Though women may try their darndest to eliminate them, fat deposits on the thighs and buttocks are often there to stay. Men don't have similar deposits, and why and when this trait emerged in human evolution has proved difficult to pinpoint. Some researchers propose that it evolved as a sexual signal to attract mates. Others posit that such fat could serve as reserves in times when food is scarce. Yet none of these hypotheses explains why the fat deposits are located where they are. Now a report in the most recent issue of Current Anthropology aims to do just that. According to Polish researcher Boguslaw Pawlowski of the University of Wroclaw, the fat deposits may help to meet the balance requirements of two-legged walking during pregnancy and lactation.

Pawlowski notes that both during advanced pregnancy and when nursing, a human female has an additional anterior load that moves her center of gravity forward and upward, making bipedal locomotion more difficult and energetically inefficient. This imbalance can be particularly burdensome for women in traditional societies, many of whom work hard gathering food right up until giving birth. The addition of weight below and behind the center of gravity reduces those ill effects by offsetting the baby's load. Thus Pawloski suggests that evolution promoted buttocks and thigh fat deposits to compensate for the biomechanical handicap imposed by carrying a baby. "Without a counterbalance, inefficient walking and foraging could have put [early human females] at much greater risk of starvation and predation," he asserts.

As to when in humanity's long and complex history this trait might have arisen, Pawlowski speculates that it appeared only after the emergence of our genus, Homo, because ours was the first genus for which bipedal walking was obligatory. Furthermore, he observes, the the brain-size increase that characterizes the emergence of Homo "made Homo fetuses and newborns relatively large and therefore created more anterior weight during and after pregnancy."