After a long day spent chasing after their small children, many exhausted parents may feel like they're raising monkeys. But while kids may act like our fellow primates, they don't grow like them. Unlike monkeys, humans stay relatively small during childhood and then experience a growth spurt during adolescence. A new study suggests that this strategy may help increase the number of offspring parents can successfully rear.
Because children are not very effective at obtaining food, their parents must provide for them for many years. So parents would bear a lighter burden if their kids stayed small and consequently ate less during those years of dependency, which might also be a good evolutionary strategy. In contrast, other primates rely more on brawn and less on brains to find food, so they would seem to benefit from large size at an earlier age.
Anthropologists Michael Gurven of the University of California, Santa Barbara, and Robert Walker of the University of New Mexico in Albuquerque tested this idea by studying two hunter-gatherer societies who forage for food much like early humans are believed to have done. If children of the Ache people of Paraguay grew like chimps, a mathematical model developed by the researchers indicates that, from conception to age 18, Ache girls and boys would consume 9% and 25% more calories, respectively, than they actually do. The children of Botswana's Dobe Ju/'hoansi people, who grow more slowly during childhood, would eat even more. Here, girls and boys would consume 38% and 54% more calories, respectively. Given the limited resources of early humans, such demands would have made it hard for parents to raise multiple offspring, the researchers report online this week in Proceedings of the Royal Society B.
The paper is "a welcome addition to the literature," says anthropologist Steven Leigh of the University of Illinois, Urbana, because it suggests that the unique human growth pattern "is essentially a reproductive adaptation, enabling mothers and other adults to maximize fertility rates." Leigh also suggests that this growth pattern "could be even more effective in an agricultural setting" because of the skills and physical strengths required for farming.