Final uplift of Ethiopian plateau coincided with human evolution on Earth

Geologists have pinned down the exact time of the final uplift of the Ethiopian plateau, which they say, also coincided with the evolution of humans on this planet.

Washington, Sept 19 : Geologists have pinned down the exact time of the final uplift of the Ethiopian plateau, which they say, also coincided with the evolution of humans on this planet.

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, said Nahid Gani, a geologist at the University of Utah, in her study in the September issue of GSA Today.

"I think we've proved that the Ethiopian Plateau is very young," said Gani.

Gani and her husband Royhan Gani, also a geologist, and Mohamed Abdelsalam of the University of Missouri, US, conducted the study.

The researchers said the geologic rise of the Ethiopian Plateau happened millions of years later than previously thought, and just in time to nudge along the evolution of modern humans.

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.

For their study, the scientists used 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.

To arrive at the more recent date, the researchers studied the response of the Blue Nile to effects of change in elevation.

Like all rivers, the Blue Nile naturally shifts its course as the land around it changes, to form the most natural slope from its source to the sea.

Findings revealed that as the plateau rose, it raised the river's source and steepened the river's overall slope, which made the river flow faster and with more erosive power.

That resulted in the formation of the mile-deep Nile Gorge - also referred to as the Grand Canyon of Africa.

Finally, the team integrated topographic maps collected from space with fieldwork and previous studies on the dates of volcanic eruptions and other events in the region's geological history.

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 now reflect major geologic events.

The scientists said the last phase, which began six million years ago, might have been caused by foundering of a deep part of the earth's crust, which let the plateau rise like a ship losing ballast.

"In any case, the plateau rose more than 3,000 feet in just the last few million years, creating "The Roof of Africa", with a mean elevation of 8,000 feet, 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," the researchers wrote in their study.

"So human ancestors had to walk across that savannah. And walking on two feet, as independent studies have shown, is the most efficient way for an ape to cross large distances," Discovery News quoted Royhan Gani, as saying.

Geologist Martin Williams of the University of Adelaide, Australia, who was not involved in the study, said, a simultaneous drop in the Rift Valley's elevation also contributed to the drying.

"The African Rift has been going down and widening. 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," said Williams.

ANI