To parents, it may seem as if your child grows up overnight. In reality, however, humans grow pretty slowly when compared to our other mammalian cousins, but scientists have struggled to prove why. That is until now. Researchers from Northwestern University have solved the evolutionary puzzle, finding that human children grow slowly to help ensure their developing brains always have enough energy.

Just because babies are born every day doesn’t make it any less amazing. Humans have evolved to give birth just before the infant's head gets too large to pass through the mother’s birth canal to ensure their tiny brains get as much to grow as physically possible, according to the American Scientist. At birth, a newborn’s brain makes up about 25 percent of their body weight. To put this into perspective, that would be equivalent to a 180-pound man having a 45-pound brain — an amazing feat, if anything.

Once outside the mother’s body, a child’s brain growth doesn’t stop but rather speeds up at an astonishing rate. This brain development continues even when the growth of a child’s body may seem to have paused. Scientists have long suspected this disparity between brain and body had something to do with biology prioritizing intellectual development over physical. However, it’s only recently that this long-standing hypothesis was proven to be true with a study now published in the Proceedings of the National Academy of Sciences.

The researchers used already existing data from PET and MRI brain scans to measure the correlation between brain volume and glucose intake in children. This enabled them to measure exactly how much energy a toddler’s brain actually required. The surprising results show that the brain of a 5-year-old uses twice as much glucose as that of a full-grown adult. What interested the researchers most about their findings was that the children’s peak time for glucose intake was the same as their slowest time for bodily growth. "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 Christopher Kuzawa, first author of the study explained in a press release.

William Leonard, co-author of the study, explained that at its growth peak, a child’s brain will burn through two-thirds of all the calories a body uses at rest. “To compensate for these heavy energy demands of our big brains, children grow more slowly and are less physically active during this age range.” This extreme brain growth doesn’t last forever. At age 4, the child’s brain glucose intake is at its peak. At this age, researchers observed the brain to consume glucose at a rate comparable to 66 percent of the body's resting metabolic rate. Not long after, at about 5, researchers found that synapses in the brain “max out.” Thankfully, at this age a child has already learned much of the important lessons they need in order to live.

Source: Kuzawa C, Leonard W, Chugani HT, et al. "Energetic costs and evolutionary implications of human brain development. Proceedings of the National Academy of Sciences. 2014.