WE MIGHT have something to thank major climate change for after all – though only when it occurred in the distant past. It could explain why early humans learned to walk around 3 million years ago, and why our ancestors started to migrate out of Africa around 70,000 years ago.
Royhan Gani from the University of Utah, Salt Lake City, and colleagues combined geological data from the ground with radar data from space to create a detailed three-dimensional map of the . Regular volcanic eruptions over the past 30 million years have left numerous lava layers there that are easy to date, and by studying the way that the rivers cut through these layers the team was able to estimate rates of uplift and piece together how the Ethiopian plateau changed.
This showed the region had risen more than 1 kilometre, beginning around 6 million years ago (GSA Today, ). “The only viable explanation for this sudden rise is a ,” says Gani.
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The upwelling of hot rock created a mountain barrier, transforming atmospheric circulation and creating a rain shadow, he says. This encouraged the growth of grassland rather than trees. The explanation supports the theory, originally proposed by Charles Darwin, that humans learned to walk around 3 million years ago when they were forced to abandon the trees and scurry across the savannah.
“The upwelling of hot rock created a mountain barrier, encouraging the growth of grassland rather than trees”
Another climate swing around 70,000 years ago then helped our ancestors to migrate out of Africa. Christopher Scholz at Syracuse University in New York and colleagues studied sediment from the bottom of Lake Malawi in east Africa. Older muds – from between 150,000 and 75,000 years ago – had layers containing fossils, showing that water levels must have dropped to around 100 metres, indicating periods of extreme drought. About 70,000 years ago the climate stabilised and the lake stayed around 700 metres deep, with no fossils in the mud (Proceedings of the National Academy of Sciences, ).
This change from a highly variable to a more stable and moist climate can be linked to a change in the Earth’s orbit, and this made it easier for our ancestors to explore. “More extensive zones of forest would have started to appear and they weren’t being stressed out by mega-droughts every few thousand years,” says Scholz.
