When a pregnant woman leans back, and shifts her weight to stand more comfortably, she is performing a motion that for millions of years has helped to compensate for the strain and weight of childbearing on the body. According to a new study from researchers at Harvard University and the University of Texas at Austin, women’s lower spines evolved to be more flexible and supportive than men’s to increase comfort and mobility during pregnancy, and to accommodate the special biology of carrying a baby for nine months while standing on two feet.