Endogenous retroviruses (ERVs) are viral elements found in the genome of vertebrates that closely resemble (and can be derived from) retroviruses. In humans, this 'rogue' DNA is believed to make up between 5 - 8% of the human genome.
It's suggested that the DNA was incorporated into the (pre-human) genome hundreds of millions of years ago. The DNA codes for proteins that seem to have no role in the human body - though recent research has suggested some might affect embryo development and disease proliferation.
“What do these efficient genomic colonizers do? Are they merely fossils that, like mosquitos in amber, were stuck and preserved in large host genomes while their functions decayed? Researchers have been struggling to understand their roles for as long as we have known them, postulating junk, bystander, and pathogen hypotheses.”