Few debates have polarised scientists more than the origin of our species. Most scientists now believe that humans evolved in Africa and migrated to the rest of the world. The first recognisable 'humans', Homo habilis, evolved about 2.4 million years ago, but are thought never to have left the African continent. The next ancestral humans appeared about 1.9 million years ago. Those who stayed in Africa are known as Homo ergaster, while those who migrated and eventually spread around the world are known as Homo erectus. The out-of-Africa, or African Eve, theory goes on to say that anatomically modern humans, Homo sapiens, left Africa about 100,000 years ago and spread around the globe, ultimately replacing Homo erectus. Many Chinese scientists, however, subscribe to the multi-regional theory, which holds that different populations of Homo erectus evolved discretely over more than a million years into modern humans. Regional differences developed, they say, but eventual interbreeding ensured the whole species worldwide evolved together. So multi-regionalists argue that the 25,000-year-old bones on Tianyuan mountain would belong to a descendant of the 500,000-year-old Peking Man, whereas out-of-Africa advocates say Peking Man died out and was replaced by migrating Homo sapiens, again from Africa, about 100,000 years ago.