Viviane we never said analyze why some could mate with others. The point was that if you can define human you can find the first human. Mate was just one example it might not work or prove accurate.
So what if population A evolved faster than B. Even if B produced humans eventually if A produced humans first thats the winner. It doesn't matter if they lived on the same timeline that was not the problem presented. For my example tests between population A and B are irrelevant, the test would be from current population and a few thousand years to the rest.