EFCHED
Palaeoinformatic approach to the context of the earliest human dispersals (PACED)
Humans evolved in Africa over the past seven or so million years, but are now found all over the world.
When did they move from Africa and what routes did they take? Did they travel alone or with other animals, and were these movements related to climate or something else? Did they disperse once, twice or many times?
Our project set out to investigate the ecological context of the earliest out-of-Africa migrations in human evolution, since the number and timing of such events remains unclear and contentious.
We argued that efforts to assess the reality and patterning of the very earliest movements must consider not only the archaeological and human skeletal evidence within a clearly established chronology, but also the larger context of human dispersion as one facet of the evolution of the terrestrial mammalian fauna of the Plio-Pleistocene.
Image: Map of potential dispersal routes out of Africa.
We felt that arguments for a later Pliocene movement remained plausible but unproven, and the project therefore considered the past three million years as a possible time frame for actual and potential biotic contact and movements between Africa and Eurasia.
We collaborated with the online Paleobiology Database (PBD) in the USA to make our dataset of 815 Afro-Eurasian sites, localities and levels available to the wider academic community and the general public through the PBD website.
Using these data and published studies of modern bio- and phylo-geography we have examined the four plausible routes out of Africa to see which are most likely.
We have concluded that the Gibraltar Straits are highly unlikely to have been a large mammal dispersal route in the Plio-Pleistocene.
The most likely migration routes would have been through the Levant, comprising the Sinai Peninsula and Israel, or across the Bab-al-Mandab Straits between the Horn of Africa and the southern end of the Arabian Peninsula.
However, we have also determined that reaching the Levant/Arabian Peninsula was not a crucial aspect of hominin and other mammalian migrations. Rather, the key variable is how far they managed to move beyond this area.
In many cases, African genera known from the few sites in the Levant are also found in Europe, Central or Southern Asia (ie India, China), making the relative lack of sites on the Arabian Peninsula less important than it at first appears.
In many cases the first finds of African genera are in Asia. They only appear in Europe later, perhaps suggesting that for these animals a two stage migration 'Out of Africa' to Asia and then 'Out of Asia' to Europe is as plausible as a direct 'Out of Africa' movement to Europe in the Early Pleistocene.
Photograph: The hippopotamus is just one of a handful of animals which migrated out of Africa during the last three million years when conditions were favourable.
Relatively few animals have migrated into and out of Africa during the past three million years, with most of these leaving rather than entering.
Moreover, most of the species on the move preferred very different habitats to one another. For example, the hippopotamus needs water whilst the modern oryx is usually found in deserts. This suggests that these animals did not move as a group, but as individual species when conditions for their migration were favourable. This was probably as true for early humans as for other large mammals.
Nine of our major findings
1. Fewer than 20 large mammal genera have migrated from Africa to Asia during the past three million years.
2. Most movements appear to have been out of, rather than into, Africa for the past three million years.
3. The animals that left Africa have a wide range of environmental tolerances, implying isolated movements of individual taxa instead of the mass migration that is often suggested.
4. Therefore hominins probably moved alone, but this does not make them special, as it appears all taxa dispersed on their own rather than as a group.
5. Many genera including Homo and large carnivores such as Crocuta and Panthera are first recorded in Asia and then later in Europe, perhaps suggesting that for these animals a two stage 'Out of Africa' to Asia and then an 'Out of Asia' migration to Europe is as plausible as a direct 'Out of Africa' movement to Europe in the Early Pleistocene.
6. Africa remains the most likely origin for the hominins on current evidence.
7. The Gibraltar Straits are unlikely to have been a gateway for Plio-Pleistocene terrestrial mammals, including early humans, to enter or leave Africa.
8. The Bab-al-Mandab Straits at the southern end of the Red Sea are most likely to have functioned as a gateway for terrestrial mammals in the Late Pliocene, but not since.
9. The route across Sinai and the Levant is the most probable Plio-Pleistocene gateway into and out of Africa.
This summary was compiled by Hannah O'Regan (Liverpool John Moores) with input from all other members of the group. Alan Turner (Liverpool John Moores) is the PI for this project.
The research was undertaken in collaboration with Laura Bishop (Liverpool John Moores), Sarah Elton (Hull York Medical School) and Angela Lamb (NERC Isotope Geosciences Laboratory) who were all co-investigators.