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Geologists: Ethiopian Plateau Spurred Human Evolution

18.09.2007.
Source: Larry O'Hanlon, Discovery News

The geologic rise of the Ethiopian Plateau may have happened millions of years later than thought, and just in time to nudge along the evolution of modern humans, say the authors of a new study.

Using elevation data collected by the space shuttle, orbiting satellites and radioisotope dating of various rock layers found in the walls of the Nile River Gorge, the researchers believe they have narrowed down the timing of the final phase of the plateau's uplift to less than three million years ago.

That's just in time to cause the drying out of east Africa and the creation of the pedestrian-friendly savannas on which humans evolved.

"I think we've proved that [the Ethiopian Plateau] is very young," said Nahid Gani, a geologist at the University of Utah. She and her husband Royhan Gani, also a geologist, and Mohamed Abdelsalam of the University of Missouri, report their findings in the September issue of GSA Today, published by the Geological Society of America.

Some previous estimates of the plateau's age put its rise at around 30 million years ago far too early for it to have directly affected the immediate ancestors of Homo sapiens, who arose in the valley just a few million years ago.

To arrive at the more recent date, the researchers turned the question upside down. Rather than dating the plateau's uplift directly, they studied the effects of the one thing in the region that responds instantly to any change in elevation the Blue Nile River. Like all rivers, it naturally shifts its course as the land around it changes, to form the most natural slope from its source to the sea.

As the plateau rose, it raised the river's source and steepened the river's overall slope. That steepening made the river flow faster and with more erosive power. The result is the mile-deep Nile Gorge what Nahid Gani refers to as the Grand Canyon of Africa.

The history of the gorge is one key to uncovering the timing of the plateau's uplift. To uncover that history, the team integrated topographic maps collected from space with field work and previous studies on the dates of volcanic eruptions and other events in the region's geological history.

Ultimately, they determined that the Blue Nile eventually carved out more than 22,400 cubic miles of rock from the area, not at a constant rate, but in three pulses that reflect major geologic events. The last phase, which began six million years ago, may have been caused by foundering of a deep part of the earth's crust, which let the plateau rise like ship losing ballast.

In any case, the researchers estimate that the plateau rose more than 3,000 feet in just the last few million years, creating "The Roof of Africa" a plateau with a mean elevation of 8,000 feet. That's high enough to wring moisture out of monsoonal air moving east across Africa, leaving the Ethiopian Rift Valley drier and drier, and dominated by grasslands instead of forests.

"So (human ancestors) had to walk across that savanna," said Royhan Gani. And walking on two feet, as independent studies have shown, is the most efficient way for an ape to cross large distances.

A simultaneous drop in the Rift Valley's elevation also contributed to the drying, said geologist Martin Williams of the University of Adelaide in Australia.

"The African Rift has been going down and widening," Williams told Discovery News. "The real drying out of Ethiopia proper began 3 to 2 1/2 million years ago." Just in time, perhaps, to drive the evolution of our ancestors.