Asleep: because I am afraid my post will get too long if I address every word, I will stick to what I really see as one repetitive error in your argument.
In a nutshell, such specific exclusionary claims (to the extent they are supported by evidence) might have some effect on p2, but p2 remains unknown. This is because p2 comprises all imaginable pathways that VP62 could have arrived in 22Rv1, minus the singular, mutually exclusive possibility of recombination taking place within the cell line as proposed by the authors
You commit a very basic probablilistic error. Instead of explaining again (but see the second part of
this reply to Alex, I'll adapt your own example somewhat:
Suppose we assume that Alice didn't buy a lottery ticket after all and Bob wins the lottery. One day after he returns the ticket, a policeman visits Bob's house. He says:
"Sorry Sir, but I have to arrest you. The chance of you having won the lottery in a fair way was exactly 1 in 10,000,000. Therefore it's almost certain that one of the other 9,999,999 persons who bought lottery tickets actually won. You apparently stole the lottery ticket or manipulated the draw."
Yes, I know this is silly. But this policeman could be you.
Is also similar to certain people arguing that LHO killing JFK must have been a conspiracy because the chances of JFK getting within 100 yards of LHO during JFK's presidency would be incredibly low. Irrespective of what you think of the assassination, it's a silly way of calculating the odds.
Point is that it was no 'singular possibility' that a certain recombination event of many possible events occured in a certain cell line at a certain moment in time (or that a certain lottery ticket was going to be drawn or that JFK had to drive past this madman's workplace). It only became certain to you after one of many possibilities happened to occur.
Another aspect (that's not really an independent aspect but is really derived from the above) is that, from all of those many "imaginable pathways, there would also have to be
one solution, one event that happened to be the source. For that one source (that is surely equally imaginable), the same would really apply: it would have required the same
singular, mutually exclusive (and equallly improbable) event to get in that source
and then it still had to infect people and 22Rv1 many years later.
Therefore, even at face value, without doing any other experiments, the hypothesis that XMRV arose in 22Rv1 is already more likely than the alternative. After all, the alternative would have needed exactly the same kind of "luck" XMRV had in its creation,
and then some (e.g. infecting people, staying undetected, infect 22Rv1 many years later and without having evolved over the many years).
Ironically, the probability of a second event calculated by the authors (1.3 10?12), must also serve as an estimate for p1 (the probability of their choice recombination having occurred). This is because, in making their calculation, they assume the exact same preconditions that they purport to have led to #1 (the presence of the PreXMRVs).
I thought I illustrated how the above is nonsensical with
my earlier example about getting identical kids from independent conceptions (and therefore, aside from twins).
Point is that we know that it (XMRV) happened (at least) once, because it is infectious in 22Rv1.
Contamino ergo sum.
Again, the evidence that it occured in 22Rv1 has little to do with the calculation of it occuring twice and the low odds that it happened twice have little to do with it having happened in 22Rv1.
It happened in 22Rv1 because there is evidence that it did. For instance, 22Rv1 is the best known environment on the planet for XMRV to exist and multiply which is a pretty solid evolutionary indication, its ancestors happened to "live" in the same environment, the phylogenetic evidence supports the notion, etcetera.
