I’ll try to keep this short.
For years, viruses have both fascinated and completely terrified me. I understand that bacteria are little itty bitty simple organisms. Makes sense, like when you learn about your own cells in biology class.
But a virus? From what I’ve read, they aren’t alive. They just go around hoping to get inside of you so they can hijack your cells, cause mayhem and potentially kill the host that’s keeping its cycle going.
Where did they come from? How can they not have a full life cycle on their own? Seemingly no purpose, but still exist. How did they start out? I know no life form has a ‘true purpose’ because we’re all lucky to just be here but, there are pollinators, predators to keep prey numbers in check. Organisms that work together to both benefit. Then there are mindless viruses that just exist and if you inhale them, they mess you up. Why?
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>Where did they come from?
Nobody knows exactly, because you cannot dig up some dirt and find virus skeletons. Most probably they’re with us since the earliest times of life. There are 3 ideas scientists came up with, but there’s no ultimate evidence to decide which one is true.
Idea #1 is that viruses first came when the life was already established. That means that there were living cells around, very simple bacteria-like cells. These cells certainly had some DNA and RNA (or perhaps only RNA) in them and some of this DNA/RNA might have been so loose that the ancient cells could spit it to the environment and another cell could take it from there. Such loose piece of DNAs exist even nowadays (called plasmid) and bacteria can totally pick them up or release from/to the environment. These plasmids cannot copy themselves, they use the host cells copying mechanisms. They usually bring some useful genes on them like antibiotics resistance, so it’s good for the host cell. But what if a plasmid would carry a harmful gene, a gene that for example tells the host cell to shed little bubbles out of it’s own material and spit these bubbles to the environment. Such bubbles would contain this hostile plasmid, so it’s basically a bubble that has the DNA code to make bubble. If another cell comes across and picks up the bubble, now it also has a hostile DNA that hijacks it to make more bubbles and pout the bubble maker DNA in it.
Maybe it wasn’t the form of bubbles, but as you see the simplest virus only needs an escape mechanism and the code for the escape mechanism *within* the escape mechanism. The rest is done by the host and taken care by the evolution.
Idea #2 is that viruses, again, come from the time when life was already established. But this idea says there’s no loose DNA escaping, but instead imagine a cell that attacks another cell. There are already different sizes of cells, and the smaller one is going inside the big one and lives as an “internal” or in fact intracellular parasite. Again, there are such intracellular parasites nowadays so the idea is not some crazy sci-fi, and often these real intracellular parasites lose some important genes because they don’t need them anymore. Such parasite cells are still considered as living beings even though they can’t do some important biological processes and thus they rely on the host. But what if such an intracellular parasite goes to the extreme with losing unnecessary genes and keeps only the ones needed for attacking and escaping the host? Then here you are, you have a virus that was once a living being but not anymore.
(At this point I also want to add that viruses are not living beings because humans *decided* to not call them living beings. What is considered life and what isn’t, is an arbitrary categorization and one day we could change our minds and call them living beings. Like Pluto is a planet or not. They don’t really care about our labeling system.)
Idea #3 goes back in time before life was even formed. You see, a living cell as we know today is a lot of very difficult chemistry in a bag made of a sort of fat. These fat-bags contain all the chemistry that is needed to eventually make two fully equipped fat-bags from one, which is called cell division. This is life, basically a fat-bag with chemistry in it to produce more copies of the same fat-bag. Nowadays life contains a lot of things that are not necessarily needed for basic life, but it’s a nice luxury to have. Like nowadays all cars have airbags which is not strictly needed to move the car forward. So the first life was certainly super simple compared to what we have now. And there was an even simpler world before that, when most of the self-copying chemistry already existed in a kind of soup that also had some nutrients and other chemistry in it, but without the bag. Later a fat bag came along and started containing the chemistry, making it more efficient, and it simply ate up the bagless proto-life so it doesn’t exist anymore.
In this idea the first viruses are basically the remainders of the bagless proto-life that could only live if a bag somehow picked them up. Because the inside of the bag is still the same chemistry ever since, so the virus only needs some way to enter and exit the bags, because it finds a familiar world inside. And from there they evolved together with the bagged life we call cells today.
>How can they not have a full life cycle on their own?
Life and virus are the same chemistry. It’s a bit like having a tent in your own garden. The tent does the really basic protection, and the house can supply with everything. Tents are not made from some alien material, they are made of things you would find in your house too. Viruses are made of the same material as cells: RNA/DNA, proteins and sometimes lipids. They can snatch those components from the cell.
> predators to keep prey numbers in check.
Actually, viruses and other pathogens often do the same job. In the view of ecology, pathogens and predators take down their preys but both groups must not overdo it as this would jeopardize their own life. So as much as there is a predator-prey balance, there’s a virus-host balance too.
>Organisms that work together to both benefit.
No it’s absolutely not true. Very often only one benefits. You see organisms that fit the system because of survivor bias. Because those who didn’t fit, died out and you don’t see them. That gives you a false perception of a well designed system. No, it’s just everything that didn’t work well, went extinct. Including ancient viruses too. What you see today, you see because they fit in.
>Then there are mindless viruses that just exist
Just like majority of organisms.
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