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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.

First, the researchers used a 1987 study of PET scans of 36 people between infancy and 30 years of age to estimate age trends in glucose uptake by three major sections of the brain. Then, to calculate how uptake varied for the entire brain, they combined that data with the brain volumes and ages of more than 400 individuals between 4.5 years of age and adulthood, gathered from a National Institutes of Health study and others. Finally, to link age and brain glucose uptake to body size, they used an age series of brain and body weights of more than 1000 individuals from birth to adulthood, gathered in 1978.
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.”

The expensive tissue hypothesis was first proposed in 1995 by anthropologists Leslie Aiello of New York’s Wenner-Gren Foundation for Anthropological Research and Peter Wheeler of the United Kingdom’s Liverpool John Moores University. Although it was initially thought that bigger brains were supported by smaller digestive systems, later studies revealed that other mechanisms could also be at work. Isler and primatologist Carel Van Schaik from the University of Zurich suggested that energy-rich diets, delayed growth and reproduction, and energy-efficient locomotion could also help feed the energy-hungry brain. Humans show signs of all three: We cook our food and eat meat, boosting caloric intake; we grow up more slowly and reproduce later; and we walk on two feet, saving energy compared with quadrupedal chimpanzees. The PNAS study supports the trade-off between delayed growth and larger brains, Isler says.

The ideal next step would be to see if a similar trade-off with growth happens in other primates, too. But that will hard, Kuzawa says. “Obtaining PET data on brain glucose use across the full growing years in other closely related primates would be fascinating but difficult, and likely impossible for the more relevant comparative species like chimpanzees,” he says.