I've had a few people that I have taught about endogenous retroviruses, or ERVs, (if you don't know about ERVs, see 'ERV FAQ' at the top of the page then come back), who have expressed incredulity that we can all have around 200,000 of these viral genetic markers fixed in our genomes, testifying to common ancestry. Here, "fixed" means that they have spread throughout the entire population, and now, it is inevitable that our descendants will inherit them from us.
Now there are many things in nature that confound our sense of what can possibly be true, but are, nevertheless true*. There are also things we don't (at least as yet) understand. Using either of these things to doubt reality is a well known fallacy, the "Argument from Personal Incredulity". But is the fact that there are some 200,000 ERVs in our genomes so difficult to find credible? Let's do some simple arithmetic. I'll use nice rounded off figures to make this easier to follow. All we are looking for is a ball in the ballpark.
Life has been around in our local neighbourhood for roughly 4,000,000,000 years. For 200,000 ERVs to become fixed in a population it has taken on average 4,000,000,000/200,000 years. That's one, on average, of course, every 20,000 years.
Chimps and humans went their separate ways some 6,000,000 years ago. Between the two species, there are roughly 350 ERVs that are unique, either to chimps or to humans. They must have been acquired after speciation. That means that the rate of fixation of these new ERVs over that period is around 6,000,000/350, roughly one, on average, every 17,000 years or so.
Now I know that these figures are very approximate, and many factors, such as population sizes and bottlenecks, retroviral pandemics, reproductive habits etc. can affect the real figures. I am also aware that over deep time, much DNA can become so eroded such as to make identification difficult or even impossible, but as the cdesign proponentsists love to point out, ERV descended material can be beneficial, and even essential. Such material would be preserved through positive selection. Much of these things are also solo long terminal repeats, or solo LTRs, which are easy to identify.
I think you must be able to see from the above that 200,000 ERVs becoming fixed in a species is not beyond the bounds of credibility, at least for those of you who can evaluate these things impartially.
*The universe does not owe it to us to respect our parochial intuitions. It used to be intuitively obvious that the earth does not move. That the heavier object would fall faster than the lighter one. That when you measure the speed of light, it depends on the relative speed of the source and the detector. That when you pass a single quantum of light through an apparatus with two apertures, it will pass through only one of them. All obvious. All false.
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