.

Chimpanzees, whether they are knuckle walking or upright, have a very crouched gait with bent knees and hips.

Humans, on the other hand, take long steps on straight long legs, requiring less muscle activation—which in turn means less energy spent.

Energy savings could be invested in other activities that ensure survival and improve fitness, such as reproduction and caring for offspring, said William Jungers, an anatomist at New York State's Stony Brook University in Stony Brook who is unaffiliated with the study.

The researchers also found high individual variation in chimpanzee leg length, which impacts the efficiency of the chimp's stride. Such diversity offers a species greater chances of evolving successfully when pressures to adapt—such as walking on two legs—enter the picture.

There could have been that kind of variation among early human ancestors, Pontzer said.

"Evolution needed a foot in the door, and we kind of got a snapshot of that here, which is kind of cool," he said.

From Jungle to Jungle Gym

"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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