Any thoughts?
All I have is my 7 yr old textbook.....BUT, dealing with populations in PRE-recorded history is going to get you a bunch of "estimates". It's estimated that there were 6 million humans around 5000 BC. It's estimated that there were 500 million people around 1500 AD and 1 billion people around 1800 AD. If you plot the population over time, you will see that it is not linear. There are some that want there to be a direct exponential population growth so that they can find an inception point, but complex life in complex ecosystems on a complex planet doesn't follow an equation.
Someone tried it and, mathematically, showed that the first 2 humans could have been around 4300 BC by using the 1800AD estimated population of 1 billion and backcalculating it.....doesn't explain how there are remains found that outdate that by tens of thousands of years....but mathematics is not going to be the answer anyway. Mathematically, that is correct, but the problem is that when the population was small (say a million), the variables that act on that population have huge and longer lasting negative impacts when compared to a larger population (like a billion). Wars, diseases, carrying capacities of the land, droughts, famines...you name it. When the population was small, these things stagnated global population growth. Now, these things don't even make a dent in the total population.....though a lot of little dents may add up.
Actually, now that I think of it, I don't know if you want population biology anyway. It deals more with genetics and genetic fitness (your ability to pass on your genes) more than anything else. Good luck hunting for your answers.