What limits human population? Like other animals, we are capable of reproducing at a rate fast enough to cause the population to grow exponentially. Like other animals, our populations are limited by predation, accident, disease, starvation and competition with each other. By the late hunter-gatherer period we had become the top predators, so predation by other animals was a minor problem. Disease was probably a much less serious threat than in the later agricultural period when domestication of animals brought in a host of devastating diseases.
Our ancestor's lifestyle was dangerous. Big game hunting was a very dangerous activity, and all sorts of natural accidents are always around - drowning, falling, getting lost in snowstorms, etc. Homicide was almost certainly a major killer. Competition between neighboring bands, as well as in band disputes, frequently ended in murder. In addition, HGs practice a kind of natural birth or population control, since a woman can't carry more than one child. Still, a child living beyond the first few years probably had a good chance of living a fairly long life.
Agriculture introduced a more productive but also more arduous lifestyle. Women could have more babies in these sedentary societies but the disease problem became much more severe. At least two other societal innovations provided other means of population control: war and the introduction of social classes. Social classes seem to be a feature of all large scale agricultural societies, at least until recently, and the lower classes of slaves and peasants are the designated casualties. They do the starving, get the worst effects of disease and suffer all sorts of other casualties as well.