Ancient viruses played a role in the advanced development of our brains, study finds

Ancient viruses that infected vertebrates hundreds of millions of years ago played a crucial role in the evolution of our advanced brains and large bodies, a new study suggests.

This work, published Thursday in the journal Cell, examines the origins of myelin, an insulating fatty membrane that forms around nerves and allows electrical impulses to be distributed more quickly.

According to the authors, a genetic sequence acquired from retroviruses – viruses that invade their host's DNA – is crucial for myelin production. And this code is found today in modern mammals, amphibians and fish. “What I find most remarkable is that all this diversity of known modern vertebrates, and the sizes they have reached – elephants, giraffes, anacondas… – would not have happened” without the infection of these retroviruses, neuroscientist Robin Franklin, co-author of the study, told AFP.

Researchers searched genome databases to try to uncover genetic factors associated with myelin production. Tanay Ghosh, a biologist and geneticist working with Mr. Franklin, was particularly interested in the mysterious "non-coding" regions of the genome, which have no apparent function and were at one time considered useless, but which are now recognized as having importance in evolution.

His research resulted in a sequence derived from a retrovirus, which has long been in our genes, and which the researchers named “RetroMyelin”. To verify their discovery, they carried out experiments consisting of deleting this sequence in rats, and observed that they then no longer produced a protein necessary for the formation of myelin.

The scientists then looked for similar sequences in the genomes of other species, and found similar code in jawed vertebrates - mammals, birds, fish, reptiles and amphibians - but not in jawless vertebrates or invertebrates. They concluded that the sequence appeared in the tree of life around the same time as jaws, that is, around 360 million years ago.

The study was called a “fascinating insight” into the history of our jawed ancestors by Brad Zuchero of Stanford University, who was not involved in the work. “There has always been a selection pressure to make nerve fibers conduct electrical impulses more quickly,” emphasized Robin Franklin. “By doing that faster, then you can act faster,” he explained, which is useful for predators chasing prey, or prey trying to flee.

Myelin allows rapid conduction of these signals without increasing the diameter of nerve cells, allowing them to be brought closer together. It also provides structural support, meaning the nerves can grow further, allowing the development of larger limbs. In the absence of myelin, invertebrates have found other ways to transmit electrical signals quickly: giant squid, for example, are equipped with larger nerve cells.

Finally, the team of researchers wanted to understand whether the viral infection had occurred once, in a single ancestral species, or several times. To answer this question, they analyzed RetroMyelin sequences from 22 species of jawed vertebrates. These sequences were more similar within a species than between different species. This suggests that multiple waves of infection occurred, contributing to the diversity of vertebrate species known today, according to the researchers.

“We tend to think of viruses as pathogens, agents causing disease,” noted Robin Franklin. But the reality is more complicated, he says: at different times in history, retroviruses entered the genome and integrated into the reproductive cells of species, allowing them to be passed on to subsequent generations. One of the best-known examples is the placenta - characteristic of most mammals - acquired from a pathogen integrated into the genome a long time ago.

For Tanay Ghosh, this discovery on myelin could only be a first step in an emerging field. “There is still a lot to understand about how these sequences influence different evolutionary processes,” he said.