Humans are late bloomers when compared with other primates. For example, they spend almost twice as long in childhood and adolescence as chimps, gibbons, or macaques do. Researchers claim to have found out why human children grow slowly and childhood lasts so long in a new study.
The study led by anthropologists at Northwestern University in Evanston, Illinois and published in the Proceedings of the National Academy of Sciences (PNAS) on August 25th 2014, shows that a child’s brain is “an energy monster,” consuming twice as much glucose, the energy that fuels the brain, as that of a full-grown adult.
Horses are up and running soon after birth and thoroughbreds are racing by age two. Chimps are adults at between 12 and 15 years. But human toddlers seem to grow particularly slowly and researchers believe this is because the brain claims most of the calories consumed.
“Our findings suggest that our bodies can’t afford to grow faster during the toddler and childhood years because a huge quantity of resources is required to fuel the developing human brain,” said Prof. Christopher Kuzawa a professor of anthropology at Northwestern University’s Weinberg College of Arts and Sciences. “As humans we have so much to learn, and that learning requires a complex and energy-hungry brain,” he added.
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 parts. 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.
Kuzawa 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.
However, the costs of the human cognitive development are still unknown. “The mid-childhood peak in brain costs has to do with the fact that synapses, connections in the brain, max out at this age, when we learn so many of the things we need to know to be successful humans,” said Kuzawa.
“To compensate for these heavy energy demands of our big brains, children grow more slowly and are less physically active during this age range,” said co-author William Leonard of the Northwestern University.
“Our findings strongly suggest that humans evolved to grow slowly during this time in order to free up fuel for our expensive, busy childhood brains,” Leonard added.
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