New studies have revealed that ancient viruses, which became a part of the DNA of jawed vertebrates, played a vital role in the development of myelin. Myelin allows for swift impulse conduction without the need to increase the diameter of nerve cells. This made it possible for nerves to be packed together more closely and grow longer, which was beneficial for both larger body sizes and more complex brains.
https://www.sciencenews.org/article/ancient-retrovirus-myelin-speedy-nerves-evolve
Ancient viruses helped speedy nerves evolve
A retrovirus embedded in the DNA of some vertebrates helps turn on production of a protein needed to insulate nerve cells, aiding speedy thoughts.Tina Hesman Saey (Science News Magazine)