Greening of Sahara desert triggered early human migrations out of Africa

Dust in marine sediment cores

The researchers studied marine sediments covering nearly 200,000 years collected from the seafloor off the coast of Guinea in West Africa. Strong off-shore winds transport large volumes of dust from the Sahara and Sahel to the study area. Mixed in with the dust are plant leaf waxes, which are blown long distances across the African continent to the Atlantic Ocean, where they were ultimately deposited on the seafloor at about 3 km depth.

Over thousands of years, layers of sediment accumulated on the seafloor, each layer containing evidence of past environmental conditions in Northern Africa. The plant leaf waxes are resistant to degradation and when trapped within layers of sediment, they can be very well-preserved for millions of years.

Vegetation changes in the Sahara

Based on analysis of plant leaf waxes the researchers could determine the relative importance of trees and grasses in the Sahara and Sahel regions. Trees generally require more water to survive than do tropical grasses, and so by analysing the plant leaf waxes to determine if they were produced by trees or grasses, the scientists could examine past precipitation changes in tropical Africa over the last 200,000 years.

During three discrete periods, ca. 120,000-110,000 years, 50,000- 45,000 and 10,000-8,000 years ago, substantially more trees grew in Sahara and the Sahel, indicating significantly wetter conditions than at present. The two oldest periods exactly coincide with times when the earliest humans were migrating out of East Africa to northern Africa, the Middle East, Asia and eventually Europe. At these times, the wetter conditions in central North Africa likely enabled humans to cross this normally inhospitable region, allowing them to migrate into other continents. When climate in the Sahara and Sahel turned dry again, humans were forced out of these areas causing genetic and cultural changes in already inhabited regions such as Northern Africa and the Middle East.

Changes in ocean circulation caused a wetter Sahara

The researchers also looked for the causes of these major climate shifts to much wetter conditions in the Sahara and found that they were indirectly related to an increase in the strength of the major current system, the Atlantic Overturning Circulation (AOC). The researchers could assess the strength of this current by analysing fossilized tiny shells of small animals (benthic foraminifera).When the intensity of the AOC changes, this leads to changes in the chemical composition of the deep water masses, which is then reflected in the shells of benthic foraminifera. The researchers found that when the AOC weakened, more grasses were present in central North Africa indicating a drier climate. Likely, the weakening of the AOC was caused by increased freshwater input to the high-latitudes, leading to less saline surface waters. This freshwater input also caused surface cooling in these regions, in turn leading to movement of cold air from the high-latitudes to the tropics, and causing drier conditions in central North Africa.

Thus, early human migrations from the African continent were likely triggered by events originating far away in the North Atlantic.

This research project was funded by the Netherlands Organisation for Scientific Research (NWO) and the Deutsche Forschungsgemeinschaft Research Centre/Excellence Cluster "The Ocean in the Earth System".