A Virus Switched Off a Brain Gene in Chimpanzees
Humans never caught it. The gene it silences in chimps is still speaking in us.
Humans and chimpanzees share more than 96 percent of their protein-coding DNA. Brains diverged anyway. Johan Jakobsson's group at Lund says part of the split is a virus the other lineage caught and we did not.
PTERV1 invaded the germline of African ape ancestors about 5 to 7 million years ago. Chimpanzees carry at least 150 insertions. The human genome has none. One insertion switches off LINC00662 in chimpanzees. The same gene is active in early human fetal brain development. In human brain organoids, silencing it weakened the staining for young neurons where new cells extend fibres, at the 30-day mark. Genes involved in axon and dendrite growth became more active, which the team reads as a possible compensation. Sci.News carried the Lund release on 28 October. ScienceAlert's 23 October account names Science Advances.
The Rational Explanation
Organoids are not brains. A gene that matters in a dish at 30 days does not explain a threefold difference in brain size. The paper's language is contributor, not cause.
What We Don't Know
How much of the real developmental split this one silenced gene accounts for. The weird step is already solid: the difference is an absence. We did not gain a miracle gene. They caught a virus, and a switch went quiet.