An ancient viral infection may have given animals the tools to become fast, coordinated and smart, a study has found.
According to a paper published Thursday in Cell, complex nervous systems emerged 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.
In itself, this ‘invasion’ is unremarkable; Inserting such code is the main way that viruses – which cannot reproduce themselves 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 discovered underlying core animal activities such as 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 of the Cambridge Institute of Science.
The new bits of injected code helped guide the cellular machinery to produce myelin – a protective sheath around nerve cells that helps speed up the transmission of the 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 wire (or a nerve fiber), it allows that signal to travel faster and with fewer errors. are shipped. .
In evolutionary terms, this trait produces other powerful effects.
Because myelin allows nerves to transmit faster, it also enables new forms of simultaneous communication. And that enabled the evolution of complex neural networks with more connections and more interactions within a given amount of space. (Although not all nerve cells have myelin sheaths, those that do – especially in the white matter of the brain and spinal cord – are located in areas where speed and density of connections are crucial.)
Without those faster signals, Ghosh said, “all the predator and prey mechanisms — all that enormous diversity — would not have evolved.”
The team’s research found that the infection of ancestral vertebrates by myelin-encoding viruses likely occurred many times, because the closely related family of viruses modified the genomes of the ancestors of today’s fish, amphibians and mammals – each of which reused the new lines of code. to build complexity.
This required a complex evolutionary dance. The viral infection did not code for myelin production; another mutation did that. Instead, it helped the proteins that read and interpret the genome bind to the precise region where the instructions for myelin are found.
Scientists know this because some simple vertebrates – such as the sea lamprey – have the mutation for myelin, but not that extra piece of viral genome. And the sea lamprey’s relatively simple nervous system also has no myelin. Ghosh likens 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 did not intend to change the structure of their hosts, Ghosh pointed out. Instead, the way this evolutionary concert played out reveals something about cells that laypeople often miss.
“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 from what seemed like small boxes – doesn’t really capture the complexity of how cells interpret and respond to every aspect. of their environments. A box of molecules and tiny organs in a microscopic shell of fats isn’t enough to make a cell, Ghosh said. “You need a lot more things.”
This complexity plays out across many domains: in the highly efficient way cells create and maintain the systems that power our bodies, and in their careful self-pruning to find and fix errors in their code. All this points to the idea that cells don’t store “waste,” Ghosh said. “If they don’t need something, they just throw it away.”
This idea has major implications for the human genome in general, about 8 percent of which consists of sequences of such ancient injected viral code, 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, virus-derived DNA helps form the placenta, which holds the fetus in most mammals – as well as a similar structure in marsupials, and another in a type of lizard that produces live young.
Humans and other primates also use recycled viral DNA to help regulate a hormone that controls the timing of birth. And at the other end of the pregnancy process, viral DNA appears to control the crucial transition where an embryo’s newly fertilized cells change from the ability to create any structure — including those outside the fetus’s body, such as the placenta itself — to being dedicated to building the fetus itself. (This stage occurs a few days after fertilization, once the new one-celled embryo has divided repeatedly to create a blastocyst of several hundred cells.)
To make their way to us, these changes couldn’t simply take place in the bodies of individual animals. They had to somehow find their way to the ‘germline’: a potentially immortal progression of sperm and egg cells that codes for – and is passed on by – the cells that make up individuals’ bodies.
This process of infection, repurposing and transformation is not limited to the ancient history of our species, Ghosh noted – it is still ongoing, with unknown future outcomes. “More things could happen to our DNA in the future – we don’t know,” Ghosh said.
“Evolution takes a long time,” he said. “It is a dynamic process, not a fixed process.”
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