Humans are late bloomers when compared with other primates—they spend almost twice as long in childhood and adolescence as chimps, gibbons, or macaques do. But why? One widely accepted but hard-to-test theory is that children’s brains consume so much energy that they divert glucose from the rest of the body, slowing growth. Now, a clever study of glucose uptake and body growth in children confirms this “expensive tissue” hypothesis.
Previous studies have shown that our brains guzzle between 44% and 87% of the total energy consumed by our resting bodies during infancy and childhood. Could that be why we take so long to grow up? One way to find out is with more precise studies of brain metabolism throughout childhood, but those studies don’t exist yet. However, a new study published online today in the Proceedings of the National Academy of Sciences (PNAS) spliced together three older data sets to provide a test of this hypothesis.
The researchers, led by Christopher Kuzawa, an anthropologist at Northwestern University in Evanston, Illinois, found that when the brain demands lots of energy, body growth slows. For example, the period of highest brain glucose uptake—between 4.5 and 5 years of age—coincides with the period of lowest weight gain. This strongly suggested that the brain’s high energy needs during childhood are compensated for by slower growth. “This is a very, very cool paper,” says Karin Isler, a biological anthropologist at the University of Zurich in Switzerland. “It very convincingly shows that the conflicting demands of the brain’s and the body’s energy requirements for growth are met, in humans, by a temporal sequence of delayed growth.”