Stand upright, cool off. That's long been touted as one of the benefits of our ancestors becoming bipedal in a hot and sunny world. But now researchers have poured cold water on the idea. A team examined how our ancient relatives, who were most likely covered with a thick pelt of hair, would fare while walking briskly in a sizzling place like the African savanna. The body dimensions used in the model—30 kg for females, 55 kg for males—were based on a group of early human ancestors, or hominins, such as Australopithicus afarensis, the species that includes the famous Kenyan fossil “Lucy.” The models showed that a 30-minute trek put hairy hominins at risk of heat stroke whether they were four-legged or erect , according paper published online today in the Proceedings of the National Academy of Sciences. Earlier models supporting the connection between bipedalism and heat loss examined ancient humans standing still in the sun, which the new paper's authors argue is less realistic.