Hamilton, MJ; Walker, RS

Background: All biological populations exhibit fluctuations in size over time due to stochastic variation in growth rates that result in local extinctions, but these dynamics are poorly understood in traditional human populations. Here, we explore extinction processes over human evolutionary history due to the effects of (i) demographic stochasticity, (ii) environmental stochasticity, and (iii) periodic catastrophes. Organism: Modern humans, Homo sapiens. Methods: We built a mathematical model of stochastic population dynamics, estimated its parameters using ethnographic data, and simulated probability distributions of extinction outcomes using Monte Carlo methods. Results: Although expected times to extinction vary under different conditions, hunter-gatherer populations likely experienced local extinction events every few hundred years (or every few generations). Conclusions: Played out over the evolutionary time scale of the human species (similar to 250,000 years), stochastic variation in growth rates resulted in a near-constant turnover of local populations, contributing to the near-zero growth of human populations over much of our evolutionary history.