Despite the widespread belief that natural is better when it comes to sex, pregnancy, and parenting, most of us have no idea what "natural" really means; the origins of our reproductive lives remain a mystery. Why are a quarter of a billion sperm cells needed to fertilize one egg? Are women really fertile for only a few days each month? How long should babies be breast-fed?

In How We Do It, primatologist Robert Martin draws on forty years of research to locate the roots of everything from our sex cells to the way we care for newborns. He examines the procreative history of humans...