How did an ancient virus make our brains so complex?

An ancient viral infection may have given animals the tools to become fast, coordinated and intelligent, a study has found.

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

This “invasion” itself is ordinary; Inserting such code is the main method used by viruses – which do not have the ability to reproduce themselves without the support of a protected cell – to force cells to carry out their commands.

But in this case, the cells transformed the new code to achieve their own goals, a dynamic that scientists have also found at the roots of basic animal activities like fertilization and pregnancy.

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

The new pieces of injected code helped direct the cellular machinery to produce myelin, a protective sheath around nerve cells that helps speed up the transmission of electrical signals that power our nervous systems.

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

Evolutionarily, this property allows for other powerful effects.

Because myelin allows nerves to travel more quickly, it also allows new forms of simultaneous communication. This has enabled the evolution of complex neural networks characterized by more connections and more interactions within a given space. (Although not all neurons have myelin sheaths, the cells 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 critical.)

Without these faster signals, “all the predator-prey mechanisms — all this enormous diversity — would not have evolved,” Ghosh said.

The team's research found that infection of vertebrate ancestors with myelin-coding viruses likely occurred multiple times, as the closely related family of viruses modified the genomes of the ancestors of today's fish, amphibians and mammals — each reusing new lines of code. To build complexity.

This requires a complex evolutionary dance. The viral infection did not code for myelin production, but another mutation did. Instead, it helped the proteins that read and interpret the genome bind to the specific region where myelin instructions could be found.

Scientists know this because some simple vertebrates – such as sea lampreys – have the myelin mutation but do not have this extra part of the viral genome. The sea lamprey's relatively simple nervous system also contains no myelin. Ghosh compares this primitive nervous system to an orchestra waiting to start playing. “All the instruments were there, but they needed a trigger. Violins – or viruses.

Ghosh emphasized that these ancient viruses did not intend to change the structure of their hosts. Instead, the way this evolutionary ceremony took place shows something about cells that ordinary people often overlook.

“Cells are smart,” he said. “They have a lot of mechanisms that we don't understand – we don't know how they do everything. Sometimes we say they are too smart for us.

In a very real sense, the word “cell” — derived from the 17th-century discovery that plant and animal tissues were made of what looked like little boxes — doesn't really capture the complexity of how cells interpret and interact with each aspect. From their environments. A box of small molecules and organs crammed into a microscopic shell of fat is not enough to form a cell, Ghosh said. “You must have many other things.”

This complexity is evident in a wide range of areas: in the highly efficient means by which cells make and maintain the systems that supply our bodies with energy, and in their precise self-tuning to find and fix errors in their code. All of this points to the idea that cells don't retain waste, Ghosh said. “If there's something they don't need, they throw it away.”

This idea has stark implications for the human genome as a whole. About 8 percent Which consist of strings of this ancient viral code that are injected, according to the Proceedings of the National Academy of Sciences.

Much of this code may also be functional, or repurposed by animals to do new things, many of them surprisingly intimate. For example, DNA derived from a virus. Helping in the formation of the placentawhich carries the fetus in most mammals — as well as A Similar structure in marsupialsAnd another In some kind of lizard That gives birth so that young people can live.

Humans and other primates also use repurposed viral DNA to help with hormone regulation Which controls the timing of birth. At the other end of the pregnancy process, viral DNA appears to control the crucial switch by which the newly fertilized cells of the fetus go from being able to form any structure — including those outside the fetus's body, such as the placenta itself — to becoming Designed for the construction of the fetus itself. (This stage occurs a few days after fertilization, once the new one-cell embryo divides repeatedly to form a blastocyst of several hundred cells.)

For these changes to reach us, they cannot only occur in the bodies of individual animals. They had to somehow make their way to the “germ line”: a Potentially immortal progress Sperm and egg cells encode – and transport – the cells that make up individuals' bodies.

Ghosh noted that this process of infection, reuse and transformation is not limited to the ancient history of our species, it is still ongoing, with unknown future outcomes. “In the future, more things could happen to our DNA — we don't know,” Ghosh said.

“Evolution is long,” he said. “It's a dynamic process, not a static process.”