Evolution: How viruses gave rise to complex brains

Nerve fibers of humans and other vertebrates are covered with an insulating layer Myelin surround. This allows nerve stimuli to travel quickly and effectively over long distances. This was the only way vertebrates could grow to their current size over the course of evolution.

“Without myelin and the ability to conduct stimuli over longer distances, all terrestrial creatures today would be at most the size of our thumb,” explains the neuroscientist. Robin Franklin From Altos Labs Cambridge Institute of Science (UK) to science.ORF.at.

Complex nervous systems are possible

But what was even more important for vertebrate evolution was that the insulating layer of myelin meant that many thin nerve fibers could line up closely together without getting in the way of each other. Myelin was the basic requirement for the formation of highly interconnected nervous systems, and thus was also the basis for the development of complex brains.

Franklin, in collaboration with a British-French research team, has now searched for the exact reasons that stimulated myelin production in the ancestors of today's vertebrates. The result The team recently presented the study in the specialized journal “Cell”.

Viruses are crucial

The researchers examined gene activity in Oligodendrocytes – Cells that produce myelin in the body of vertebrates. They were particularly interested in non-coding regions, that is, parts of the genome that are not themselves converted into proteins, but some of which undertake important regulatory tasks.

In studies on mice, the team actually discovered such a non-coding section that regulates the production of myelin protein. If the researchers inhibited the corresponding area, the cells also stopped producing the protein.

Comparisons with other genetic sequences ultimately showed that this regulatory unit did not arise by chance. According to the research team's findings, it likely came from a prehistoric infection Retroviruses.

There is no myelin without viruses

Retroviruses work differently from other common pathogens because they integrate their own DNA into the vector's genome and can thus permanently alter it. “Over the course of evolution, vertebrates have repeatedly been infected with such viruses,” Franklin says. Virus-associated changes in the genome have occurred repeatedly, but many of the affected regions have lost function over time, while others remain active today.

This also applies to the genetic region that plays a crucial role in producing myelin and stimulating myelination, i.e. the lining of nerve cells. “If it weren't for retroviruses, myelin wouldn't exist today and life on Earth would look very different,” says Franklin.

minute. 400 million years ago

However, it is now difficult to determine when retrovirus infection first stimulated myelin production in vertebrates. In further studies, the team demonstrated that the required genetic region is found not only in mammals, but also in all other classes of jawed vertebrates. It was absent in jawless vertebrates and invertebrates.

Franklin therefore hypothesizes that the evolution of jaws occurred at approximately the same time as the beginning of myelin production in vertebrates. It is estimated that this was at least 400 million years ago.

Independent infections

The team also wanted to clarify whether myelin formation was caused by a single retrovirus infection or whether there were multiple contacts with the viruses responsible over the course of evolution. To do this, the researchers compared the genomes of 22 different vertebrate species.

It turns out that DNA sequences related to myelin production were genetically divergent in different classes of vertebrates. For the team, this was a clear indication that retrovirus infections that can still be detected today probably only occurred after different classes of vertebrates separated from each other. “This contradicts previous assumptions that myelin production can be traced back to a common ancestor of all vertebrates,” says Franklin.

Further effects of retroviruses

But retroviruses were not only important for the development of complex brains. In other studies, researchers were able to find evidence that some of the genetic regions altered by the viruses supported the formation of the placenta. Other similar areas are also involved in making male muscles grow faster.

On the other hand, there are also negative consequences of genetic regions altered by viruses. Some parts derived from retroviruses have been linked to an increased risk of cancer, dementia and multiple sclerosis in previous research. Franklin cites HIV (HIV).

According to the neuroscientist, it is also very likely that retroviruses will continue to have a significant impact on the genomes of humans and animals in the future: “We are constantly infected with retroviruses of some kind, and it would be very unusual for this to happen.” “It doesn't affect us in some way in the future that it would affect the genetic material.”