A new study seems to  confirm the theory that early humans started to walking on two legs in order to reduce locomotor energy costs.

In thee 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.

The study was 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 researchers trained five chimpanzees to walk on a treadmill while wearing masks that allowed measurement of their oxygen consumption. The chimps were measured both while walking upright and while moving on their legs and knuckles

The only other research study on chimpanzee locomotor cost, conducted in 1973, used juvenile chimpanzees, which have different locomotor mechanics and costs than adults.

“We were able to tie the energetic cost in chimps to their anatomy. We were able to show exactly why certain individuals were able to walk bipedally more cheaply than others, and we did that with biomechanical modelling. What those results allowed us to do was to look at the fossil record and see whether fossil hominins [ancestors] show adaptations that would have reduced bipedal energy expenditure” said David Raichlen.

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, 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 Herman Pontzer. “It distinguishes our entire lineage from all other apes.”