WASHINGTON : Scientists have found that drought has decreased in the African continent over the past 1.3 million years, contradicting the prevailing notion that the continent has been getting progressively drier over time.
A new study found that the African continent is on a 100,000-year cycle of wet and dry conditions.
These new findings add a wrinkle to one of the keys to human evolutionary theory, the savannah hypothesis, which states that the progressively drier conditions in Africa led to prehuman ancestors migrating from forests and moving into grasslands.
Josef Werne, associate professor at the University of Pittsburgh in the US and colleagues made the discovery by examining core samples extracted from the bottom of Lake Malawi, one of the world’s largest lakes, located between Malawi, Mozambique and Tanzania in southeastern Africa.
Previous studies of the climate of Africa focused on the northern part of the continent, Werne explained, and were responsible for the origin of the savannah hypothesis that the continent was getting drier.
The 100,000-year cycles the researchers found correspond with the beginnings and endings of the great ice ages.
Lake Malawi had not been explored previously because the depth of the waters – 700 feet – exceeded researchers’ ability to get core samples from the bottom.
The researchers were able to overcome that limitation by using a barge and modifying oil-rig equipment to obtain a 380-meter-long sediment core sample.
The core was dated using a combination of radiocarbon, volcanic ash, and magnetic polarity reversals and examined for “molecular fossils” indicating changing temperature and rainfall.
Temperature was derived by studying the distribution of the membrane lipids of a single-celled microbe, which was analysed by mass spectroscopy, and the aridity and rainfall were measured by calcium content and the distribution and carbon isotope composition of fossil leaf waxes, which differ between those originating in trees and shrubs, which thrive in wetter conditions, and those originating in grasses, which can outcompete trees in dry conditions.
By noting the changes in temperature records and especially rainfall, the team determined that the continent was getting wetter over time in southern East Africa, as well as identifying the 100,000-year climate cycles.
The study was published in the journal Nature. (AGENCIES)
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