The story of our species is one of remarkable success. From a starting point in Africa, our ancestors spread across the planet, harnessing local resources and adapting to the harshest environments. While some societies collapsed once they had exhausted local resources, and other groups migrated to new lands or fought wars over diminishing local resources, the overall picture was one of continued growth. As technological advances enabled our ancestors to increase agricultural output, their numbers grew exponentially. The lack of open discussion about this topic means most people are not aware that our high numbers today are such a recent phenomenon. As recently as 1930, in our parents’ or grandparents’ youth, world population was some two billion compared with the seven billion living on the planet now.