Im glad this reader was persistent in poking me with this Q, its a really good un!
I always thought that ERVs were some of the best evidence for evolution, but I always wondered if there was an additional aspect to the story that is not usually told. I imagined that different strains of viruses have existed at different times over the last few million years, so if some of the ERVs we share with chimps (or other animals) were caused by retroviruses that HAVE NOT existed since modern humans evolved, this would really put the nail in the coffin to some of the Creationist arguments. Can we recognize the specific strain of virus that caused a particular ERV, and if so, are some of our inherited ERVs the result of ancient viruses that are no longer around? Any information would be appreciated.
This is a really good question.
Some endogenous retroviruses are so young, they have very closely related infectious exogenous relatives. A good example would be mouse mammary tumor virus— mice have endogenous MMTV, and mice are still infected with exogenous MMTV.
All ERVs in humans are extinct retroviruses. The viruses in your genome right now have no homologues in our population that infect modern humans. The only two retroviruses that are real ‘normal’ human pathogens are HIV and HTLV. HIV is a lentivirus— there are very, very few endogenous lentiviruses (found one in bunbuns, another in lemurs). HTLV is a deltaretrovirus— I am not aware of any endogenous deltaretroviruses.
Our ERVs are only distantly related to exogenous viruses that infect other organisms. That is, MLV is a gammaretrovirus, but our Class I retroviruses (related to gamma and epsilonretroviruses) are not literally MLV. Likewise, our Class II (related to alphas and betas) are not literally ALV or MMTV.
So to put it a different way, our youngest ERVs, HERV-Ks, are as similar to HIV-1, a modern infectious virus, as humans are to Methanobacterium thermoautotrophicum.
Yes, there are some ERVs that are specific to humans (again, the HERV-K family). But the ERVs we have are nothing like modern human viruses.