We became very nomadic for quite some time - therefore during these nomadic stages of our history we would have had a lower population rate. We also spread out when we could. At times when humans left Africa for the Middle East were times when they were able to cross much larger areas of land - at a time when water levels were lower. There's a fantastic mini series by Dr Alice Roberts on this - and it's quite evidential that what she's stating is a very likely theory. She also talks about population from time to time - but I'd need to re-watch it again to see what is said.