Why Are Our Brains So Big? Scientists Have a Few Possible Answers

Human brains are big. They account for about 2 percent of our body mass—a higher percentage than most other species—and they’re triple the size of those belonging to our common ancestor, the ape-like Australopithecines. While there’s no consensus on how and why our brains got to be so big, scientists have suggested a new theory that contradicts earlier ideas.

As phys.org reports, a study published in the journal Nature contains evidence that the human brain evolved in response to ecological stress. Faced with a harsh environment, the brain grew to help humans better survive and provide for their offspring, the scientists believe. Previously, studies suggested that the brain evolved as humans began to engage in more complex social interactions. The researchers behind this new study say their findings suggest otherwise.

“The findings are intriguing because they suggest that some aspects of social complexity are more likely to be consequences rather than causes of our large brain size,” Mauricio González-Forero, the paper’s co-author and a mathematical evolutionary biologist at Scotland’s University of St. Andrews, tells phys.org. “The large human brain is more likely to stem from ecological problem-solving and cumulative culture than it is from social maneuvering.”

They reached this conclusion by using computer software to simulate evolution. Data relating to how much energy an adult human female needs to support the brain was plugged in, as well as energy requirements relative to body size, the Indiana Gazette reports.

Overall, researchers determined through their model that the main challenges that spurred brain growth were overwhelmingly (60 percent) ecological in nature, but it also may have resulted from cooperation (30 percent) and competition between groups (10 percent). Researchers said the ability to acquire new skills is also a crucial component when considering how human brains evolved.

“Ecological challenges can only lead to brain size of this scale if they are coupled with the ability to continue learning from others,” González-Forero said. “The paper is suggesting that human brain development is an interaction between these two things—ecological pressures and culture.”