THis is a classificationist/essentialist/didactic ontology or way of dividing the world and unlike most simple divisions it seems like it holds up well. Except for viruses.
Ask probably any teacher or professor and they'll tell you that viruses are non-living because they don't meet part of some list of criteria. Only a few will engage in an consideration of them being alive; for comparison only the most philosophical will suspend disbelief and entertain the idea that rocks are alive.
But the people who agree that viruses are non-living will also easily say that they evolve. This for a long time would've been contradictory, but now where we consider memes to be subject to evolution and definitely not alive, the tension between non-living and evolving isn't apparent. There should be tension because evolution came out of biology, no one looking at non-living but evolving things would've ever come up with the idea of evolution, I suspect.
Nothing that isn't man-made or alive evolves, except viruses. Perhaps rather than being an exception to the general rule they're subject to it and were in the past somehow connected to living things.
Living things reproduce, move, grow, and have some sort of metabolism. In the debates about the origin of life the main focus in fact is between reproduction and metabolism, rather than movement and growth. So perhaps those main features of living things are expressed very different across the whole tree of life. Some things practically don't grow at all. There's a gigantic (essentialist) division between life that reproduces by fission and life that reproduces sexually. Some living things don't move at all, like bacterial mats. And as far as growth, there are things that have the same form and simply grow bigger, and things that take radically different forms as part of growth.
But there's nothing that doesn't have metabolism. What would something without a metabolism look like? It probably can't move much and probably doesn't grow much and probably doesn't have much of an ability to reproduce. It'd look like a virus. So what if viruses evolved from very primitive life forms that slowly lost their metabolism as they became more evolved to live off of and inside bacterial cells?
Alternatively, what if viruses are the product of living cells? Cells could've produced a viral like protein coat/cluster that they pushed out into the world, in order to alter the environment around them, or perhaps even as a type of waste or holdfast structure at the earliest stages. If these holdfasts instead of attached to a neutral substrate were interlocking with proteins expressed on the surface of other cells, they could evolve with even more specificity. Perhaps viral capsules are a bizarre evolution of whatever mechanisms allowed cells to hold on to each other, either in making bacterial mats or as a way to leach off of other cells.
Viruses do move. Beyond their brownian motion through a media, once they meet a host cell they attach and undergo molecular movements to flex and introduce their genetic material into the host cell. They attack not by a normal sort of grabbing though but by a lock-and-key like interaction between molecules on the viral surface and molecules on the cellular surface, and in this way have evolved to become host-cell specific.
Everyone takes for granted that viruses evolved this specificity in order to be injected into the cell and hijack it for reproduction. But what would an intermediate stage of that evolution look like? We've learned a tremendous amount about the evolutionary history of birds and reptiles and primates and humans because we have access to intermediate stages, and those intermediate stages have shown us that we should reject orthogenetic evolution or directed and striving evolution. Monkey's didn't evolve into humans with large brains because of some pre-programmed tendency within all of monekydom to have larger brains. Primates didn't rearrange the functional capsules of their face and skull in order to make room for bulbous brains. Reptiles didn't slowly extend the length and lightness of their scales because eventually it'd pay off in flight. The intermediate stages were undirected and functional on their own often in ways that were totally different than what we recognize the current function is. Viral latching mechanisms didn't have to evolve in order to inject viral genetic material into a host cell.
Perhaps an imaginary cell growing in competition with early bacteria that were forming matts or volvox like sphere developed viral particles that interrupted the bacterial cellular adhesion of those structures. THis could be a tremendous advantage for a non-colonial organism against a colonial one. Later packaging off some genetic material in it, which isn't so surprising in cells without nuclear organisation and with plasmids circulating in the intercellular fluid, could end up making the interrupting capsules more host/target specific. It'd be an investment with a heck of a payoff, evolutionarily speaking, because the producing cells wouldn't have to invest all that much of their own energy in it, produce a few replicating capsules, and then let their own reproduction propel them on. A free defense mechanism.
Alternatively, something more like a modern virus could've evolved as a type of machinery that a living cell sends out to attack and lyse nearby competing cells, allowing the producing cell to live off the spilled and lysed materials, like a fly spreading digestive juices onto it's prey and sucking up the mottled remains, or a starfish everting its stomach onto it's prey. An evolutionary scenario like that would provide selection for attachment specificity and the ability to reproduce inside a host-cell.
In any case once these molecular machines are produced, their evolutionary history is decoupled form the producing cells. Perhaps the producing cells went extinct in the Archean for all we know and the machinery has been whirring on ever since. 90% or more of all life has gone extinct so the odds are simply in favor of the producing cells being extinct. The intermediate forms and the producing cells, unlike whole dinosaurs in the case of birds or ape teeth in the case of humans, won't preserve well and would be in rocks that are potentially astoundingly old and rare and metamorphosed when still extant, we'll likely never find those intermediate forms. And evolution sometimes seems to repeat, flight evolved several times, in birds, pterosaurs, bats, and insects. But notice that besides insects the other groups are somewhat closely related, used relatively similar mechanisms and anatomical structures to evolve flight, and probably did so in very similar environmental circumstances (like arboreality). So if viral producing cells are still extant we'd have to look in environments that would select for it, just like the environments that select for flight. WHich means we need to have better ideas about what those environments and functions are. If it's for lysing cells to digest them, we perhaps need to look at places where autotrophy doesn't work (deep sea environments away from hydrothermal vents? Deeply buried anaerobic soils?) and heterotrophy has some extreme requirements (like preying on bactieral cells that are too big or too durable for phagocytosis). We'd also want to look for groups of cells that are best candidates for this sort of behavior, just like we'd want to look at vertebrates when studying flight. "Vertebrates" might seem like an absurdly wide group to look for, but the bacterial world is much more diverse than the world of normal macroscopic organism that we're used to (which is basically vertebrates, arthropods, molluscs, and 'plants'), so having something like that would probably help greatly in finding if these types of cells are still alive. Find the target environment, then do something like environmental DNA studies to even see if the 'right' groups are present at all before doing more detailed study.
So in this way we're applying the major themes of evolution across life to the evolution of viruses.
[this is an off the cuff set of thoughts that I'm not even going to bother editing and may return to to wildly change/revise things or may never look at again, just getting some dumb thoughts on 'paper'.)
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