Interesting -- apparently there are thousands upon thousands of sequences in our DNA with the characteristics of retroviruses, and evolutionists use this as an argument for Common Descent.
First some definitions -- a retrovirus is a virus that codes itself into the DNA of the host cell and hijacks the host cell to create copies of the virus.
An endogenous retrovirus, then, is a retrovirus that is permanently in an organism's DNA.
Estimates of the percentage of our DNA that is retroviruses range from 6-10%. A small number of these retroviruses are in almost exactly the same place as they appear in chimps. Evolutionists use this fact to argue for common descent, because, they argue, if these retroviruses are in exactly the same place in both humans and chimps, that retrovirus must have been in place prior to chimps and humans branching, because the chances are too slim that both would be infected in exactly the same place, independently.
This argument rests on a number of key unstated assumptions:
1) That the endogenous retroviruses appear (and appear in exactly the same place) in the genes of all humans. If they weren't, then we have an interesting question: are some humans related to apes and others not? Or, given the 30,000 ERVs in the genome, aren't some bound to turn up in the same location in apes + humans, even if they're not related and were infected independently?
2) These actually are retroviruses. Most of these estimates are just based on segments that have similar coding to viruses. Isn't it possible for DNA to have coding similar to virus coding without actually containing viruses?
3) These viruses weren't intentionally inserted into our DNA. We use retroviruses to perform genetic engineering. Who's to say the Designer didn't use them to genetically engineer us? This is especially interesting, given the fact that some (many) ERVs are indispensible to life.
4) ERVs can actually infect germ lines. In order for an ERV to be passed on to the next generation, it would need to infect the testes or egg of a parent. While we've seen these ERVs in DNA, we've never actually seen them infect a germ line. We assume that what we see are actually old viruses. But we still haven't seen an infection.
5) It also leaves us with an interesting question: if we do all share the same ERVs, how did they come to be set in the entire population? If a single individual is infected with an ERV, that ERV is going to be wiped out by genetic drift over time, unless there is some distinct survival advantage to the ERV. What are the chances, seriously, that viruses that bungle their way into our gene pool are so advantageous to our survival that everybody without them dies? That is, unless the viruses were designed to be good for us ...
What does all this mean? I don't know. But I think it's cool.