From MSNBC:
Humans walking on two legs consume only a quarter of the energy that chimpanzees use while “knuckle-walking” on all fours, according to a new study.
The finding, detailed 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 moving about.
“Walking upright on two legs is a defining feature that makes us human,” said study leader Herman Pontzer, an anthropologist at Washington University in St. Louis. “It distinguishes our entire lineage from all other apes.”
More here.