An ancient viral infection left genetic material in ancestral ape DNA that permanently silenced a key human-active gene, according to a study published on October 23, 2026 in Science Advances. The finding offers a new explanation for why chimpanzee and human brains evolved along drastically different paths.
How Retroviruses Rewrote the Primate Genome Millions of Years Ago
About 7 million years ago, the evolutionary paths of humans and chimpanzees diverged from a common ancestor. Despite that split, our modern genetic code remains astonishingly similar. Humans and chimpanzees have only 1.2 percent of their DNA different.
Yet the physical and cognitive outcomes could not be more distinct. The human brain is roughly three times larger than that of a chimpanzee, fueling complex thought, language, and advanced technology. Retroviruses like HIV do not replicate on their own; they hijack cellular machinery by slipping viral RNA into host cells, where it gets reverse-engineered into DNA. When this process occurs in germline cells, those viral snippets pass down through generations.
Those leftover bits—known as endogenous retroviruses—make up at least 8 percent of the genomes of both humans and chimpanzees. While many remain dormant, they often alter how nearby genes are regulated.
Viral Insertions Alter Chimpanzee DNA
A team led by researchers at Lund University in Sweden turned their attention to a specific retroviral remnant called PTERV1. This ancient pathogen infected African apes about 4 million years ago, inserting viral sequences in the chimpanzee genome. Curiously, PTERV1 is entirely absent in humans.
Molecular neurogeneticist Patricia Gerdes and her colleagues note that our ancestors may have lived in a different habitat during the pandemic, or perhaps a unique antiviral protein known as TRIM5α protected human lineages from infection.
PTERV1 (Pan troglodytes endogenous retrovirus-1) was a particularly interesting candidate because it is found in chimpanzees and other African apes but not in humans.
Patricia Gerdes, Lund University, via Yahoo
Within chimpanzee DNA, these repetitive viral sequences are heavily modified by methylation, an epigenetic silencing mechanism that controls whether specific genes turn on or off. The research team set out to discover what happened when those viral insertions went to work on neighboring genetic instructions.
Stem Cell Organoids Reveal Gene Expression Differences
To test how the viral remnant impacts neural growth, the researchers grew three-dimensional miniature brain organoids from human and chimpanzee stem cells. Comparing these lab-grown tissues revealed a contrast in gene expression.
A molecule made from DNA called LINC00662 was active in human organoids, providing instructions that help neurons mature and grow extensions. But in chimpanzee organoids, LINC00662 was completely shut down. The PTERV1 viral insertions blocked its genetic readout.
The gene was strongly expressed in the human organoids, but not at all in the chimpanzee organoids.
Patricia Gerdes, postdoc in molecular neurogenetics at Lund University, via Mirage News
To confirm that the retroviral DNA was directly responsible for silencing the gene, the researchers used CRISPR gene-editing technology to remove the viral sequences from chimpanzee cells. Once the viral remnants were gone, LINC00662 sprang back to life. Conversely, when the team used CRISPR to silence LINC00662 in human brain organoids, the activity of genes tied to nerve-cell development dropped significantly.
Experts Weigh In on Evolutionary Divergence
Independent experts greeted the findings as a compelling demonstration of how viral insertions can drive evolutionary divergence, though they urge caution regarding how far the conclusions extend. Kanda, who was not involved in the project, noted that the team definitively proved the viral leftovers influence developmental differences in neurons, adding that at the moment we’ve only touched the tip of the iceberg
regarding undiscovered retroviruses.
The study does not suggest that removing viral insertions from an adult chimpanzee brain would alter its development or trigger dramatic behavioral changes. Brain architecture relies on countless genetic and environmental influences spanning early development.
Why humans avoided the virus while our closest ape relatives contracted it remains an open question.