Though as to exactly when humans started walking on two legs is something yet to be ascertained, scientists have nevertheless hypothesized that bipedalism evolved as a way of reducing locomotor energy costs.
In the first study to fully examine this theory among humans and adult chimpanzees, the team of scientists from Washington University in St. Louis, University of California and University of Arizona, 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 analysis of bipedal walking in chimpanzees, revealed that human walking was around 75 percent less costly, in terms of energy and caloric expenditure than bipedal walking and quadrupedal knucklewalking in chimpanzees.
The energy savings could have provided early hominids with an evolutionary advantage over other apes by reducing the cost of foraging for food, said Herman Pontzer, Ph.D., assistant professor of Anthropology in Arts & Sciences at Washington University in St. Louis.
"Walking upright on two legs is a defining feature that makes us human. It distinguishes our entire lineage from all other apes," said Prof. Pontzer.
The study "Chimpanzee Locomotor Energetics and the Origin of Human Bipedalism" appears in online in the July 17 issue of the Proceedings of the National Academy of Sciences