Transcriptase

. . . . The spare DNA would have features that made it good at getting itself duplicated and re-inserted into chromosomes. Bingo. The commonest gene in the human genome is the recipe for reverse TRANSCRIPTASE, is the key part of any DNA sequence that can copy itself and distribute the copies around the genome. It's a sign of a digital parasite. Most of the copies are inert these days, and some are even put to good use, helping to regulate real genes or bind proteins. But they are there because they are good at being there.

. . . . The alternative view, penetratingly articulated by Dawkins, takes the perspective of the gene itself: how DNA would behave if it could. Close to half of the human genome consists of so-called transposable elements designed to use transcriptase. Some of the commonest are known by names like LINE s (17 percent of the genome), SINEs (11 percent) and LTR retrotransposons (8 percent). Actual genes, by contrast fill just 2 percent of the genome. These transposons are sequences that are good at getting themselves copies, and there is no longer smidgen of doubt that they are (mostly inert) digital parasites. They are not there for the needs of the body at all. ~ Page 68