An Ancient Pandemic Left a Gene Silent in Chimpanzees, Which May Offer Clues as to Why Their Brains Differ From Ours

An ancient virus left a silent gene in chimpanzees, potentially explaining why their brains differ from humans, according to a new study published in Science Advances. Roughly 5 million years ago, a retrovirus called PTERV1 infected their ancestors and left behind pieces of itself in their DNA, which humans did not inherit. Researchers found that one such leftover, the LINC00662 gene, sits inside a gene that keeps it turned off in chimpanzees, while in humans, it is turned on early in brain development and helps young nerve cells grow.

LINC00662's suppression in chimpanzees may contribute to the differences between their and human brains. The virus, a retrovirus that inserts itself into an animal's DNA, has at least 150 copies in chimpanzee cells, but why humans avoided infection remains unclear. One possibility is that early human ancestors either lived elsewhere or had a defense mechanism against the virus.

The study involved growing organoids, tiny balls of cells that mimic early brain development, from human and chimpanzee cell lines, revealing that the gene was turned on in human cells and off in chimpanzee cells. Removing the viral DNA from chimpanzee cells caused the gene to turn back on, supporting the connection between the leftover virus and the gene's state.

The gene's influence on nerve cell growth was observed in human organoids, but not in those with the gene turned off. Further research is needed to determine if other viral leftovers have similar effects on genes and if such DNA is linked to brain disorders. Both chimpanzees and bonobos carry the same viral DNA in the same spot, with the gene remaining turned off in their brain-cell models as well.