"One element common to both human and fugu is a DNA sequence called LINE, short for Long Interspersed Element. LINE, itself a gene about 6,000 base pairs long, codes for an enzyme called a reverse transcriptase. The key here is that it recognizes its own RNA sequence, and repeatedly inserts copies of itself into our genome. LINEs seem to be relics of the "copying machinery" of old viral infections wherein a virus would embed a portion of itself into our genome, not enough to propagate the full, infectious virus, but enough to continue copying itself. It is a classic example of a selfish gene: It has no purpose but to do only that, without benefit to us.