In a major breakthrough in evolutionary research, scientists have identified for the first time the gene responsible for humans’ large brain size. While it has long been accepted that higher bran function in humans developed from neurological differences, this is the first time that scientists have been able to pinpoint a specific genetic cause.

Researchers recently identified the human-accelerated regulatory enhancer – or HARE5 for short – by inserting fragments of human DNA into mice. There are 16 major differences in the genetic sequence between humans and our closest genetic relatives, chimpanzees, making it extremely difficult to identify which genetic sequence controls the growth and complexity of the brain (human brains are, on average, about three times heavier than that of a chimp, and its interconnections much more intricate). Mice, however, possess a completely different gene sequence, allowing researchers to isolate the purposes of individual sequences when human DNA is introduced.

The research revealed that the brains of mice implanted with the human DNA grew larger and developed more quickly than those of untreated mice. The gene worked by regulating the number of neural stem cells produced by the mouse embryo, ultimately increasing the number of neurons produced in the brain.

Study author Debra Silver, an assistant professor of molecular genetics & microbiology at the Duke University Medical School, said that the research findings could be crucial to understanding how human evolution has differed from that of other animals, and the roles that various parts of our DNA played in that change.

However, there are doubts and fears around the experiments, which have given rise to speculation of eventual results resembling something out of Planet of the Apes. “The prospect of, sort of, tearing down the barriers between humans and other nonhuman species in ways that really threaten our sense of ourselves as special is disturbing,” said Ruth Faden, director of the Johns Hopkins Berman Institute of Bioethics. “One can never say never, but I think it’s a pretty long-shot, far-fetched type concern.”