It's only relatively recently that the scientific community has begun to accept that early humans interbred with our Neanderthal cousins. Now it looks as if it was not only possible, it was essential for providing us with immunity from strange diseases.
For the first time ever, we can track the number of genetic mutations in each generation of a human family. Humans are mutating at a downright glacial pace, with a shockingly tiny number of mutations passed on in each generation.
One of the problems with decoding the human genome was discovering how little of the human genome was actually... well, human. Are we really made up by mostly virus DNA?