Compared with most other animals, chimpanzees are incredibly intelligent: They work with tools, communicate with complex vocalizations, and are good problem-solvers. But as smart as chimps are, their brain power pales in comparison with our own. A multitude of factors help makes the human brain superior to the chimps’, but new research indicates that looser genetic control of brain development in humans allows us to learn and adapt to our environment with more flexibility than our primate cousins.
The neocortex—the outermost layer of the brain characterized by the squiggly sulci, or brain folds—is the region that gives all primates their exceptional intelligence. In both chimps and humans, this brain region continues to grow and organize for years after birth, allowing us to learn and develop socially. The brain’s ability to reorganize in response to environmental cues is known as plasticity, and it is this flexibility that allows us to learn things we never knew at birth; how to tie our shoes, for example, or do calculus problems. Chimps demonstrate plasticity when they pick up things like cooperative grooming practices. But new research, published online today in the Proceedings of the National Academy of Sciences, suggests that genetics dictate the organization of a chimp’s brain much more rigidly than in humans, allowing the environment to play a larger role in our neural development.
“I can appreciate these results because they illustrate and reinforce, using comparisons of closely related species, something that has been doubted by some evolutionary biologists in the past, namely the fact that plasticity itself can evolve,” says Smithsonian Tropical Research Institute theoretical biologist Mary Jane West-Eberhard, who was not involved in the study.
To find out how genetics may shape the brain, scientists at the George Washington University (GWU) in Washington, D.C., examined the brains of 218 humans and 206 chimpanzees using MRI scans. Crucially, the researchers had access to detailed family trees for the chimpanzees, allowing them to measure similarity in the brains of genetically related individuals. Similarly, MRI scans from related humans—including twins—were intentionally selected from a database for the analysis. Researchers measured differences in brain size and in sulci shape and location—factors that have been shown to reflect the underlying cortical organization of the brain in previous studies.
In terms of size, there wasn’t much difference in how brains varied among family members: Closely related individuals of both species tended to have very similar brain volumes. However, the sulci told a different story: Closely related humans had considerably more variation in shape and placement of the squiggly grooves in their cortexes than did chimpanzees. Two chimp brothers would have more similar sulci than two human brothers, for instance. This means that chimps have greater limitations on the ways in which their brains can develop and on their capacity to learn new behaviors or skills compared with humans.
With genetics taking a back seat in humans, our brains are more susceptible to external influences. This allows the environment, experience, and social interactions with other individuals to play a more dramatic role in organizing the cerebral cortex. And this increase in plasticity may very well be one of the defining features that propelled our hominid ancestors past other primates in terms of intelligence, says GWU anthropologist Aida Gómez-Robles, the study’s lead author. The team also speculates that being born with underdeveloped brains may contribute to our increased neural plasticity. Relative to newborn chimpanzees, human babies are born with less developed brains, rendering us more helpless, but allowing more brain growth to occur postnatally, where the outside world can play a larger role.
The scientists also point out that this pattern of delayed development appears to have increased over evolutionary time, with our hominid ancestors presumably slowly gaining larger, more plastic brains relative to modern chimpanzees. The proof for this comes from fossil evidence, which shows that the neocortex expanded and reorganized over time in early hominins.
The new results indicate that as brains grew larger and entered the world in a less developed state, it became increasingly advantageous to relax the genetic control of their organization, essentially providing a bigger, blanker canvas for adapting and learning. This greater ability to fashion our brains in response to our environment, the study maintains, could provide a link between biological evolution and cultural evolution.