Humans and chimpanzees share more than 96% of their protein-coding DNA, yet the human brain is about three times larger and considerably more complex. A study led by scientists at Lund University suggests that one factor in this divergence may be a retroviral infection that entered the ancestors of chimpanzees millions of years ago but did not affect the human lineage.
A retrovirus found in chimpanzees
“Over the past five to seven million years, humans and chimpanzees have diverged in brain size, structure, and cognitive abilities, despite a high degree of conservation of protein-coding genes,” said the study’s lead author, Professor Johan Jakobsson of Lund University.
“The majority of the primate genome sequence is made up of transposable elements, one major family of which is the endogenous retroviruses, which have expanded throughout primate evolution,” Jakobsson said.
At least 8% of both the human and chimpanzee genomes consists of retroviral sequences. The researchers focused on one such retrovirus, called PTERV1.
PTERV1 entered the germ line of the ancestors of African great apes about 5 to 7 million years ago, leaving copies of its genetic code behind. The chimpanzee genome contains at least 150 PTERV1 insertions, while the human genome contains none.
“The virus we studied particularly affected chimpanzees,” Jakobsson said. “But humans and chimpanzees have almost the same genetic makeup, which is why it’s important to understand the differences in how genes are regulated.”
Viral DNA and gene regulation
The researchers grew small clusters of developing brain tissue, known as neural organoids, from human and chimpanzee stem cells. Using long-read DNA sequencing, they found that nearly all of the PTERV1 insertions they identified in the chimpanzee genome were heavily marked with methyl groups. DNA methylation is commonly associated with gene silencing.
The scientists then searched for genes containing PTERV1 insertions that behaved differently between the two species. One prominent candidate was LINC00662, a gene that produces a long non-coding RNA.
LINC00662 was highly active in human organoids but silent in chimpanzee organoids. In chimpanzees, the approximately 7,000-letter viral insertion lies within the gene, and the surrounding DNA is highly methylated.
The insertion appears to be present in chimpanzees and bonobos, but not in the other great apes examined in the study.
“This gene was strongly expressed in human organoids, but not at all in chimpanzee organoids,” said first author Patricia Gerdes, a postdoctoral researcher at Lund University.
In human cells, LINC00662 was most active during early fetal brain development. When the researchers silenced the gene in human organoids, the tissue showed weaker staining 30 days later for markers of young neurons in regions where new neurons were growing fibers.
Genes involved in axon and dendrite growth became more active after the gene was silenced. The researchers interpreted this change as a possible compensatory response.
Implications for brain evolution
“Our findings reveal a mechanism by which endogenous retroviral insertions contribute to human and chimpanzee brain divergence,” the scientists concluded.
The study was published on October 23 in Science Advances.
Patricia Gerdes et al. 2026. “Retroviral insertions contributed to the divergence of human and chimpanzee brains.” Science Advances 12(43). doi: 10.1126/sciadv.aeg5791.
Source: www.sci.news
