I am about halfway through the book now, and I got to the part where he talks about these retroviruses. I had heard about these in the technical gobble-dee-gook sense, but never read an explanation on what they really were. According to Ridley (well, his sources anyway–Dawkins?), approximately 35% of the genome is this “selfish” genetic material that is capable of copying and pasting itself repeatedly back into DNA using reverse transcriptase. He indicates that without reverse transcriptase, viruses wouldn’t work, and we would be much better off without it.
On the contrary, I find it quite interesting that much of our genetic code is made of old, broken viruses that had copied themselves into our DNA, and that when these pieces get switched on accidentally, they might cause some forms of cancer. It’s also quite interesting that the number and location of some of these junk sequences are used for forensic identification. (So THAT’s what those little genetic barcode things are!)