Over the past few million years, the brains of humans and chimpanzees have taken different paths, growing in size, complexity, and abilities despite sharing most of their genes. A preprint in BioRxiv suggests this split may trace back to an ancient viral infection that struck chimpanzee ancestors but spared humans, inserting genetic material that silenced a key RNA molecule involved in brain development.
“Our results support a scenario where PTERV1 infection and endogenization had two effects that influenced hominid speciation,” the researchers write in their report. They point out that the virus not only caused a population bottleneck in early chimpanzees but also left behind genetic changes that altered how brain cells mature.
For years, scientists have puzzled over why human and chimpanzee brains differ so much when their protein-coding genes are nearly identical. The team turned to lab-grown brain models from both species to explore this. They discovered that a family of retroviruses, called PTERV1, invaded the chimpanzee genome around 5 to 7 million years ago, adding more than 150 insertions absent in humans. These viral remnants attract a chemical tag known as DNA methylation, which packs them into silent, inactive regions.
One such insertion landed in a stretch of DNA that controls a long noncoding RNA called LINC00662. In chimpanzees, this blocks the RNA’s production, but in humans—free of the virus—LINC00662 thrives during early brain growth. To test the link, the scientists used gene-editing tools to remove the viral insertion from chimpanzee cells, restoring the RNA’s activity and confirming the virus’s role in the shutdown.
LINC00662 resides in the cell’s outer compartment, where it interacts with proteins that guide the growth of neuron extensions, helping young brain cells connect and mature. When the team silenced this RNA in human brain models, they saw reduced outgrowth of these extensions, along with shifts in genes tied to neuron development—hinting at a feedback loop to compensate for the loss.
The findings reveal how viral remnants can reshape gene activity over generations. “In summary, this study describes how retroviral insertions contributed to the functional divergence of the human and chimpanzee brain, providing a new mechanism by which retroviral infections influenced primate brain speciation,” the authors conclude. While the full impact on brain evolution remains to be explored, it underscores how past infections may have helped forge the unique human mind.
Citations
Johan Jakobsson et al. Retroviral insertions contributed to the divergence of human and chimpanzee brains. BioRxiv. Published online December 12, 2025. DOI: 10.64898/2025.12.12.693858
