mathematically illustrates what led to the evolution of humans’ abnormally large brains.

Evolutionary biologists devised these equations to tease apart the relationship between human brain size and the cost of maintaining a large brain. Over the last few decades, the pace and stages of brain growth in humans have become clearer. From birth to preschool, our brains quadruple in size. Our brains reach 90 percent of their final size by six years old, and they continue to grow slowly through adolescence until stopping in our mid-20’s.

Anthropologists have hypothesized — made educated speculations — about what factors in human evolution drive this pace. For example, newborns heavily rely on their families, so they can develop strong social bonds during their youth. As humans get older, we increasingly learn to be self-sufficient, use tools and learn of our environments. Scientists speculate both of these habits contribute to brain growth, but they don’t know which of these factors or others have the greatest bearing. A standard mathematical model could provide clarity by quantitatively comparing hypotheses.

Anthropologists can plug in their hypotheses to the model, which then predicts brain size from birth to adulthood based on those numbers. If those numbers match what we know about the pace of human brain development, then the model supports the hypothesis.

“With this model, you can obtain predictions for each of the hypotheses to see which hypothesis yields a better prediction,” said evolutionary biologist Mauricio González-Forero of Université de Lausanne in France, who led the study.

The final model states that adult skill level equals adult brain mass times the cost of maintaining brain tissue divided by the cost of memory times a constant. Stated in laymen’s terms, this idea means as adult brain mass increases, so too does adult skill, assuming that the costs of maintaining the brain mass and memory stay constant.

These costs include eating a lot in order to maintain the brain. Brains make up 2 percent of our bodies, but consume 20 percent of our oxygen and sugars in our food to sustain the activity of billions of neurons. This mental gorging could have been a disadvantage for early humans thousands of years ago, because bigger diets, consisting of more calories, means having to spend more time hunting and foraging for food. If their evolving brains drained too much food and oxygen, then they might have been too tired to fend for themselves.

While there is debate among anthropologists, many believe that social interaction is a major factor in increasing brain size. Knowing people, communicating with them and maintaining relationships takes a lot of brainpower.

González-Forero’s model counters this narrative and asserts that humans gain more intelligence as they learn to use technology, which University of Wisconsin-Madison evolutionary anthropologist John Hawks describes as a controversial but revealing take on brain development.

Many anthropologists look at the pace of brain growth in terms of social interactions, he added, but “this paper is saying maybe social relationships don’t have anything to do with it. It’s really neat to see such a cool, clear statement of that because it gives us a target.”

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