How an ancient virus complexed our brains | Top Vip News

One study found that an ancient viral infection may have given animals the tools to become fast, coordinated and intelligent.

According to a paper published Thursday in Cell, complex nervous systems emerged in the distant past after viruses inserted fragments of code into the genomes of vertebrates, animals with spinal cords, from humans to frogs and salmon.

In itself, this “invasion” is nothing special; Inserting such a code is the main way that viruses, which have no ability to replicate without the support of a protective cell, force cells to do their bidding.

But in this case, the cells used the new code for their own purposes, a dynamic that scientists have also found at the root of central animal activities such as fertilization and pregnancy.

“The cells got sick and thought, ‘We can use this sequence for our own purpose,'” said co-author Tanay Ghosh of the Cambridge Institute of Science.

The new injected code fragments helped guide cellular machinery to produce myelin, a protective sheath around nerve cells that helps speed up the transmission of electrical signals by which our nervous system functions.

Myelin in our nervous system works much like the plastic insulation that covers a fiber optic cable: by blocking the ability of a signal to escape through the walls of a cable (or a nerve fiber), it allows that signal to transmitted faster and with fewer errors. .

In evolutionary terms, this property allows for other powerful effects.

Because myelin allows nerves to transmit more quickly, it also allows for new forms of simultaneous communication. And that allowed the evolution of complex neural networks that have more connections and more interactions within a given amount of space. (Although not all nerve cells have myelin sheaths, those that do, particularly in the white matter of the brain and spinal cord, are found in areas where the speed and density of connections are crucial.)

Without those faster signals, Ghosh said, “all the mechanisms of predators and prey, all that enormous diversity, would not have developed.”

The team’s research found that infection of ancestral vertebrates by myelin-encoding viruses likely occurred many times, as the closely related virus family modified the genomes of the ancestors of extant fish, amphibians and mammals, each of which reused the new lines of code. to build complexity.

This required a complex evolutionary dance. Viral infection did not encode myelin production; another mutation did it. Instead, it helped proteins that read and interpret the genome bind to the precise region where the instructions for myelin can be found.

Scientists know this because some simple vertebrates, like the sea lamprey, have the myelin mutation but don’t have that extra part of the viral genome. And the sea lamprey’s comparatively simple nervous system also lacks myelin. Ghosh compares this primordial nervous system to an orchestra waiting to start playing. “All the musical instruments were there, but they needed the trigger. “The violins… or the viruses.”

These ancient viruses were not intended to change the structure of their hosts, Ghosh emphasized. Instead, the way this evolutionary concert unfolded shows something about cells that laypeople often overlook.

“Cells are smart,” he said. “They have many mechanisms that we do not understand; We don’t know how they do it all. “Sometimes we say they are too smart for us.”

In a very real sense, the word “cell” (derived from the discovery in the 17th century that plant and animal tissues were made of what appeared to be tiny boxes) does not really capture the complexity of how cells interpret and react to each. aspect. of their environments. A box of tiny molecules and organs packed into a microscopic envelope of fat is not enough to form a cell, Ghosh said. “You need to have a lot of other things.”

This complexity manifests itself in a wide range of domains: in the highly efficient means by which cells create and maintain the systems that provide energy to our bodies, and in their careful self-pruning to find and correct errors in their code. All of this points to the idea that cells don’t store “waste,” Ghosh said. “If there’s something they don’t need, they just throw it away.”

This idea has serious implications for the human genome in general, about 8 percent of which is composed of strings of such an ancient injected viral code, according to the Proceedings of the National Academy of Sciences.

Much of this code may also be functional, or have been reused by animals to make new things, many of them surprisingly intimate. DNA derived from viruses, for example, help form the placentathat supports the fetus in most mammals, as well as a similar structure in marsupialsand other in a kind of lizard that gives birth to live young.

Humans and other primates also use repurposed viral DNA to help regulate a hormone that controls the moment of birth. And at the other end of the pregnancy process, viral DNA appears to govern the crucial transition by which an embryo’s newly fertilized cells go from being able to create any structure (including those outside the fetus’s body, such as the placenta itself). to become dedicated to building one’s own fetus. (This stage occurs a few days after fertilization, once the new single-celled embryo has divided repeatedly to create a blastocyst of several hundred cells.)

In order to reach us, these changes could not simply occur in the bodies of individual animals. Somehow they had to make their way to the “germ line”: a potentially immortal progression of sperm and eggs that encodes (and is transmitted through) the cells that make up the body of individuals.

This process of infection, reuse and transformation is not limited to the ancient history of our species, Ghosh noted: it is still ongoing and future outcomes are unknown. “In the future, more things may happen to our DNA; we don’t know,” Ghosh said.

“The evolution is long,” he said. “It’s a dynamic process, not a fixed process.”