Apart from Homo sapiens, primate babies cling to the fur of their mothers' stomachs. Later they hang from their mothers' backs.
During that time, it's advantageous for moms to walk on all fours; if they walked upright, their babies would slide right off.
Writing in Naturwissenschaften, Amaral theorizes that bipedalism evolved in response to our ancestors' hair loss.
Why'd they lose their hair?
Perhaps to stay cool under a hot savannah sun, or to shed themselves of disease-carrying parasites. Maybe hairlessness became a sign of fitness, and therefore a sexual turn-on.
Whatever the reason, our ancestors grew steadily less hairy. Their babies ostensibly had a tough time hanging on to mom.
And the more mom could stay upright on two legs, the better the chances of her newly-encradled baby surviving and passing on those upright genes.
Is there any way of knowing whether Amaral's speculation is true? Probably not, but it makes sense and is -- for lack of a more scientific descriptor -- comforting.
And on a related note, a study published today in Nature shows that women's spines evolved differently from men's, allowing them to keep their balance during pregnancy.
Not even chimpanzees have that
adaptation; it's all ours!
