via Psychology Today:

‘A retroviral infection is responsible for evolution of the human brain from chimpanzee ancestors. The viral DNA regulates genes that drive brain development. The viral DNA is not a gene, so it eluded researchers looking for genes responsible for human brain evolution…’

The viral pandemic, between 5-7 million years ago, may be the cause of heretofore unexplained differences between the human and chimpanzee brain. Despite sharing 99% of our genome with chimpanzees, our closest evolutionary ancestors, it has been a source of puzzlement that the human brain is roughly three times larger. The new study identified a viral DNA fragment in the chimpanzee but absent in humans that appears to impair genes vital to brain development. Researchers believe that an ancient retrovirus inserted this genetic material into the chimpanzee genome while humans were protected by an antiviral protein the chimpanzees lacked, conferring a strong evolutionary advantage.