Viruses may be minuscule, but one could have influenced major differences between human and chimpanzee brains.

Millions of years ago, a virus left DNA behind in an African ape’s genome, and the genetic material stuck around in modern day chimps. Those ghosts of infections past appear to influence genes involved in neuron development, which may help explain how human and chimp brains ended up so different, researchers report October 23 in Science Advances .

Compared with the brains of their chimpanzee kin, human brains are three times larger and more cognitively powerful. Yet “most of their protein-making genes are very similar,” says Johan Jakobsson, a neuroscientist at Lund University in Sweden. In the absence of large differences in the genetic code, Jakobsson and his colleagues decided to look at the parts of the human and chimp genome that control when and where genes are turned on or off.

Changes in gene regulation can come from viruses.…