Wetter Sahara allowed early humans to leave Africa
Early human migrations out of Africa were made easier by changes to the climate which saw the Sahara become much wetter than it is today, according to new research by Dutch and German scientists. The study, published online by the Proceedings of the National Academy of Sciences (PNAS), also suggests that the changes were triggered by alterations in the strength of a major current in the Atlantic Ocean. The Sahara has not always been the desert it is today; earlier research has revealed that around 10,000 years ago, during a period known as the African Humid Period (AHP), the region was covered in forests, grasslands and lakes and was inhabited by humans. However, when the AHP ended around 5,500 years ago, the Sahara became a desert once again. In this study, scientists from the Royal Netherlands Institute for Sea Research (NIOZ) and the University of Bremen in Germany studied sediments taken from the sea bed off the coast of west Africa to investigate how the environment of the Sahara has changed over the past 200,000 years. Strong winds blow dust from the Sahara and Sahel regions out over the Atlantic. The dust eventually falls to the sea floor where it builds up in layers over the millennia. Mixed in with the dust are plant leaf waxes, which become trapped in the sediment layers and remain well-preserved for millions of years. The chemical composition of trees, shrubs and grasses that grow when the Sahara is wetter differs from that of the grasses and sedges found in the region when it is more desert-like. By studying the chemical composition of these waxes, the researchers were able to determine when the Sahara was dry and when it was wet. Their analyses revealed 3 periods during the last 200 millennia when the Sahara was covered in trees, suggesting a wet environment. The first period, between 120,000 and 110,000 years ago, coincides roughly with the dispersal of anatomically modern humans out of Africa and into south-west Asia and Europe between 130,000 and 100,000 years ago. This new research therefore supports the idea that the Sahara could have provided these ancient people with a route out of Africa. The second wet period lasted from around 50,000 years ago to 45,000 years ago. Again, this period coincides with another wave of migration out of Africa between 60,000 and 40,000 years ago. There is also evidence that people may have moved from south-west Asia back to Africa at around this time. The third wet period occurred between 10,000 and 8,000 years ago; as has already been mentioned, the Sahara is known to have been occupied by human populations at this time. The researchers were also keen to find out what had caused these dramatic changes in the Sahara's environment. To do this, they studied the chemical composition of the shells of tiny animals called foraminifera that are also trapped in marine sediments. The chemical make-up of these shells is linked to the chemical composition of the depths of the ocean in which the minute creatures live. Key to the Saharan climate is a major Atlantic current system called the Atlantic Meridional Overturning Circulation (AMOC), which carries warm waters northwards towards the Arctic at the surface and transports cold water back south in the depths of the ocean. The strength of this system varies over time. The team found that when the AMOC was weaker, north Africa was drier. The researchers suspect that the weakening of the AMOC was caused by an increased flow of freshwater into the Arctic region of the Atlantic. When the AMOC is weaker, the sea surface temperature in the north Atlantic falls, the trade winds become stronger and this, combined with the movement of cold air from the high latitudes to the tropics, shifts the north African monsoon rains further south and so leads to more arid conditions in the Sahara. 'Our results thus suggest that changes in AMOC influenced North African climate and, at times, contributed to amenable conditions in the central Sahara/Sahel, allowing humans to cross this otherwise inhospitable region,' the researchers conclude.
Countries
Germany, Netherlands