Brain’s Evolutionary Explosion Linked to Ancient Retrovirus Infection

A new study has unraveled a mystery that is key to understanding the course of human and animal evolution. A team of genetic researchers from Altos Labs at the Cambridge Institute of Science in the United Kingdom explain how infection by an incredibly ancient form of retrovirus jumpstarted the process of animal evolution.

The Science Linking Retrovirus to Evolution

When retroviruses infect an organism, they will invade that organism’s cellular DNA and plant their own genetic material inside of it. The host organism’s genome may then begin to function differently, possibly causing that organism and its offspring to develop some entirely new characteristics.

In the study just published in the journal Cell, scientists reported their discovery of a section of DNA in the cells of humans and other vertebrates (animals with a spine or backbone) that help stimulate the production of myelin, a substance concentrated in the central nervous system that is strongly linked to the evolution of complex mental and physical traits. The scientists have been able to demonstrate that this section of DNA came from a retrovirus, and that it entered the collective genetic pools of mammals, amphibians and fish a long time ago.

“The cells got sick, and the cells thought — ‘We can use this sequence for our own purpose’,” is how study co-author Tanay Ghosh, a computational biologist at Alto Labs—Cambridge Institute of Science, described this monumental development in an interview with The Hill.

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In this case, the purpose was to activate previously existing sections of DNA that coded for myelin production. These DNA sequences evolved in the ancestors of modern mammals, amphibians and fish sometime before the retrovirus came along, surviving in a completely dormant or inactive state as they awaited activation. And when the right ancient retroviral infections occurred, suddenly mammals, amphibians and fish all developed the capacity to produce myelin.

The impact of myelin on the central nervous system of vertebrates was profound. It allowed for the proliferation of a dizzying variety of animal species that were increasingly fast, agile and smart, and suited to occupy different niches along an incredibly complex interconnected chain of predators and prey.

It is believed that myelin was first produced by a group of fish species known as placoderms, which lived on the earth approximately 425 million years ago, during the Silurian Period of the Paleozoic Era. From those humble beginnings myelin production eventually became a standard feature of vertebrates, where it spurred evolution in innumerable ways.

Fittingly, the 425-million-year-old DNA sequence has been given the name RetroMyelin, in honor of its astonishing and long-lasting effects.1

Representative image of a retrovirus. (BornHappy / Adobe Stock)

Revealing the Roots of the RetroMyelin Revolution

The scientists discovered RetroMyelin while examining the characteristics of oligodendrocytes, the cells that are tasked to produce myelin in the central nervous system. Investigating its purpose, they devised experiments where the RetroMyelin DNA sequence was inhibited, or prevented from acting, and when this was done the oligodendrocytes immediately lost their ability to manufacture myelin.

After they were able to confirm that RetroMyelin was retroviral material, they began looking for indications of it in the genomes of different animals. They discovered this DNA sequence was common to all and undoubtedly linked to myelin production in all vertebrates.

Myelin is an extraordinarily useful insulating substance. Made from a mixture of proteins and fatty acids, myelin forms a sheath or coating layer that protects all the parts of the nervous system, including the brain and the spinal cord. In addition to providing extra protection, the myelin sheath also acts as a conductor of electricity, causing the electrical signals produced in the nervous system to move more rapidly and efficiently between nerve cells.

Before the retroviral infection that stimulated myelin production, the nervous system of mammals was limited in what it could do. But the addition of a protective myelin coating made it possible for far more complex networks of neurons or nerve cells to emerge, and that allowed for the evolution of more advanced physical and mental abilities over time. Significantly, it also allowed for the evolution of entirely new species, creating a far more complex and diverse animal kingdom than would otherwise have been possible.  

“Retroviruses were required for vertebrate evolution to take off,” senior study author Robin Franklin, a neuroscientist the Cambridge Institute of Science, stated in a Cell Press media release. “If we didn't have retroviruses sticking their sequences into the vertebrate genome, then myelination wouldn't have happened, and without myelination, the whole diversity of vertebrates as we know it would never have happened.”

 

Retroviruses and Evolution: The Amazing Connection

Retroviruses are a special kind of viral agent. When they infect animal cells they insert their genetic material into the genomes of those cells, creating new DNA strands that will integrate with the existing DNA sequences. A creature infected by a retrovirus will then experience a permanent change in their DNA, which will now include sections that were introduced from the outside.

Fortunately, the cells of vertebrates (animals with backbones) are robust, meaning they can usually handle the added DNA without experiencing negative effects on health or functioning. Retroviral DNA  can damage a person or animal’s health, but in most cases its impact will be minimal.

There are some instances, however, when the newly acquired genetic material has beneficial effects on the functioning of the cells and the organisms that host them. The insertion of the retroviral DNA that triggered myelin production was one of those times, and in this instance the result was not just positive, but transformative.

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Retroviruses will initially insert their genetic material into individual vertebrates. But eventually the changes can be passed on to the entire gene pool of a species, as a result of “infected” members of the species interbreeding with others from the species and producing offspring that possess the retroviral genetic material. If the changes caused by the retroviral DNA are beneficial, natural selection will ensure they become more common over the passage of millions of years (or possibly more quickly if the survival benefits are significant).

This is what happened with RetroMyelin, which likely entered the mammal, amphibian and fish gene pools at different points between 425 and 200 million years ago (the first mammals appeared on earth a few million years before the latter date).

The study has implications for how scientists understand how ancient retrovirus infections could have affected evolution of the brain and other physical traits. (jolygon / Adobe Stock)

The Retroviral Invasion Continues

While the discovery of the link between retroviral activity and myelin production is important, it is not particularly surprising.

“Retrotransposons [sections of retrovirus DNA] compose about 40% of our genomes,” Tanay Ghosh noted, confirming that retrovirus infection has had a big impact on human evolution. Commenting on his team’s new study, he stated that “our motivation was to know how these molecules are helping evolutionary processes, specifically in the context of myelination.”

It should be noted that retroviral infection is still happening today, meaning that 40% figure could rise even higher in the future. The addition of more DNA through this astounding mechanism could cause the process of human evolution to continue indefinitely, and where that may ultimately lead our species is unknown and unknowable. 

Top image: Representative image of the impact of ancient retrovirus on evolution. Source: alexkich / Adobe Stock

By Nathan Falde