Humans migrated out of Africa as the climate shifted from wet to very dry about 60,000 years ago, a research has found.
Genetic research indicates people migrated from Africa into Eurasia between 70,000 and 55,000 years ago. Previous researchers suggested the climate must have been wetter than it is now for people to migrate to Eurasia by crossing the Horn of Africa and the Middle East.
"There has always been a question about whether climate change had any influence on when our species left Africa," said Jessica Tierney, associate professor of at University of Arizona in the US.
"Our data suggest that when most of our species left Africa, it was dry and not wet in northeast Africa," said Tierney, who led the study published in the journal Geology.
Researchers 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.
They traced the Horn of Africa's climate 200,000 years into the past by analysing a core of ocean sediment taken in the western end of the Gulf of Aden.
"Our data say the migration comes after a big environmental change. Perhaps people left because the environment was deteriorating," Tierney said.
"There was a big shift to dry and that could have been a motivating force for migration," she said.
Earlier, the researchers had successfully revealed the Horn of Africa's climate back to 40,000 years ago by studying cores of marine sediment.
The team hoped to use the same means to reconstruct the region's climate back to the time 55,000 to 70,000 years ago when our ancestors left Africa.
The first challenge was finding a core from that region with sediments that old.
The researchers enlisted the help of the curators of the Lamont-Doherty Core Repository, which has sediment cores from every major ocean and sea.
The curators found a core collected off the Horn of Africa in 1965 that might be suitable.
Researchers studied and dated the layers of the 1965 core and found it had sediments going back as far as 200,000 years.
They then teased out temperature and rainfall records from the organic matter preserved in the sediment layers.
The scientists took samples from the core about every four inches, a distance that represents about 1,600 years.
They analysed 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, Tierney said.
To figure out the region's ancient rainfall patterns from the sediment core, the researchers analysed the ancient leaf wax that had blown into the ocean from terrestrial plants.
Since 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 analyses showed that the time people migrated out of Africa coincided with a big shift to a much drier and colder climate, Tierney said.
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