ERVs are endogenous retrovises that insert themselves into the DNA of the host organism. Most of the time, this is successful and there is more than some suspicion that some ERVs are responsible for MS and some forms of cancer. Sometimes the insertion doesn't work, though and fragments of the non-functional ERV are left over in the host's DNA and can be spotted as markers. Wellllllllll.....it turns out that humans and the higher apes share some of these markers, adding yet more evidence for common descent. The author writes:
Some have argued that ERV’s are not the result of viral infections but instead are the result of a supernatural design process. Firstly, the viral genes found in mammals today are very similar (99% to 75% homology) to viral genes that we see today. This evidence supports the origins of ERV’s as viral. No other mechanism, besides viral infection, has ever been observed that results in an ERV. Secondly, the argument could be made that if ERV’s are all focused on the design of the organism that a difference in ERV’s would make a noticeable difference in design. As it turns out, humans differ in genomic ERV content. That is, some people have ERV’s that other people don’t.  Since we don’t see any difference in design between these groups of people it would seem that ERV’s are not necessarily there for design purposes only. However, ERV’s have been suspected as playing a role in many functions, such as placental development in mammals. On the other hand, ERV’s have also been suspected of causing multiple sclerosis  and susceptibility to cancer. Most scientists have come to the conclusion that ERV’s act as random mutation events in that most ERV’s are neutral and some have beneficial or detrimental effects on the individual. If they are part of some design process then they are ineffective in causing design differences in most cases. Also, a designer would not be forced to make the pattern of similar ERV’s between species follow what is found in the fossil record. That is, a designer would not be forced to follow the rules set forth by common ancestory and the theory of evolution.Good points, all. It is interesting when a bit of evidence not only supports evolution but is also antithetical to ID. the comments are also quite interesting.