Ancient viruses gave us our brains.

“Language is a virus,” said William S. Burroughs. Now, Science Daily reports that researchers at Altos Labs-Cambridge Institute of Science have found that intelligence itself may be a viral byproduct – because it was ancient retroviruses that created the genetic sequence necessary for prehistoric vertebrates to grow 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.

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. Specifically, the team was investigating the role of noncoding regions including retrotransposons in these gene networks — something that hasn’t previously been explored in the context of myelin biology.

“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.”

In rodents, the researchers found that the RNA transcript of RetroMyelin regulates the expression of myelin basic protein, one of the key components of myelin. When they experimentally inhibited RetroMyelin in oligodendrocytes and oligodendrocyte progenitor cells (the stem cells from which oligodendrocytes are derived), the cells could no longer produce myelin basic protein.

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.

You can read more of the RetroMyelin research here, in Cell.