Question: How did viruses start?

Ciarán O'Brien answered on 20 Nov 2014:

That’s a really good question! It’s so good that scientists don’t have one straight answer for it, viruses are tiny and really diverse, and because they’re so different from other living things (in fact scientists scientists still have trouble deciding if they’re really alive or not) they’re still a bit of a mystery…

There are 3 ways scientists think viruses might have started. They each have problems, so we’re not quite there yet.

1- Viruses might have started off as bacteria that were parasites of larger bacteria. Over time, they lost the genes that didn’t involve being a parasite, and they got better at it, getting smaller and smaller, their cell membrane becoming a harder coat of protein, until they became what we think are viruses.

Problems: Even the smallest parasitic bacteria has no resemblance to a virus at all, you’d expect a few things in common if they shared an ancestor.

2- Viruses might have evolved from bits of DNA and RNA that “escaped” the cells of bacteria. Bacteria have sort of add-on chromosomes called plasmids which they can freely exchange, a virus might have come about from some of these. Cells also have bits of DNA called transposons, which are sequences that can cut themselves out of your DNA and insert themselves somewhere else, those might have been the first viruses.

Problems: If viruses evolved from DNA like this, it doesn’t explain where they got the ability to make their protein shells and some of the complicated structures we see on viruses that let them penetrate a cell’s defenses.

3- Viruses might have evolved from complex molecules at about the same time the first living cells developed. Scientists have found molecules they call viroids, they’re DNA/RNA like viruses, but they don’t have genes to make a protein shell and mechanisms to get into a cell, but if you put one in a cell, it does everything else a virus does. They’re a big cause of diseases in plants, but usually require help to get into the cell. There are also prions- proteins that didn’t fold up correctly, and make more of themselves by nudging other proteins into the same incorrect shape. They’re what cause mad cow disease. Prions are good evidence that viruses could have evolved from complex self-replicating molecules.

Problems: This idea that viruses evolved separately this way goes against one of the main definitions of a virus, which is that they need a host cell. Of course, maybe our definition of a virus isn’t quite good enough…

Ever since scientists have been able to look closely at DNA and RNA, we’ve been getting a better picture of viruses, how they evolve, and where they might have come from. We still don’t know which of the above ideas is correct (or if it’s a combination of the three, or even if there’s a fourth idea we haven’t discovered yet), but we have found out enough to say that viruses probably didn’t have a single common ancestor like cell-based life does.

Viruses probably evolved in several different places using different mechanisms.