An international team of geologists led by Dr. Eric Roberts of Australia’s James Cook University has suggested that the Great Rift Valley of East Africa – the birthplace of the human species – may have taken much longer to develop than previously believed.
The study, published in the journal Nature Geoscience, shows that initiation of rifting in the western branch of the East African Rift began more than 14 million years earlier than previously thought, contemporaneously with the eastern branch.
“We now believe that the western portion of the rift formed about 25 million years ago, and is approximately as old as the eastern part, instead of much younger as other studies have maintained,” said Dr. Michael Gottfried, an associate professor of geological sciences at Michigan State University and a co-author on the study.
“The significance is that the Rift Valley is the setting for the most crucial steps in primate and ultimately human evolution, and our study has major implications for the environmental and landscape changes that form the backdrop for that evolutionary story.”
The Rukwa Rift (a segment of the western branch) is an example of a divergent plate boundary, where the Earth’s tectonic forces are pulling plates apart and creating new continental crust. The East African Rift System is composed of two main segments: the eastern branch that passes through Ethiopia and Kenya, and a western branch that forms a giant arc from Uganda to Malawi, interconnecting the famous rift lakes of eastern Africa.
Traditionally, the eastern branch is considered much older, having developed 15 to 25 million years earlier than the western branch.
This study suggests that the two rift segments developed at about the same time, nearly doubling the initiation age of the western branch and the timing of uplift in this region of East Africa.
“A key piece of evidence in this study is the discovery of approximately 25 million-year-old lake and river deposits in the Rukwa Rift that preserve abundant volcanic ash and vertebrate fossils,” Dr. Roberts explained.
“These deposits include some of the earliest anthropoid primates yet found in the rift,” said Dr. Nancy Stevens of Ohio University, a co-author on the study.
The results imply that around 25 to 30 million years ago, the broad uplift of East Africa occurred and re-arranged the flow of large rivers such as the Congo and the Nile to create the distinct landscapes and climates that mark Africa today.
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