When anthropologist Aida Gómez-Robles began studying human evolution, her work revolved around comparing the dental structures of early human species. But before long, she switched gears, to focus on the brain instead. "The most important trait that defines us as a species is our brain," she says. "I felt that if I wanted to understand what was special about human evolution, I needed to study the brain and not the teeth."
But how does one study the evolution of a structure that doesn't fossilize? One way is to compare human brains to those of one of our closest living relatives: the chimpanzee. "The idea is that if we can identify some traits that are present in humans and not in chimpanzees," says Gómez-Robles, a post-doctoral scientist at George Washington University, "these are traits that must have evolved since we separated from our last common ancestor."
"If we can identify some traits that are present in humans and not in chimpanzees, these are traits that must have evolved since we separated from our last common ancestor."
In a new study out today in Proceedings of the National Academy of Sciences, Gómez-Robles and her colleagues parsed the role of genetics and the environment in shaping the brains of both species. The researchers analyzed MRI scans of 218 humans and 206 chimpanzees, with an eye on brain size and organization—the latter of which they assessed by noting for each brain the location of 16 landmark anatomical structures that humans and chimps share. Because the researchers knew the respective biological relationships of both the chimps and the humans in the study, they were able to estimate the heritability—in other words, how much of a role genetics had to play—for both traits.
The team found that, for both humans and our primate cousins, brain size was largely determined by genetics. Genes also had a leading role in the brain organization of chimps. But in humans, cerebral anatomy was more strongly controlled by the environment, providing more evidence that human brains are uniquely flexible. It's this plasticity that gives rise to humans' characteristic adaptability.
"That is something that may facilitate things like cultural evolution," says Gomez-Robles. "If we know that the environment is really important in shaping the [large-scale] anatomy of the brain, we can infer that it's also going to be important in shaping the microstructure of the brain—in shaping the circuits of the brain—and eventually in shaping behavior."
So where does this plasticity come from? One explanation, according to Gómez-Robles, stems from the fact that, when humans are born, our brains are still in the throes of development. That's because, as our ancestors' brains evolved to become larger, humans had to be born before development was complete, to ensure that our growing brains and the heads that encapsulated them would fit through the birth canal. "What we've seen is that having a very plastic brain is the result of having a very big brain," she says.
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