This is the exact opposite of truth. There is no sharp line between Canis lupus (wolf) and Canis familiaris (dog): the dogs have acquired several genetic changes from the wolves over tens of thousands of years, but not quite enough for reproductive isolation; that would take a hundred thousand years. Similarly there was no sharp line where we could say "Now, in this generation, these are Homo sapiens where the last generation was Homo erectus", nor was there any sharp line marking genus Homo from genus Australopithecus, etc. These changes are an accumulation of multiple genetic alterations over the course of hundreds of thousands of years, within a breeding population that never numbered less than the thousands even during the most severe crashes.