Ancient Humans Left Africa to Escape Drying Climate, Says Study

Ancient Humans Left Africa to Escape Drying Climate, Says Study

Humans migrated out of Africa to escape a drying climate, about 60,000 years ago, according to a new study in the journal Geology. The study contradicts previous suggestions to the opposite: that the climate must have been wet in order for people to be able to cross the Horn of Africa and the Middle East.

While humans may have left Africa earlier, genetic research indicates that the main migration into Eurasia that eventually peopled the rest of the world 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,” said study coauthor Peter deMenocal, a climatologist at Columbia University’s Lamont-Doherty Earth Observatory. “We may have to revise this model to one where people were pushed out, due to unfavorable conditions.”

The researchers 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. The study’s lead author, Jessica Tierney of the University of Arizona, said the core showed that 70,000 years ago, the region also became cooler. “Our data say the migration comes after a big environmental change. Perhaps people left because the environment was deteriorating,” she said.

Tierney, deMenocal and colleagues had previously charted the Horn of Africa’s climate back 40,000 years by studying cores of marine sediment. They hoped to use the same means to reconstruct the region’s climate back 55,000 to 70,000 years ago, but the first challenge was finding a core with sediments that old. They found one at the Lamont-Doherty Core Repository, collected off the Horn of Africa in 1965 by the Lamont research ship Robert D. Conrad. DeMenocal studied and dated the layers of the 1965 core and found it had sediments going back as far as 200,000 years.

At the University of Arizona, Tierney and coauthor Paul Zander teased out temperature and rainfall records from organic matter preserved in the sediment layers. The scientists took samples from the core about every four inches, a distance that represented about every 1,600 years.

To construct a long-term temperature record for the Horn of Africa, 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.

To figure out the region’s ancient rainfall patterns, the researchers analyzed leaf wax that had blown into the ocean from terrestrial plants. Because plants alter the chemical composition of the wax depending on how dry or wet the climate is, this 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.

The team’s findings are corroborated by research from other investigators who reconstructed past regional climate by using data gathered from a cave formation in Israel and a sediment core from the eastern Mediterranean. Those findings suggest that it was dry everywhere in northeast Africa, she said.

“Our main point is kind of simple,” Tierney said. “We think it was dry when people left Africa and went on to other parts of the world, and that the transition from a green Sahara to dry was a motivating force for people to leave.”

The National Science Foundation and the David and Lucile Packard Foundation funded the research.

— Kevin Krajick, Earth Institute