Estimating the ancestral population of anatomically modern humans, Colin McEvedy and Richard Jones chose bounds based on gorilla and chimpanzee population densities of 1/km² and 3-4/km², respectively, then assumed that as "Homo erectus" moved up the food chain, they lost an order of magnitude in density. With a habitat of 68 million km² ("the Old World south of latitude 50° north, minus Australia"), "Homo erectus" could have numbered around 1.7 million individuals. After being replaced by "Homo sapiens" and moving into the New World and de-glaciated territory, by 10,000 BC world population was approaching four million people. McEvedy and Jones argue that, after populating the maximum available range, this was the limit of our food-gathering ancestors, with further population growth requiring food-producing activities.