The cover of the May 21 issue of Time magazine, featuring a woman breast-feeding her 3-year-old child, has reignited the debate over the appropriate age to stop nursing. Without the influence of lactation consultants, parenting magazines, and judgmental acquaintances, for how long did prehistoric women breast-feed their little cave-babies?

Probably for two to four years. The fossil record provides little indication of weaning times, so the best evidence we have comes from our closest living relatives in the animal kingdom. Chimpanzees and bonobos wean their young at the age of 4 or 5, while gorillas do so about a year earlier. It’s tricky to translate those ages into human years, though. A couple of developmental milestones coincide with the cessation of nursing in the great apes: the emergence of adult teeth and the growth of the offspring to about four times its original birth weight. Humans develop their adult teeth at age 5 or 6, but they’ve quadrupled in size by 2 1/2years old. (Children in developing countries tend to grow more slowly, and would nurse until age 3 by this measure.)

The numbers produced by the animal analogy are likely a bit too high, because they ignore important differences between man and ape. Female chimpanzees, for example, leave their families upon reaching sexual maturity and are famously promiscuous, which means they don’t get a lot of help in rearing children. Given this social structure, it may be easier for them to hold off on weaning infants until the age of 4 or 5.

A Homo sapiens baby, on the other hand, enjoys the support of relatives on both sides of the family tree, including the preparation and sharing of food. In addition, humans are unique among the primates for their reliance on “baby food”—a diet specialized to ease the transition from breast milk. (It’s not clear when people started using baby food, but it may go back a long, long time.) The existence of a transitional diet would make it easier for humans to start weaning at an early age, which would in turn allow human women to have children more regularly than our ape relatives.

Anthropologists have studied the weaning customs of hunter gatherer societies, and found significant variation. The !Kung San of the Kalahari desert stop breast-feeding at around 4 or 5 years of age. The Hadza of Tanzania wean about six months earlier than the !Kung. The Aché people of Paraguay wean earlier still, at around 3 years of age. This suggests that human culture and lifestyles have influenced the practice of breast-feeding since the beginning of civilization, although anthropologists still aren’t sure exactly what factors account for the differences.

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Explainer thanks Katherine Dettwyler of the University of Delaware and co-editor of Breast-Feeding: Biocultural Perspectives, Katherine Hinde of Harvard University and author of the blog Mammals Suck, and Meredith Small of Cornell University and author of Our Babies, Ourselves: How Biology and Culture Shape the Way We Parent.

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