Leaving home

Research from Chew Bahir Basin (pictured) in Southern Ethiopia reveals climate background to the modern human ‘Out of Africa’ event.

One of the great debates in archaeology surrounds the ‘Out of Africa’ event – the date when some of our direct genetic (Homo sapiens) ancestors left the continent to permanently settle in other parts of the world. For the first time, a climate reconstruction covering the past 200,000 years in Ethiopia has given us more accurate portraits of the climate background to this momentous event.

In 2014, researchers from the University of Cologne drilled a 300-metre long drill core through the Chew Bahir Basin in southern Ethiopia. The core spans an astonishing 620,000 years of climate history and environmental change, the longest such sample in tropical Africa. The last 200,000 years of the core provide a unique record of short-term climate change. It reveals that from 200,000 to 125,000 years ago, the climate was relatively favourable. The lowlands supported abundant animal and plant life. This meant that humans could travel longer distances, and even reach the Arabian Peninsula, where fossil finds date to about 175,000 years before present.

Then the climate became gradually drier, culminating in a very arid period between about 60,000 and 40,000 years ago.

Furthermore, the very precise core record reveals striking, short-term moisture fluctuations, which are reminiscent of warm/cold shifts revealed in Greenland ice cores. Environmental shifts were extreme, so much so that people moved from the lowlands to the highlands, where numerous archaeological finds are found. This was also the period when tools and weapons became smaller and more sophisticated. The researchers speculate that these innovations resulted from environmental stress at lower elevations. There’s more. The last major wet phase in the core agrees with the latest genetic findings, which records that some of our direct genetic ancestors left Africa to permanently settle elsewhere about 70,000 to 50,000 years ago. Ultimately, they arrived in Europe between 40,000 and 50,000 years before present. At last, science is zeroing in on the date of a major event in the human past.

Image: The Chew Bahir Basin, as pictured by NASA from space.