Up until about 12,000 years ago, human populations were primarily limited by

Local infrastructure of the surrounding lands

Food sources would be a major limiting factor for humans. About 12,000 years ago, humans were hunter-gathers. It was at that point that we begin to see the domestication of some animals (goats) and semi-permanent settlements arise. Within 2,000 years, we see the first evidence of farming which leads to permanent settlements.