A recent UA-linked study hypothesize that walking on two legs evolved because it used a smaller amount of energy than knuckle walking or walking on all fours.

The study, presented in the July 17 issue of the journal for the Proceedings of the National Academy of Sciences, supports the idea that early humans became bipedal as a way to reduce energy costs associated with the body.

David Raichlen, an assistant professor of anthropology at the UA, conducted the study with Michael Sockol from the University of California and Herman Pontzer from Washington University in St. Louis.

The energy saved by walking upright gave early humans an evolutionary improvement over other apes by reducing the expenses of looking for food, according to the study.

“”For decades now, researchers have debated the role of energetics and the evolution of bipedalism,”” Sockol said.

To conduct its research, the team trained five chimpanzees to walk upright and on all fours while on a treadmill. The chimps wore masks and energy consumption was measured by how much oxygen was used during the experiment. The results were then compared to those of four humans, who walked upright on the treadmills wearing the same gear.

The researchers collected metabolic, kinematics and kinetic data from the chimps and humans. They determined that humans walking on two legs used only one-fourth the energy used by chimpanzees who knuckle-walked on four legs.

Walking upright is an essential characteristic that makes us human, said Pontzer, the study’s leader.

“”It makes a distinction from our entire ancestry of apes,”” he said.

“”Our results allow us to look at the fossil records and see if fossil hominines show similar adaptations that would have reduced bipedal energy disbursements,”” Raichlen said. “”The results tell us that energetics played a considerable part in the evolution of bipedalism.””

On average, the chimpanzees used the same amount of energy using two legs as they did when they used four legs.

“”We were able to tie the energetic cost in chimps to their anatomy,”” Pontzer said. “”We were able to show precisely why some individuals were able to walk bipedally with less energy than others.””

The study was able to explain all of the differences in costs using biomechanical modeling, Raichlen said, adding that costs were explained by differences in the amount of muscle used to move.

“”Chimps walk with a crouched posture and recruited more muscle than humans, who walk on very straight limbs. Adaptations in the pelvis and hind limb of humans allow us to walk with very straight limbs and we see evidence of these adaptations in the hominid fossil record.””