Xphobe probably has it right. Given that each generation is in transition, there did there need to be a specific first couple who got all jiggy with each other. There is nothing that would require that the first human male could only have sex with the first human female for all of this to happen. The difference between species at that point would have been so small as to be indiscernible either by them or us. At some point the gene pool we call human would have emerged from others, but not necessarily with a bang (no pun intended. Well, maybe it was).
We are still evolving. Our average brain size is decreasing, wisdom teeth are disappearing, the appendix apparently no longer does what it used to (whatever that was). At what point is the species that created Aristotle going to be different than the one that creates our grandchildren a few hundred thousand or million years down the road?
Some species can interbreed, and certainly many pre-human versions could do just that because they were so closely related. Presumably we became who we are in part because of those oft-intermingled genes.
Those who came up with punctuated equalibrium, something UP thinks supports his thesis, have pointed out that by "fast" they mean a change happening within a 50,000 year time span, not literally overnight. Fifty thousand years is zippity quick in geologic time, but not really all that fast in the internet age. It's hard for a human living 1/625th of that time to comprehend how long it is. Or put it in perspective.
Edit: got rid of all the extra empty space at the bottom of this post. People sometimes use that as evidence that I have nothing to say