I'm adding this post before I forget about it. We will discuss it more when we cover viruses.
Human Endogenous Retroviruses. These pieces of DNA are 'fossil's of retroviruses that were incorporated into the human germline DNA over thousands to hundreds of thousands to millions of years. They make up about 1% of the human genome. The first HERV was identified in 1981. So far, at least 20 HERV families have been identified (find free text article by M. Tristem here). There are lots of things cool about HERVs. First, they may be involved in the pathogenesis of some diseases, such as cancer, Sjogren's syndrome, Rheumatoid arthritis, Multiple sclerosis or Systemic Lupus Erythematosus, to name a few.
HERVs have been classified into 3 groups based on sequence identity with animal retroviruses.
Class I HERVs share sequence homology with infectious mammalian type C viruses. Examples would be porcine endogenous retrovirus, murine leukemia virus and baboon endogenous virus.
Class II HERVs share sequence homology to mammalian type A, B and D related retroviruses. HERV-K is a member of this group and is one of the most active HERVs. HERV-K can actually form virus-like particles.
Class III HERVs are foamy virus related HERVs and include only HERV-L. This is the oldest known genus of Retroviridae and more can be found about them here.
Why have HERVs been so successfully maintained in our genome? This person has one hypothesis. Is is possible that HERV's somehow serve the needs of the host? Also here.
Science fiction novelist Greg Bear has a series of novels in which HERV's play an important role in evolution.