Ancient viruses that infected vertebrates hundreds of millions of years ago played a key role in the evolution of our developed brains and large bodies, says a study cited by AFP.
The study, published in the journal Cell, looked at the origin of myelin, an insulating layer of fatty tissue that forms around nerves and allows electrical impulses to propagate faster.
According to the authors, a gene sequence acquired from retroviruses - viruses that invade the DNA of their host - is crucial for myelin production, and this code is now found in modern mammals, amphibians and fish.
"What I find most remarkable is that the whole variety of modern vertebrates that we know and the sizes that they have reached: elephants, giraffes, anacondas, bulls, condors, would not have happened," said senior author and neuroscientist Robin Franklin of Altos Labs-Cambridge Institute of Science.
In the new study, led by Tanai Ghosh, a biologist and geneticist in Franklin's lab, analysts searched genomic databases to try to find the genes likely linked to myelin-producing cells.
Specifically, he is interested in exploring the mysterious "noncoding regions" of the genome that have no obvious function and were once dismissed as "junk" but are now recognized as having evolutionary significance.
Ghosh's search came across a particular sequence originating from an endogenous retrovirus that has long lurked in our genes, and which the team dubbed "RetroMyelin".
To test their discovery, the researchers conducted experiments in which they removed the RetroMyelin sequence in rat cells, and found that they no longer produced an essential protein needed for myelin formation.
They then looked for sequences similar to RetroMyelin in the genomes of other species, finding a similar code in jawed vertebrates - mammals, birds, fish, reptiles and amphibians - but not in jawless vertebrates or invertebrates.
This leads them to believe that the sequence appeared in the tree of life at the same time as jaws, which first evolved about 360 million years ago during the Devonian period called the Age of Fishes.
"There has always been evolutionary pressure for nerve fibres to conduct electrical impulses faster. If they do it faster, then you can act faster," he added, which is useful for both predators trying to catch something and prey trying to escape," Franklin said.
Myelin allows for the rapid conduction of impulses without expanding the diameter of nerve cells, allowing them to be spaced closer together.
It also provides structural support, which means nerves can grow longer, allowing for longer limbs.
In the absence of myelin, invertebrates have found other ways to transmit signals faster - giant squid, for example, have evolved wider nerve cells.
Finally, the team wanted to find out whether the retroviral infection was a one-off, in a single ancestral species, or if it occurred more than once.
To answer this question, they used computational methods to analyze the retromyelin sequences of 22 species of jawed vertebrates, finding that the sequences were more similar within a species than between species.
The discovery suggests that multiple waves of infection have led to the diversity of vertebrate species we see today, the team says.
"Humans tend to think of viruses as pathogens or disease-causing agents," Franklin says.
But the reality is more complex, he says: at various points in history, retroviruses have entered the genome and integrated into a species' reproductive cells, allowing them to be passed on to subsequent generations.
One of the most famous examples is the placenta - one of the defining characteristics of most mammals - which we acquired from a pathogen embedded in our genome in the distant past.
Ghosh said the discovery of myelin could be just another step in an evolving field. "There's still a lot that needs to be understood in terms of biology, about how these sequences drive different processes of evolution," he said. / BGNES
