 Complete the following phrase:  Estimating the ancestral population of anatomically modern humans, Colin McEvedy and Richard Jones chose bounds based on gorilla and chimpanzee population densities of 1km2 and 3-4km2, 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 