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

An ancient virus that infected vertebrates billions of years ago played a crucial role in the evolution of our advanced brains and large bodies, a new study suggests.

The work, published Thursday in the journal cellExplore the origins of myelin, an insulating fatty membrane that forms around nerves and allows electrical impulses to travel faster.

Importance in evolution

According to the authors, the genetic sequence derived 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. “The most remarkable thing to me is that all this diversity of known modern vertebrates, and the sizes they have reached – elephants, giraffes, anacondas… – would not have happened” Without infection with this retrovirus, study co-author neuroscientist Robin Franklin 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 mysterious regions. “Non-Coding” of the genome, which has no apparent function and was once considered useless, but is now recognized as having evolutionary significance.

Their research led to a sequence from a retrovirus, which has long been in our genes, and which the researchers called “Retromyelin”. To test their findings, they conducted experiments involving the deletion of this sequence in mice, and observed that they then no longer produced the protein required for the formation of myelin.

Faster reactions, bigger body

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

The study was described as“Attractive Lighting” on the ancestral history of our jaws by Brad Zucchero of Stanford University, who was not involved in this work. “There has always been a selective pressure for nerve fibers to conduct electrical impulses faster”Robin Franklin underlined. “By doing this quickly, you can then act quickly”This, he explained, is useful for hunters chasing prey, or trying to escape prey.

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

Viruses are not just bad

Finally, the team of researchers wanted to understand whether the viral infection occurred once, in a single ancestral species, or multiple 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 think of viruses as pathogens, disease-causing agents”, noted Robin Franklin. But the reality is more complex, 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—a feature of most mammals—acquired from a pathogen integrated into the genome long ago.

For Tanay Ghosh, this discovery on myelin may be just the first step in an emerging field. “Much remains to be understood about how these sequences influence various evolutionary processes.”he said.