Question: what is the origin of viruses? due to their inability to reproduce on their own how did they come to be? Do all viruses share a common ancestor or are there some examples of convergent evolution?

Zoe Vance answered on 29 Apr 2024:

So I don’t think this is necessarily a fully answered/settled question, but there are a few ideas on the origins of viruses. They may have originally been able to reproduce themselves but it was advantageous to become parasitic and they lost the ability over time – it’s typically ‘expensive’ for something to keep an ability it doesn’t need in evolutionary terms.

Another possibility is that they were originally ‘mobile genetic elements’, kind of like a piece of DNA that is able to hop in and out of the main genome and insert elsewhere (these still exist). Under that idea the element managed to evolve the ability to get out of the cell it originally belonged to and spread to other cells. These elements are usually considered to be ‘selfish’ – they multiply in a genome even if it isn’t good for the main cell, so not a million miles from a virus!

Then there’s another idea again that says viruses actually came before the rest of the tree of life. I couldn’t really get into it with quick answers in the chat, but this seems more plausible when you know that biology was quite different around the time life was getting started. Again, lots of ideas on that but I tend to lean towards the idea of an ‘RNA world’, that the initial pre-cursors to life were RNA molecules that became self-replicating (we know that some RNA molecules can catalyse reactions). So the idea would be that viruses arose from that ancestral state and cells like hours, or even bacteria, came later and then the ancestral virus became able to parasitise them.

Single origin vs multiple origin is also an area of debate still! There are a lot of types of virus that have very different set-ups to each other in terms of how they reproduce and store their genetic material so it’s quite plausible that there are multiple origins for different ones (and maybe then different ones of the ideas above apply to different origins…)

Hopefully it’s not too unsatisfying to not have a concrete answer! Any evolutionary question that deals with deep time is very difficult to answer. There are still some things about deep animal evolution we don’t know and viruses are just even more difficult to work out again. They mutate a lot so that makes it hard for us to work very far back in time without the signal getting blurry. Usually we make sort of “family trees” where the number of mutations different between two organisms tells us how far apart they are. Between us and chimps, about 98% of the genome is the same after about 6/7 million years of evolving separately. But if you take two viruses that have been evolving separately for that long you might not even think they’re related!