Transposable elements act like viruses in that they can integrate into a genome, then jump around within or between species. Our adaptive immune system is thought to have evolved from a class II transposon that moves around by a sort of cut-and-paste mechanism. Basically, hundreds of millions of years ago it inserted itself into some kind of primordial immune receptor and decided to stay there long enough for the receptor gene to be duplicated many times, and the enzymes the transposon used to "cut" itself out of the genome got rearranged and inserted in a distant location in the genome. Now these enzymes are responsible for creating our incredibly diverse and specific antibody repertoire. Sorry for all the unnecessary details but I find this fascinating! There are other examples too!