Humans and chimpanzees are remarkably similar, despite the major differences in how our societies have panned out.
After about 5-6 million years of evolution from a common ancestor, only 1.2 percent of our DNA is different.
A virus that infected chimpanzees (Pan troglodytes) – but not humans – about 4 million years ago is partly responsible.
This ancient retrovirus made a tiny tweak to chimp DNA that has led to some major differences in how our brains develop, according to a new study published in Science Advances today.
“We wanted to understand how human and chimpanzee brains could have evolved differently when so many of their protein-making genes are similar,” lead author Patricia Gerdes told ScienceAlert.
Gerdes is a molecular neurogeneticist at Lund University in Sweden.
“We knew that ancient viruses can leave DNA behind in the genome, and those remnants can affect nearby genes,” she said.
“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.”
Brain development is a known point of difference between chimpanzees and humans: For instance, our brains are about three times larger.
Perhaps the ancient virus could have even played a role in the parting of our species.
It is unclear why humans never contracted PTERV1 when so many of our ape relatives did, but there are a few theories.
Perhaps a unique antiviral protein, TRIM5α, protected us against it. Perhaps chimp and human ancestors lived in different places when the viral pandemic hit.
The new study doesn’t seek to answer that question, but it does reveal a specific effect the infection had on chimp brain development.
All those years ago, PTERV1 inserted DNA sequences into the chimpanzee genome that are still active today.
Both humans and chimpanzees have a molecule made from DNA called LINC00662, which provides our cells with instructions for building a particular protein.
“We found it active during early human brain development, and our experiments suggest it helps [human] brain cells mature and grow extensions,” Gerdes explained.
Chimpanzees have it too, and that’s exactly where the virus left its mark.
The virus’s DNA insertion switches off the chimpanzee’s LINC00662.
In experiments with human brain organoids (tiny clusters of cells grown in a lab as a research model), when the team switched off LINC00662, neurons and their extensions (the dendrites and axons that transmit information across the brain) did not mature and grow as prolifically as they do with LINC00662 intact.
“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 said.
An organoid experiment like this is a great way for scientists to get a quick preview of what might happen if, for example, PTERV1 one day made its way into the human genome to make a similar edit.
“If humans had the same insertion, it probably would have switched off LINC00662 and altered the development of brain cells,” Gerdes explained.
“But we cannot predict that this would produce a particular brain shape, ability, or behavior. The effect could also depend on the surrounding human DNA and other genes.”
And what if chimpanzees were freed from the ancient viral DNA that’s holding back the full potential of their version of LINC00662?
“In our experiment, removing the insertion from chimpanzee stem cells restored LINC00662 activity. That shows the insertion contributes to switching off this gene. It does not show that removing it would transform brain development,” Gerdes said.
“Brain differences arise from many genetic and developmental influences, and removing the insertion from an adult brain would not undo the brain’s earlier development.”
So, it seems there’s no risk of a sudden Planet of the Apes scenario.
But it does drive home just how influential viruses can be in the evolution of life on Earth, adding further evidence that retroviral insertions contributed to differences between humans and chimps.
Weird to think how different things could have been…
The research has been published in Science Advances.
This article was fact-checked by Fiona MacDonald and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.