About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.
Unless Thank.Darwin's hypothesis is correct, yes you can. But even though it is unlikely that viruses came from outside this planet, we still cannot be totally sure.
Viruses use the exact same bases and amino acids that Terran life uses. While it is not impossible that extraterrestrial life could (some scientists say "must") create a genetic sequence and biochemistry, I find it hard to believe that it would be identical to ours.
I do agree with James on this, parasites cannot evolve before their hots, then viruses probably evolved after some cellular life evolved first.
As for their extraterrestrial origin, I would just say that if you use Ockham Razor's, the answer is just that life appeared once on earth and viruses evolved from a cellular ancestor rather than to have to imagine a convergent evolution in space that would seed earth with viruses sharing the same basic mechanisms as the rest of earthly life forms (mostly DNA, RNA and genetic coding).
I also, think that they can't evolve before cells originated. But, we cannot say surely that convergent evolution has not occured here, merely because it takes longer time.
Also, how can we explain their origin [ origin of viruses] on the basis of theory of mutation and natural selection.
Or their evolution was similar to the origin of life [origin of life for second time , but not entirely new life forms , some what new...] ???
Their origin can probably be explained by loss of functions due to a parasitic way of life. Quite extreme in its own way, but still start from the same basic chemistry. Convergent evolutions to explain why life will arise twice using exactly the same chemicals, the same coding and so on looks much less probable to me, don't you think?
That they once were living organism is a sure thing for mem, are they still? I have really no idea if they could still be considered such though. But I still consider them as biological objects/beings.
Do they constitute a sixth kingdom? Well it would be interesting to know if all viruses emerged from a common ancestor, and then the answer would be yes, or if they are a case of convergent evolution of parasites, then I would have to say no. But tracing back their origins, when they lost so many of their original genetic material is probably impossible
So I stay in doubt and avoid definitive answers.
Yes IMO they were organism and they lost everything non essential to be better parasites. Over simplifiaction like tape worm who lose its digestive tract. Or some mycobacteria that become endo parasites and can no longer be isolated outside of a cell.
The problem of living thing is mostly a question of definition. You can include them or not, as long as you give the definition you are using, so that anybody can understand you. I haven't made up my mind on this topic, that is all.
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