Ancient Virus in Chimpanzee DNA Silences Gene Active in Human Brain Evolution

When an ancient retrovirus infected chimpanzee ancestors roughly four million years ago, it inserted its genetic material into their DNA and triggered a defensive cellular lockdown known as methylation. That chemical silencing did more than neutralize the viral threat; it permanently switched off a neighboring gene called LINC00662, halting a critical driver of brain cell maturation.[1][4]

Because human ancestors never contracted the virus, the LINC00662 gene remained fully active in our lineage. This divergence at the regulatory level allowed human neurons to grow longer extensions and form denser networks, ultimately helping to build a brain that is roughly three times larger than that of a chimpanzee.[4][5]

The discovery, published in October 2026 in the journal Science Advances, provides a concrete mechanism for how human and chimpanzee brains evolved along such different paths. Humans and chimpanzees split from a common ancestor between five and six million years ago, and today the two species share 98.8 percent of their DNA.[1][4]

Researchers have long struggled to explain how such profound cognitive differences could arise from nearly identical genetic blueprints. The Lund University team hypothesized that ancient viral infections, which leave permanent DNA remnants in the host genome, might have altered how those shared genes are regulated.[1][2]

A Pandemic That Missed Humans

The virus in question, known as Pan troglodytes endogenous retrovirus-1 (PTERV1), swept through African ape populations millions of years ago. It generated at least 150 fixed insertions across the chimpanzee genome, making it one of the most abundant endogenous retroviruses in the species.[1][4][5]

Human ancestors escaped the PTERV1 pandemic entirely, leaving our genome free of the virus. Scientists theorize that early hominins either lived in a different geographic habitat when the outbreak occurred, or they possessed a unique antiviral protein, such as TRIM5α, that provided natural immunity.[4][5]

To understand the impact of this viral absence, the researchers focused on a specific PTERV1 insertion located on chromosome 19 in chimpanzees. They found that the viral sequence sits adjacent to LINC00662, a gene that produces a long noncoding RNA transcript involved in cellular development.[1][4]

"We wanted to understand how human and chimpanzee brains could have evolved differently when so many of their protein-making genes are similar," said Patricia Gerdes, a molecular neurogeneticist at Lund University and lead author of the study.[4]

"We knew that ancient viruses can leave DNA behind in the genome, and those remnants can affect nearby genes," Gerdes added. The team discovered that the chimpanzee's cellular machinery systematically silences the PTERV1 viral sequences through methylation to prevent the virus from replicating.[1][3][4]

However, this epigenetic silencing spreads beyond the viral DNA, inadvertently turning off the adjacent LINC00662 gene. To prove the virus was directly responsible for the gene's inactivity, the researchers utilized the CRISPR-Cas9 gene-editing system to manipulate chimpanzee induced pluripotent stem cells.[1][3]

Reversing the Evolutionary Clock

By precisely snipping out the ancient viral remnant from chromosome 19, the researchers prompted the chimpanzee cells to immediately resume expressing LINC00662. The experiment confirmed that the chimpanzee genome still possesses a fully functional copy of the gene, but it remains trapped behind a viral roadblock.[1][3]

The researchers then sought to observe the gene's function in real time by growing three-dimensional human and chimpanzee brain organoids. These miniature, lab-grown tissue clusters mimic the early stages of neural development, allowing scientists to track how roughly 2,500 differentially expressed genes shape the brain.[1][4]

In the human organoids, LINC00662 was strongly expressed in the cytoplasm of developing brain cells. "We found it active during early human brain development, and our experiments suggest it helps brain cells mature and grow extensions," Gerdes explained.[4]

Conversely, the gene remained completely dormant in the chimpanzee organoids. To simulate the chimpanzee condition in human cells, the team artificially silenced LINC00662 in the human organoids and watched how the neural networks responded to the sudden genetic deficit.[3][4]

The Architecture of Cognition

Without the active gene, the human neurons failed to mature properly. Their axons and dendrites—the vital extensions that transmit electrical signals across the brain—did not branch out or grow as prolifically, resulting in a significantly less connected neural architecture.[4]

The stunted growth mirrored the neural development seen in the chimpanzee organoids, demonstrating the gene's critical role in building complex brain structures. The findings underscore that evolutionary divergence often relies on which genes are turned on, rather than which genes are present.[3][5]

While the PTERV1 insertion offers a compelling explanation for neural differences, researchers caution that it is not the sole factor behind human cognition. The human brain's expansion involved a cascade of genetic and environmental shifts over millions of years.[4][5]

"We do not yet know all of LINC00662's roles in the human body, or what the loss of its activity means for chimpanzees as a whole," Gerdes noted. She added that predicting how a living chimpanzee might change if the gene were restored remains impossible.[4]

Ultimately, the study highlights the profound and sometimes accidental role that ancient pathogens played in shaping life on Earth. The cognitive complexity that defines the human species may owe a significant debt to a random viral immunity acquired millions of years ago.[2][5]