Actually, scientists are now playing around with the theory that the human race were the result of a single man's borderline nuclear potency- "Adam", if you will. Quite simply, this is thought to be because of a supervolcanic eruption in Indonesia 74,000 years ago. Hence the human race is actually very restricted genetically.
And here's a creepy thought...to have a "pure" line (you'll see what I mean in a bit), you need to have 2 ancestors for every generation...taking 25 years as a generation, the human race "restarted" 3,000 generations ago, meaning that the number of ancestors you need is 2 to the power of 2,999, which is more than the number of humans that have ever lived, in all certainty. How to explain the discrepancy? At some point, someone has been gettin' it on with his sister....
Just thought I'd share that with you all.