Baby brains reveal how we evolved from apes to humans

Human infants start life with ape-like brains. Now scientists are saying that the parts of the brain that develop the most during childhood are the exact same parts that changed most dramatically when apes evolved into humans.

The surprising similarity between infant brain development and the evolution of the human brain was discovered accidentally. Scientists at Washington University in St. Louis were researching the effects of premature birth on long-term development, which first required putting together a comprehensive guide to how normal brain development works. Amazingly, such a fundamental topic as how our brains develop from infancy to adulthood had been largely neglected until very recently.

Putting together dozens of brain scans of full-term infants and healthy young adults, the researchers were able to track what parts of the brain are born fully-formed and which change the most over time. What had simply been meant to provide a baseline for further research actually revealed a huge part of humanity's evolutionary record. Most strikingly, the cerebral cortex, the seat of higher mental functions in the human brain, expands unevenly, with about a third of the cortex growing twice as much as any other area.

David Van Essen, head of the Department of Anatomy and Neurobiology, explains how they made the connection between baby brains and those of apes and monkeys:

"Through comparisons between humans and macaque monkeys, my lab previously showed that many of these high-growth regions are expanded in humans as a result of recent evolutionary changes that made the human brain much larger than that of any other primate. The correlation isn't perfect, but it's much too good to put down to chance."

The researchers speculate that a lot of what makes us human - higher functions like language and reasoning - are centered on the parts of the cerebral cortex that grow the most. Based on the similarities in development and evolutionary changes, it's as though infants are born with an earlier evolutionary version of the human brain, a simpler model that can deal with the much more basic tasks infants have to deal with before our more uniquely human traits kick in. They speculate this process may occur because humans need these more sophisticated regions of the brain to be shaped by life experiences, or because a fully-formed adult brain would be much too big to pass through the mother's pelvis during childbirth.

[Proceedings of the Natural Academy of Sciences]