We know that humans first started to migrate our of Africa around 60,000 years ago, what we’ve never been entirely sure of is what caused the to do it.
Using the world’s most important collection of sediment cores from the deep sea the researchers were, incredibly, able to actually determine the temperature and climate from 60,000 years ago.
Previous research has suggested that when humans moved into Eurasia around 40,000-70,000 years ago north Africa needed to be wetter than it is now. What they found was very different.
Using the sediment samples the team found that Africa had undergone a major transformation. Its previously fertile ‘Green Sahara’ had started to dry out, in fact at around the time humanity started to leave the Sahara was even drier than it is now, and a lot colder.
“Our data say the migration comes after a big environmental change. Perhaps people left because the environment was deteriorating,” explains Jessica Tierney, UA associate professor of geosciences.
“There was a big shift to dry and that could have been a motivating force for migration.”
What’s almost as impressive as their discovery is how they discovered it in the first place.
To create a long-term temperature record for the Horn of Africa the team analysed 4-inch segments of the sediment core with each section accounting for around 1,600 years.
They then analysed the layers for chemicals called alkenones which are made by a very specific type of marine algae. As the temperature changes, so too does the composition of the chemicals being made by the algae, effectively allowing the team to take a temperature reading from 60,000 years later.
To figure out the rainfall the team did something equally as impressive.
They analysed the leaf wax that had blown into the ocean. Plants alter the chemical composition of their leaf wax depending on how wet or dry the climate is. By looking at the composition of the wax from that precise period in time they could tell exactly how wet or dry it was.
What they found was the last piece of the puzzle which in turn led them to one easy conclusion.
“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.”