f two species have very similar DNA/Genes, . . . . doesn't it stand to reason that at some point in the past, the same viral infection could have had similar effects on both species?
Similar genetic makeup is a requirement for being eligible to infection, e.g. that is the reason why infections that affect fish don't affect humans, but humans and chimps can be infected by the same diseases.
That however isn't the end of the story, the relevant point here are the details of the infection.
A virus just inserts its own genome into that of a cell, somewhere into it. While there are places which are more susceptible than others, the location where the viral DNA is inserted is pretty much random.
In case of the shared ERV sequences e.g. chimps and humans not only would have to be infected by the same virus, but that virus infection would have to have happened in the exact same location of the genome, down to the base pair. The point is that it is more likely that humans and chimps inherited this bit of viral DNA fom a common ancestor than for them both to suffer the exact same infection.
Note that this is not an ordinary every-day infection - it had to affect a sex cell which later produced a sperm cell or ovary which resulted in a pregnancy, and something had to go wrong with that infection as it otherwise would have killed the cell, preventing it from producing sperm or ovaries.
Such a cell being infected, surviving the infection and later producing the one out of 100 million sperm cells which fertilizes the ovary already is a very very rare occurence.
Other species suffering the same event and even in the same out of 3 billion locations in the genome is extremely unlikely. And there isn't just one such sequence, but there are about a dozen which have been compared to the genomes of related primates. All fit the phylogenic tree regarding their distibution.
And even if they were likely to happen, there still would be no reason why the sequences should be distributed that way. That distribution alone already puts a 99.999% or so certainty on common descent, not even taking into account the low chance of identical infections occuring, which later results in the astronomical number which i mentioned in my last post.
This is the distribution of such sequences. E.g. the two arrows left of the middle indicate that after the speciation event that divided new world monkeys from the rest but before other speciation events three such insertions occured, all of them are shared by humans, chimps, gorillas, orang utans, gibbons and old world monkeys but not by new world monkeys.
I gave humans and old world monkeys for granted in my calculation, as if they wouldn't have this sequence, evolution wouldn't predict them being found in chimps, gorillas, orang utans and gibbons.
A chimp having that sequence in that location - if an infection and survival of the cell and pregancy are given as granted as i don't know their odds (i am being *very* generous to your position here), then the probability that this occurs in the right location for it to fit with the prediction of evolution is 1:3.000.000.000 as there are that many base pairs in the genome where the infection could have happened.
So the odds that chimps, gorillas, orang utans and gibbons (four species) all share this sequence without common descent are 1:3.000.000.000^4
And that's only the chance of that single sequence...in that timespan twoof them happened, making that 1:3.000.000.000^8
When one takes into account all the sequences on that diagram one arrives at a chance of 1:3.000.000.000^33, roughly 1:10^312.
Compare it to the "merely" 10^78 atoms that the whole universe consists of.
If every atom in our universe contained another universe of which in every atom another universe is found and then this whole thing once more, then you have roughly that number of atoms. It's really an insanely big number...
We're talking about a huge amount of time. I can see how this could have happened!
I'm glad you mention this, as it brings up another point (i know you're not a YEC, but i'll mention it for the others): As explained, the requirements for such a sequence becoming fixed in the genome are quite tough. If humans only had been on the earth for 6000 years, they wouldn't have accumulated the number of such sequences which they have in their genome.
However, I'm out of my element now, but can only continue to hold to my convictions that we don't have all the information to make life decisions on, unless you are holding to some level of faith that you ('you' is generic in this context) are right.
Well...it is not a life decision, is it?
If one believes in creation ex nihilo or theistic evolution is not a salvation issue.