Chair made some good points on p.1. Populations evolve. One of the difficulties of understanding biological evolution is to appreciate the scale of time involved. As I age I find it easier to comprehend what it might be like to live for a century, but that's still a stretch. I don't really have any idea what it would be like to live for 1000 years, it's not in my experience to understand it. So ask me what 100,000 years means and I have nothing to relate to it. Modern humans have existed for about 200,000 years.
I'm going to set a completely arbitrary line by defining 'human' as all individuals descended from the first bipedal apes. There is a whole population 'coming down from the trees', but they don't all do that together. Some are better at walking upright than others because their physiology is better suited, but it's not easy for any of them. The ones who can walk upright on the forest floor or savannah can carry food because their arms are freed up. They have an advantage. They are the ones that have more children and so the next generation has more individuals with greater ability to walk upright. There will be still more with the next generation and more again with the next. The lines of descent of those with unlucky physiology will tend to die out. After many generations of living in a new environment, this whole population will start to look quite different. There will always be genetic variation, and the most advantageous traits will become more common over time.
By my definition of 'human' in this case, you can see there never was a time of only two humans. But maybe my definition isn't very good. This diagram from the Holy Wikipedia is updated quite often, and currently looks like this (the vertical scale is in millions of years):
So the situation really is that there was a time of many thousands or millions of homo erectus, and if you leap forward in time not by a single day, but by hundreds of thousands of years, sufficient changes have accumulated that you give them a different name, Heidelberg man. And leap again and we are Sapient man. This diagram has no hard borders between the different species in any line of descent. You can make legitimate arguments for us being modern humans for 200,000 years, or for 185,000 years, or for 100,000 years. Some would take it back 500,000 years. I could make a case for bipedalism four million years ago as a criterion for being called human, but others could made perfectly good arguments for other definitions involving other points in prehistory. The point is, naming an ever-changing population is arbitrary.
The OP is an irrelevant question. It is not a problem for biology because there never was a time when there were only two of any sexually reproducing species that has arisen since the evolution of sexual reproduction itself. Adam and Eve each had two parents, and four grandparents, and eight great-grandparents, and so on. And that has been how it has been for hundreds of millions of years.
Stuart
