While no one has an authoritative answer, anthropologists have long theorized that early humans began walking on two legs as a way to reduce locomotor energy costs.
In the first study to fully examine this theory among humans and adult chimpanzees, published online July 17 in the Proceedings of the National Academy of Sciences, researchers have found that human walking is around 75 percent less costly, in terms of energy and caloric expenditure, than quadrupedal and bipedal walking in chimpanzees.
That energy savings could have provided early hominids with an evolutionary advantage over other apes by reducing the cost of foraging for food.
Conducted by Herman Pontzer, Ph.D., assistant professor of Anthropology in Arts & Sciences at Washington University in St. Louis; Michael Sokol of University of California, Davis; and David Raichlen of University of Arizona, the study used treadmill trials to analyze walking energetics and biomechanics for adult chimpanzees and humans.
The only other research study on chimpanzee locomotor cost, conducted in 1973, used juvenile chimpanzees, which have different locomotor mechanics and costs than adults.
The team also examined the early hominin fossil record, which they found to include predicted changes consistent with lower energy cost- longer hind legs compared to body mass and structural changes to the pelvic bone allowing for more upright walking.
Analysis of these features in early fossil hominins, coupled with with analysis of bipedal walking in chimpanzees, indicate that bipedalism in early, ape-like hominins could indeed have been less costly than quadrupedal knucklewalking.
"Walking upright on two legs is a defining feature that makes us human," said Pontzer. "It distinguishes our entire lineage from all other apes."
PNAS article titled "Chimpanzee Locomotor Energetics and the Origin of Human Bipedalism," by Michael Sockol, David A. Raichlen and Herman Pontzer.
David A. Raichlen, University of Arizona
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Michael Sockol, University of California, Davis