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The oldest known virus is known to have infected prehistoric insects 300 million years ago. A virus is basically a parasitic strand if DNA or RNA encapsulated in a protein coat. It enters cells by "wearing" a protein coat made out of proteins needed by a certain cell (e.g. Lung cells and flu virus).

Now, my question:

  • Do we know how viruses naturally formed? To simplify the question, if we went back to the first virus, how would it form?

Note that I am looking for a scientific answer, and not wild-ass speculation. While I can accept some degree of speculation the accepted answer will NOT be based entirely on speculation if it is avoidable.

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    $\begingroup$ And most likely the first viruses would be much older than the 300 million year old you mention, and probably infected bacteria or whatever was floating around back then. I suppose even an RNA world could have virus like parasites that manage to hijack some replication machinery, but now I'm getting into the speculation you didn't want. $\endgroup$
    – user137
    Commented Mar 23, 2015 at 16:17
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    $\begingroup$ After further research I came upon this question which seems closely related.....do you think mine is a duplicate? $\endgroup$
    – Jax
    Commented Mar 23, 2015 at 18:08
  • $\begingroup$ I'm not sure any of the answers will be proven. You're talking about an age we simply do not know much about. 300 million years ago is a very long time to be sure about such things. $\endgroup$
    – Mast
    Commented Mar 23, 2015 at 21:02

2 Answers 2

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One of the main points of contention in the study of virus evolution is whether or not they appear before the last universal cellular ancestor (LUCA) or afterwards (commonly accepted: genes that "escaped" from host organisms aka the escape hypothesis or vagrancy hypothesis). Basically though, the LUCA is the most recent ancestor that all organisms living on Earth are derived from.

Increasing evidence support that ancestry of viruses may predate the LUCA, for example an idea is that there "was an 'ancient virus world' of primordial replicators that existed before any cellular organisms and that both RNA (first) and DNA (later) viruses originated at this time, donating some features to the first cellular organisms. However, "a competing theory is that RNA cells existed before the LUCA and that RNA viruses were parasites on these RNA cells that later evolved DNA as a way of escaping host cell responses."

So basically, some theories support the pre-LUCA ancient virus origin and some support post-LUCA. Because it's impossible to replicate conditions from billions of years ago, several approaches using evolutionary genetics and probability models are recommended by the author of the paper, referenced below. He also discusses the specific supporting criteria for the theories that you can read in greater detail.

Holmes, E. C. (2011). "What does virus evolution tell us about virus origins?" J Virol 85(11): 5247-5251.

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  • $\begingroup$ Also good to keep in mind that viruses are not a monophyletic group but have multiple origins. Some viral clades like Caudovirales probably predate the divergence of the cellular domains whereas e.g mimiviridae have much later (but still quite ancient origins). Also, neither of these groups is in any way related to e.g. ssDNA viruses $\endgroup$
    – 5heikki
    Commented Mar 23, 2015 at 23:08
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Great question. There are several hypotheses, but in reality no one really "knows" because this is incredibly difficult to prove. We may never know for certain.

Anyway, on to the three main hypotheses, I got this from http://en.wikipedia.org/wiki/Viral_evolution, I think you'll find it to be helpful.

"There are three classical hypotheses on the origins of viruses: Viruses may have once been small cells that parasitized larger cells (the degeneracy hypothesis or reduction hypothesis); some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism (the vagrancy hypothesis or escape hypothesis); or viruses could have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on earth (the virus-first hypothesis)."

However, as you may have guessed, there are plenty of questions that these theories do not answer. The next paragraph:

"None of these hypotheses was fully accepted: the regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis was quickly dismissed because it contravened the definition of viruses, in that they require host cells. Virologists are, however, beginning to reconsider and re-evaluate all three hypotheses."

There is a little more information on the Wikipedia page, I would recommend that you read that and do a little research on the different theories if you would like to know more.

I hope this helps,

CDB

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