[betterunix]

Actually it takes more than one man and one woman for humanity to avoid extinction with a reasonable degree of success:

https://en.wikipedia.org/wiki/Minimum_viable_population

Of course, that number depends on things like the environment (a favorable environment will mean a smaller number) and the genetics of the "first" generation. I seriously doubt that the number could be as small as two for any species even under the most ideal circumstances.

[baddox]

So there was never only a single human in existence?

[photojosh]

Correct. There was a population of animals that at some stage started to approximate what we would call human.

[baddox]

Yes. Basically, the http://en.wikipedia.org/wiki/Species_problem.