Endongenous retroviruses (ERVs) are molecular remnants of a past parasitic viral infection. They provide molecular sequence evidence for univeral common descent. Retroviruses make a DNA copy of their own viral genome and insert it into their host's genome; these are called endogenous retroviral sequences. If this happens to sperm or egg cells, the retroviral DNA will be inherited by descendants of the host. This process is rare and random, so finding retrogenes in identical chromosomal positions in different species indicates common ancestry (Theobald, 2004). The video to the right provides a great visual explanation of ERVs and their role in evolution.
Humans have ~98,000 retroviruses embedded in each individual's DNA, or roughly eight percent of the genome (Belshaw et al., 2004). Most ERVs found are no longer capable of replication as they are merely defective traces of original viruses inherited millions of years ago (Belshaw et al, 2004). Most human ERVs are inactive, but a few have been proven to be active in the last few hundred thousand years as some humans carry one than one copy of certain ERVs (Belshaw et al., 2005). There are about seven common retrogene insertions between chimpanzees and humans (Sverdlov, 2000), proving a common ancestor somewhere along the evolutionary path. The image to the right shows a simplified version of how common ancestry can be determined by carrying the same retroviruses.
Belshaw, R. Pereira, V., Katzourakis, A. Talbot, G., Paces, J., Burt, A., and Tristem, M. (2004). Long-term
reinfection of the human genome by endogenous retroviruses. Proceedings of the National Academy of Sciences USA, 101(14), 4894-4899. doi:10.1073/pnas.0307800101.
Belsaw, R. Dawson, A.L., Woolven-Allen, J. Redding, J. Burt, A., and Tristem, M. (2005). Genomewide Screening Reveals High Levels of Insertional Polymorphism in the Human Endogenous Retrovirus Family HERV-K(HML2): Implications for Present-Day Activity. Journal of Virology, 79(19), 12507-12514. doi:10.1128/JVI.79.19.1250712514.2005
Theobald, D. (2004). 29+ Evidences for Macroevolution Part 4: The Molecular Sequence Evidence. Retrieved from http://www.talkorigins.org/faqs/comdesc/section4.html#retroviruses