The theory of a single Out of Africa event for modern humans is gradually being replaced by many Out of Africa events for the ancestors of modern humans and perhaps culminating around 100,000 years ago with the critical Out of Africarabia event(s). This consisted of a few Out of Africa events followed by an Out of Arabia expansion for most of the world’s non-African population. But even this expansion led to some interbreeding between these new arrivals with descendants of earlier expansions which gave rise to the Neanderthals and Denisovans and others who were already present in Asia and Europe.
It is likely that the genus Homo first moved Out of Africa some 2 million years ago.
Susan Anton et al, Evolution of early Homo: An integrated biological perspective, Science 4 July 2014: Vol. 345 no. 6192
Press Release: Many traits unique to humans were long thought to have originated in the genus Homo between 2.4 and 1.8 million years ago in Africa. Although scientists have recognized these characteristics for decades, they are reconsidering the true evolutionary factors that drove them. …….. suggests that these traits did not arise as a single package. Rather, several key ingredients once thought to define Homo evolved in earlier Australopithecus ancestors between 3 and 4 million years ago, while others emerged significantly later.
Potts developed a new climate framework for East African human evolution that depicts most of the era from 2.5 million to 1.5 million years ago as a time of strong climate instability and shifting intensity of annual wet and dry seasons. This framework, which is based on Earth’s astronomical cycles, provides the basis for some of the paper’s key findings, and it suggests that multiple coexisting species of Homo that overlapped geographically emerged in highly changing environments.
“Unstable climate conditions favored the evolution of the roots of human flexibility in our ancestors,” said Potts, curator of anthropology and director of the Human Origins Program at the Smithsonian’s National Museum of Natural History. “The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo.” ……
Until recently, the evolution of the genus Homo has been interpreted in the context of the onset of African aridity and the expansion of open grasslands. Homo erectus was considered to be a bona fide member of the genus Homo, but opinions diverged on the generic status of earlier, more fragmentary fossils traditionally attributed to Homo habilis and Homo rudolfensis. Arguments about generic status of these taxa rested on inferred similarities and differences in adaptive plateau. However, there was near-universal agreement that the open-country suite of features inferred for Homo erectus had evolved together and provided the adaptations for dispersal beyond Africa. These features foreshadowed those of more recent Homo sapiens and included large, linear bodies, elongated legs, large brain sizes, reduced sexual dimorphism, increased carnivory, and unique life history traits (e.g., extended ontogeny and longevity) as well as toolmaking and increased social cooperation.
Over the past decade, new fossil discoveries and new lines of interpretation have substantially altered this interpretation. New environmental data sets suggest that Homo evolved against a background of long periods of habitat unpredictability that were superimposed on the underlying aridity trend. New fossils support the presence of multiple groups of early Homo that overlap in body, brain, and tooth size and challenge the traditional interpretation of H. habilis and H. rudolfensis as representing small and large morphs, respectively. Because of a fragmentary and distorted type specimen for H. habilis two informal morphs are proposed, the 1813 group and the 1470 group, that are distinguished on the basis of facial anatomy but do not contain the same constituent fossils as the more formally designated species of early Homo. Furthermore, traits once thought to define early Homo, particularly H. erectus, did not arise as a single package. Some features once considered characteristic of Homo are found in Australopithecus (e.g., long hind limbs), whereas others do not occur until much later in time (e.g., narrow pelves and extended ontogeny). When integrated with our understanding of the biology of living humans and other mammals, the fossil and archaeological record of early Homo suggests that key factors to the success and expansion of the genus rested on dietary flexibility in unpredictable environments, which, along with cooperative breeding and flexibility in development, allowed range expansion and reduced mortality risks.
Although more fossils and archaeological finds will continue to enhance our understanding of the evolution of early Homo, the comparative biology of mammals (including humans) will continue to provide valuable frameworks for the interpretation of existing material. This comparative context enables us to formulate and test robust models of the relationships between energetics, life history, brain and body size, diet, mortality, and resource variability and thereby yield a deeper understanding of human evolution.