Humans Migrated Out of Africa to Escape Drying Climate, New Study Says
Oct 17, 2017 by
News Staff / Source
"Ancient humans left Africa to escape a drying climate, about 60,000 years ago — a finding that contradicts previous suggestions that humans were able to leave because a then-wet climate allowed them to cross the generally arid Horn of Africa and Middle East. The
study is published in the journal
Geology.
While humans may have left Africa earlier, genetic research indicates that the main out-of-Africa migration probably occurred between 70,000 and 55,000 years ago.
“It has long been proposed that the out-of-Africa migration was facilitated by wet periods that created green corridors to Eurasia — in other words, the climate pulled people out. We may have to revise this model to one where people were pushed out, due to unfavorable conditions,” said study co-author Dr. Peter deMenocal, of Columbia University.
Dr. deMenocal and colleagues traced the Horn of Africa’s climate 200,000 years into the past by analyzing a core of ocean sediment taken in the western end of the Gulf of Aden, off present-day Somalia.
“There’s always been a question about whether climate change had any influence on when our species left Africa. Our data suggest that when most of our species left Africa, it was dry and not wet in northeast Africa,” said lead author Dr. Jessica Tierney, from the University of Arizona.
The researchers analyzed the sediment layers for chemicals called alkenones made by a particular kind of marine algae.
“The algae change the composition of the alkenones depending on the water temperature. The ratio of the different alkenones indicates the sea surface temperature when the algae were alive and also reflects regional temperatures,” Dr. Tierney explained.
To figure out the region’s ancient rainfall patterns from the sediment core, the authors analyzed the ancient leaf wax that had blown into the ocean from terrestrial plants.
Because plants alter the chemical composition of the wax on their leaves depending on how dry or wet the climate is, the leaf wax from the sediment core’s layers provides a record of past fluctuations in rainfall.
The team found that around 70,000 years ago, climate in the Horn of Africa shifted from a wet phase called ‘Green Sahara’ to even drier than the region is now. The region also became colder. "
Continues...