Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

In contrast, a typical hunter-gatherer woman takes about eighteen years to mature into an adult, and when she becomes a mother, she needs at least 2,400 calories a day to take care of her body plus nurse an infant. To get more energy than the chimpanzee mother, the human mother eats a higher-quality, more diverse diet of fruits, nuts, seeds, meat, and leaves, some of which she gathers herself but some of which is provided by her husband, her mother, and others. Further, by cooking her food, she gets more energy than by eating it raw.25 Thanks to that extra energy, instead of having a baby every five or six years, the human mother weans her infants earlier and typically has another baby about every three years. Thus, human mothers typically care for and feed more than one dependent child at a time. In addition, humans take 50 percent longer than chimpanzees to reach maturity, magnifying the time and cost of raising each offspring.