One of the first little tidbits reported out at the conclusion of the human genome project (where they were mapping the genome) was that, lo and behold, they found VIRUS genes.some of these are called Endogenous RetroViruses (ERVs). The way that they are distributed in the human, chimp, and other ape and monkey genomes is yet another compelling piece of evidence supporting common descent.
Source: One of the 29+ evidences at Talk Origins.
But only for the putative parents ~ the viruses are the big news here ~ and what else have they melded with?
Until we wrap up a complete genome series for all the mammals we won't know will we?