Humans evolved in Africa around 200,000 years ago. But we didn't hang around. By 100,000 years ago we had begun spreading throughout the world.
This journey was long, arduous, and took them to unfamiliar environments. So why did they bother?
What drove our ancestors to leave their homeland in Africa and travel further than any primate has before?
Who, what, when, where?
We know a lot about our ancestors' migration out of Africa, from a lot of different sources.
Through genetics we know when they left the continent and what route they took. Fossil evidence backs this up, documenting humans appearing in the Middle East - for example - around the time they should.
Mites also back up this history. Humans have mites living on their body at all times. The family tree of these mites matches up very nicely with the family tree of humans. It shows an ancient home in Africa, followed by a migration around the world.
All sorts of data from all sorts of surprising sources attest to our spread out of Africa. As a result there's an awful lot about our species' movement we do know, yet there are still some key questions that remain a mystery. Questions like . . .
Why leave Africa?
We know a lot about our ancestors' migration out of Africa. But we don't know much about the why they undertook this difficult trip.
Part of this may be because there was no real migration out of Africa. As a population grows it naturally spills over into the surrounding area. Over time it just keeps expanding, following viable environments. Before long the humans have spread around the whole world. Just as a bi-product of demography.
Computer simulations have confirmed that this is one possible explanation for this migration. Demographic pressures would be enough to force humans on a very long journey (albeit, over a very long time). However, they also identified several other key factors that may be involved.
- Environmental change could have made it viable for people to move into new regions.
- Demography alone could have been the cause. Populations naturally filled up regions, forcing movement into new ones. This caused a gradual spread out of Africa over many generations.
- New resources could have made regions outside of Africa attractive. Other animals naturally spilled out of Africa as they filled up environments, and some humans just followed them on this journey.
- Humans could have deliberately decided to move. Perhaps some combination of the above factors encouraged people to migrate, rather than gradually, naturally expanding.
Other explanations
Of course, these simulations weren't an exhaustive list of reasons to leave Africa.
As such, there could be additional explanations not considered. For instance, Africa saw an awful lot of environmental change in the time leading up to the great human migration. This may have forced humans to adapt and develop new technology and behaviours. Effectively, they would have been "pre-prepared" for the migration.
Others have suggested that the migration out of Africa may have been a lot more deliberate than these simulations suggest. Our ancestors' prey may have been moving out of the continent. These early humans just naturally followed after them.
In short, why our ancestors left Africa is still a mystery.
Simulations have helped identify some plausible explanations. But this examination is far from exhaustive. There is still an awful lot more research on the topic that needs to be done. Perhaps the real explanation has already been identified. Perhaps it is a combination of some of them. But without more work we won't know for sure.
tl;dr
Simulations have identified several possible reasons our ancestors left Africa. However, this list is far from exhaustive; so the ultimate cause may still be a mystery.
References
Grove, M. (2014). Palaeoclimates, plasticity, and the early dispersal of Homo sapiens. Quaternary International
Hölzchen, E., Hertler, C., Timm, I. and Lorig, F., 2015. Evaluation of Out of Africa hypotheses by means of agent-based modeling. Quaternary International.
Palopoli, M.F., Fergus, D.J., Minot, S., Pei, D.T., Simison, W.B., Fernandez-Silva, I., Thoemmes, M.S., Dunn, R.R. and Trautwein, M., 2015. Global divergence of the human follicle mite Demodex folliculorum: Persistent associations between host ancestry and mite lineages. Proceedings of the National Academy of Sciences, 112(52), pp.15958-15963.
