Climate change and human migration out of Africa

An international team of scientists, including Dr Jonathan Dean (now at the University of Hull) and Prof Melanie Leng (BGS Chief Scientist, environmental change, adaptation and resilience), have reconstructed how climate has changed over the last 200 000 years in eastern Africa. They have shown how climate change could have enabled the out of Africa migration of Homo sapiens around 60 000 years ago. The paper was published in Nature Communications, Earth & Environment in June 2021.

Homo sapiens is thought to have evolved in eastern Africa sometime before 200 000 years ago. We used geochemistry to examine hundreds of metres of sediment cores taken from the bed of a lake called Chew Bahir, in southern Ethiopia, and were able to work out how the climate changed going back through time.

We found that, from 200 000 to 125 000 years ago, the climate of eastern Africa was relatively wet, with at least 20 to 30 per cent more precipitation than the region receives today, meaning conditions were favourable for early humans with plenty of food and water available. Further studies have also shown wet conditions in other parts of Africa at this time. Under such conditions, humans could move relatively easily through Africa and some even ventured out of Africa to reach the Arabian peninsula, which they did well before 100 000 years ago. However, it does appear that these intrepid humans died out before they could disperse further.

From 125 000 years ago, the climate gradually became drier in eastern Africa. This long-term drying trend was interrupted by short-lived climate shifts that lasted a few thousand years. These have been linked to changes in the circulation of the north Atlantic Ocean, showing how changes in one part of the world can impact upon the climate in another.

One of these short-lived climate shifts was a wet period that occurred 62 000 to 60 000 years ago. This roughly coincides with the occurrence of the ‘successful’ out of Africa migration of Homo sapiens — the one that went on to populate the rest of the world. This wet period would have made the out of Africa migration possible by providing sufficient food and water resources for the humans to survive the long journey.

Over the past 60 000 years, the conditions have became increasingly challenging for the humans who stayed in eastern Africa. There were some particularly dry intervals where Chew Bahir completely dried out. At these times, humans had to seek refuge at higher elevations in Ethiopia, where they could still access sufficient food and water.

About the author

Dr Jonathan Dean is a lecturer in Quaternary science at the University of Hull. He worked on this project while at BGS with Prof Melanie Leng.