Humanity's origin has been the subject of a fair amount of debate and question, not to mention controversy. Not much is really known with any amount of verifiable certainty. What can be certain is that at some point a certain group of reptiles evolved into the earliest mammal form from which all others mammals are descended. But evolution did not stop there. The earliest mammals continued to differentiate into what would become the basic classes of mammalian life. It is difficult to pinpoint anything with solid precision because: