For all its splendor, the human brain makes no bones about it’s exceptionally slow process of making bones. Compared to apes, people are the real knuckle draggers when it comes to growing up. We spend more time in developmental stages (e.g. infancy, childhood, adolescence) than any other simian primates. The advantage of being a human, however, is gloriously evident when you realize what’s going on inside our noggins as opposed to the monkey brains all those other primates are working with. We may not be as big or strong as our simian counterparts, but we are certainly more advanced when it comes to mental capacity. Could evolution be both the cause and effect of this trade-off? This question has long been the point of reference for many hypotheses about the role that the brain may play in regards to rate of growth and development within a species.
The most popular theory is that growing brains take up so much energy that they reduce the amount of glucose going to the rest of the body, causing delays in growth. This “expensive tissue” hypothesis was first proposed in 1995 by anthropologists Leslie Aiello and Peter Wheeler. Now, a new team of researchers, led by Northwestern University anthropologist Christopher Kuzawa, has used the data from three previous studies on glucose uptake and body growth to test this hypothesis. Kuzawa’s findings were published in August of this year on the Proceedings of the National Academy of Sciences (PNAS) website. This study confirmed that the timeframe of highest brain glucose uptake—between 4.5 and 5 years of age—directly coincides with the stage of lowest weight gain, thusly proving that heightened states of brain development divert energy from body growth functions.
Of course, this study doesn’t actually prove that our brains are entirely responsible for our prolonged development. That would require much more extensive and precise studies on brain metabolism throughout childhood and adolescence. Furthermore, we would need to determine if other primates experience a similar correlation between brain development and growth rate, which would be quite the daunting task for any researcher. “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,” according to Kazawa.
Perhaps we will never truly know why we take so much longer to grow up than other primates, but at least we can spend time hypothesizing about it without fear of being attacked by a leopard in our natural habitat. Chimpanzees may reach adulthood quicker than us, but they also throw their own feces at each other, so maybe the race to maturity doesn’t really offer the same finish-line for distant subtribes within a species.