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> Including a retrovirus:
> 
> https://www.sciencedaily.com/releases/2024/02/240215113551.htm
> 
> "Researchers report February 15 in the journal Cell that ancient viruses may be to thank for myelin -- and, by extension, our large, complex brains. The team found that a retrovirus-derived genetic element or "retrotransposon" is essential for myelin production in mammals, amphibians, and fish. The gene sequence, which they dubbed "RetroMyelin," is likely a result of ancient viral infection, and comparisons of RetroMyelin in mammals, amphibians, and fish suggest that retroviral infection and genome-invasion events occurred separately in each of these groups.
> 
> "'Retroviruses were required for vertebrate evolution to take off," says senior author and neuroscientist Robin Franklin of Altos Labs-Cambridge Institute of Science.
> 
> "'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."
> 
> "Myelin is a complex, fatty tissue that ensheathes vertebrate nerve axons.
> 
> "It enables rapid impulse conduction without needing to increase axonal diameter, which means nerves can be packed closer together.
> 
> "It also provides metabolic support to nerves, which means nerves can be longer.
> 
> Myelin first appeared in the tree of life around the same time as jaws, and its importance in vertebrate evolution has long been recognized, but until now, it was unclear what molecular mechanisms triggered its appearance.
> 
> "The researchers noticed RetroMyelin's role in myelin production when they were examining the gene networks utilized by oligodendrocytes, the cells that produce myelin in the central nervous system.
> 
> ***
> 
> "'Retrotransposons compose about 40% of our genomes, but nothing is known about how they might have helped animals acquire specific characteristics during evolution," says first author Tanay Ghosh, a computational biologist at Altos Labs-Cambridge Institute of Science.
> 
> "'Our motivation was to know how these molecules are helping evolutionary processes, specifically in the context of myelination."
> 
> ***
> 
> "To examine whether RetroMyelin is present in other vertebrate species, the team searched for similar sequences within the genomes of jawed vertebrates, jawless vertebrates, and several invertebrate species.
> 
> "They identified analogous sequences in all other classes of jawed vertebrates (birds, fish, reptiles, and amphibians) but did not find a similar sequence in jawless vertebrates or invertebrates.
> 
> 
> ***
> 
> "The analysis revealed that RetroMyelin sequences were more similar within than between species, which suggests that RetroMyelin was acquired multiple times through the process of convergent evolution.
> 
> "The team also showed that RetroMyelin plays a functional role in myelination in fish and amphibians.
> 
> ***
> 
> "The study highlights the importance of non-coding regions of the genome for physiology and evolution, the researchers say. "Our findings open up a new avenue of research to explore how retroviruses are more generally involved in directing evolution," says Ghosh."
> 
> Comment:  more evidence that viruses helped necessary evolutionary steps, which is more evidence viruses were necessary organisms to be on Earth. The question of why some are dangerous is answered by noting they are dangerous only if they wander into wrong places.