Ancient viruses caused big brains and bodies

Washington – Ancient viruses that infected vertebrates hundreds of millions of years ago played a pivotal role in the evolution of our advanced brains and large bodies, a study said on Thursday.

The research, published in the journal Cell, examined the origins of myelin, an insulating layer of fatty tissue around nerves that allows electrical impulses to travel faster.

According to the authors, a gene sequence acquired from retroviruses – viruses that invade their host’s DNA – is crucial for myelin production, and that code is now found in modern mammals, amphibians and fish.

“All of the diversity of modern vertebrates and the size they’ve achieved: elephants, giraffes, anacondas, bullfrogs, condors wouldn’t have happened,” senior author and neuroscientist Robin Franklin of Altos Labs-Cambridge Institute of Science said.

A team led by Tanay Ghosh, a computational biologist and geneticist in Franklin’s lab, trawled through genome databases to find the genetics associated with the cells that produce myelin.

Specifically, he was interested in exploring mysterious “non-coding regions” of the genome that have no obvious function and are now recognised as having evolutionary importance.

Ghosh’s search landed upon a particular sequence derived from an endogenous retrovirus, long lurking in our genes, which the team dubbed “RetroMyelin”.

To test their finding, researchers knocked down the RetroMyelin sequence in rat cells, and found they no longer produced a basic protein required for myelin formation.

Next, they searched for RetroMyelin-like sequences in the genomes of other species, finding similar code in jawed vertebrates – fellow mammals, birds, fish, reptiles and amphibians – but not in jawless vertebrates or invertebrates.

This led them to believe the sequence appeared around the same time as jaws, which first evolved around 360 million years ago in the Devonian period, called the Age of Fishes.

“There’s always been an evolutionary pressure to make nerve fibres conduct electrical impulses quicker,” said Franklin. “If they do that quicker, then you can act quicker,” he added, which is useful for both predators and prey.

Myelin also provides structural support, meaning nerves can grow longer, allowing for longer limbs.

Finally, the team wanted to learn if the retroviral infection happened once, to a single ancestor species, or more than once. They found the sequences were more similar within than between species, which suggested multiple waves of infection. –