While we’re all thinking about viruses, it might be interesting, to some at least, to consider how these things came into being in the first place, which in itself opens up some interesting philosophical questions.
Viruses are beautifully simple. If you believed in intelligent design, they are the ultimate in design simplicity, if you’re aiming at a ‘thing’ that can replicate itself, albeit a pretty malicious design. Just a bit of genetic material surrounded by a protein coat that protects the genetic material and gets the virus into its host cell.
In the past, it was accepted in evolutionary theory that viruses evolved later than their hosts, from more complex life forms, in common with other parasites. A tapeworm, for example, cannot live outside its host – it has no sensory organs as such, no motility, no digestive system – it just absorbs food from its host and devotes its entire existence to making tapeworm eggs, to infect more hosts. It’s not difficult to imagine how such a creature could have evolved from a more complex worm that was eaten by the host, took up residence in its gut, and gradually evolved into a simpler organism because it no longer needed eyes, a mouth, a digestive system, motility, and so on.
But for a virus, it’s difficult to imagine how such simplicity could evolve from a more complex organism. Also, fossil records (using the word ‘fossil’ broadly to mean all ways of detecting traces of different life forms in very old rocks) show viruses have been around pretty much from the beginning of life on earth. So how could that be, when there were no more complex animal or plant hosts for them to parasitise?
To ponder this, we have to go back a long way – about two billion years, when life diverged into three groups: Prokaryota, Eukaryota, and Archaea. The difference between Eukaryotes and Prokaryotes is the most fundamental division of life forms on earth. Prokaryotes are all single cells, but their cells don’t have a defined cell nucleus containing their DNA – it just sits in the centre of the cell. Prokaryotes are by far the most numerous group, including all bacteria, and the photosynthetic blue-green bacteria (often wrongly called ‘blue-green algae). Eukaryotes have a defined cell nucleus containing their DNA, enclosed by a nuclear membrane. This group includes all unicellular and multicellular plants and animals, from amoeba and single-celled plants to higher plants and mammals. There are other differences between Eukaryotes and Prokaryotes, but it’s the cell nucleus that’s the fundamental difference, and makes it possible for eukaryotic single cells to join together to form multicellular creatures – Prokaryotes are unable to do this. The Archaea group is a more recent invention (didn’t exist when I studied microbiology) but is a sort of catch-all group for all the weird and exotic lifeforms that don’t easily fit into the other two groups. They mostly live in strange places where other life forms can’t live – for example, boiling water in geysers, salt flats, or concentrated acids. They also often have exotic metabolisms, for example using the energy in sunlight to fix carbon from inorganic rock such as limestone.
All three of these groups have viral parasites. Indeed, pretty much all life forms have viral parasites. So that would imply that viruses evolved well before this fundamental evolutionary crossroads.
There are currently four main theories for how viruses evolved. These are:
- Virus-First Hypothesis: this says that viruses are the simplest life forms because they evolved first, they were the first self-replicating life forms. They later became obligatory parasites by parasitising more complex life forms. This is rejected by most scientists because if such life forms did exist, they were by definition not viruses, because they must have had some sort of metabolism to replicate themselves.
- Reduction Hypothesis: this says that viruses evolved from a more complex type of cell that started parasitising other cells, then evolved to be simpler, rather like the tapeworm. This doesn’t have much support either, as there’s no evidence for any kind of single-celled organism that parasitises cells by getting inside them, nor that such a thing has ever existed.
- Escape Hypothesis: this proposes that viruses are actually bits of genetic material that ‘dropped off’ the genetic material in a more advanced cell and started to reproduce on their own, but there’s no evidence for that; it seems unlikely and has never been observed.
- Coevolution Hypothesis, also known as ‘Bubble Theory’. This is an interesting one, and sounds to me the most plausible. It’s now thought that life on earth evolved in the deep oceans, around hydrothermal vents where the water was warm, and where there were all sorts of food and energy sources (and where some of the most primitive microorganisms are still found). This theory proposes that self-replicating bits of genetic material arose by chance near the vents, driven by energy sources near the vent. These vents also produced primitive fat-like ‘bubbles’ – if a bit of self-replicating material got stuck in one of those, it was protected and had a supply of food, so could drift further from the vent and still reproduce. This gave rise to two ways to replicate: find your own bubble and replicate in that, or use someone else’s bubble to replicate in, hi-jacking the primitive cellular structures they were beginning to evolve. So in this theory, viruses were there right from the very start of life – which makes sense because every life form we’ve detected on this planet is parasitised by viruses.
There’s also a very new theory called the Chimeric Origins Hypothesis, which is a variation of the Coevolution Hypothesis, but I won’t try to explain it as I don’t understand it! If anyone else does, please add a comment below …
So viruses have probably been around for a very long time, they could well have evolved alongside all other lifeforms 4 billion years ago.
And yet, at the very pinnacle of 4 billion years of evolution, we still don’t have a universal way of dealing with them. Sudden, spontaneous outbreaks of new viruses, such as we’ve seen with HIV, SARS, MERS and COVID-19, cause havoc, large numbers of deaths, and major disruption of our highly evolved societies. Of all the ‘disaster scenarios’ that could wipe out our species, from an asteroid impact to nuclear war, the spontaneous evolution of a truly deadly human virus still seems by far the most likely.
If you believe in an interventionist god (which I don’t, nor any other kind of god), it would not be difficult to imagine what viruses were for: to stop any one species becoming dominant, to guard against the hubris and arrogance of a species that could otherwise wreck the planet. But it doesn’t much matter whether you believe in a god or not, viruses are there and that’s what they have the capacity to do. The simplest, most primitive life form on the planet is still the one that could defeat us absolutely.