how a virus accidentally helped the evolution of intelligence
Intelligent life on Earth may owe its existence to an ancient virus which inserted its genetic code and machinery into early tetrapods and now helps their neurons talk to each other.
Viruses are very bizarre entities. While they’re all around us and play an extremely important role in the genetic history of virtually all life on Earth, there’s a legitimate debate about whether they should be considered living things. On the one hand, they certainly reproduce, experience evolutionary pressures, go through natural selection, and change over time. Yes, they require hosts to make all this happen, but it’s not like there aren’t numerous species of parasitic life forms out there. But on the other hand, they seem to lack any process we associate with life outside of reproduction and evolution. They don’t even do something as basic as feed on any source of energy, and even if they somehow evolved that ability, they couldn’t do anything with the energy they’d gather.
Even their structure is nothing more than a strand of DNA or RNA surrounded by a shell called a capsid, armed with molecules meant to help it latch on to the host’s cells and inject its genetic payload. You won’t even find any enzymes to facilitate any chemical activity carried out by any normal cell. They exist only to inject their genetic code into that of other organisms and that’s it, and they’re very, very successful at it, so much so that we’re copying their tactics to make gene editing and therapy possible. Just note that after billions of years of viruses hunting for a way to spread their genes, as much as 80% of our own genome has been either injected or influenced by viruses. And, interestingly enough, one of those changes may have allowed for the evolution of intelligence as we understand it.
how a viral gene helps us think
A primordial virus injected a gene called Arc into the DNA of ancient tetrapods, and that gene does something very strange. Every time our neurons exchange information through synapses, the Arc gene transcribes itself onto an RNA strand. Our neurons then build capsids around the resulting RNA strands, in effect creating new virions. Those little viral particles are then sent to other neurons through the synapses where they’re extracted from the capsid and decoded. We don’t know exactly what happens after that, but what we do know is that it’s critical. Stop these capsids from arriving to their destinations and the synapses will being to wither, making vital communication between the cells extremely difficult.
Mutations in Arc are often associated with autism and schizophrenia, and without it, higher order thinking would be pretty much impossible for species we consider intelligent and self-aware. So in a way, you could say that just like we owe a larger brain to an error in a gene called MYH16, that larger brain works thanks to a virus that was able to pass on a beneficial gene to complex life, prompting the question of whether we give viruses enough credit and attention when we’re talking about evolution. Remember, the most successful viruses are ones that cause no actual symptoms, that do no damage to the host’s body, and allow the hosts to live as long as possible to keep spreading itself. Billions of generations could pass for viruses living in human bodies and millions of hosts could be invaded over a typical lifetime.
how important are viruses to evolution, on earth and beyond?
It’s these viruses that may unwittingly be one of the most important forces in evolution, and it would be difficult not to ask ourselves if there are alien viruses out there, also made from the seemingly ubiquitous organic materials which end up in asteroids and comets that smash into baby planets, infecting alien organisms and changing their evolutionary trajectories. While we can’t — and really shouldn’t — say that the virus that gave us Arc is in and of itself responsible for intelligence in living things because it’s also plausible that our neurons could have evolved other mechanisms for talking to each other, it is fair to wonder how likely similar events could take place in alien biochemistry.
If there are multiple mechanisms for the evolution of intelligence, that may mean that self-aware, curious, clever extraterrestrial creatures may be slightly more common than we thought. Alien neurons stuck in an evolutionary rut might get an assist from viruses that found a way to spread to whatever cells are used to facilitate decision making and response to stimuli, and get swept up by selective pressures of the exoworld’s environment. Although just because there may be more intelligent life out there, that doesn’t mean they’ll be in a rush to talk to us, so even the most beneficial extraterrestrial viruses are probably not going to help us with SETI.
See: Ashley, J., et. al., Retrovirus-like Gag Protein Arc1 Binds RNA and Traffics across Synaptic Boutons, Cell (2018) DOI: 10.1016/j.cell.2017.12.022
Pastuzyn., E., et. al., The Neuronal Gene Arc Encodes a Repurposed Retrotransposon Gag Protein that Mediates Intercellular RNA Transfer, Cell (2018) DOI: 10.1016/j.cell.2017.12.024