Earlier date for Ethiopian fossils sheds light on rise of Homo sapiens

By Will Dunham

(Reuters) – Volcanic ash left by a huge ancient eruption has helped scientists determine that the first significant Homo sapiens fossils discovered in Ethiopia in 1967 are older than previously thought, offering new insight into the dawn of our species.

Researchers said on Wednesday they used geochemical fingerprints of a thick layer of ash found above the sediments containing the fossils to determine that it was the result of an eruption that spewed volcanic fallout across a wide swath of Ethiopia there about 233,000 years ago.

Because the fossils lay beneath that ash, they predate the eruption, the researchers said, though the number of years remains uncertain. The fossils were previously believed to be no more than around 200,000 years old.

The fossils, called Omo I, were discovered in southwestern Ethiopia in an area called the Omo Kibish geological formation during an expedition led by the late paleoanthropologist Richard Leakey. They include a fairly complete cranial vault and lower jaw, some vertebrae, and parts of arms and legs.

Scientists have been looking for more clarity on the timeline of our species’ origins in Africa.

The new findings are in line with the most recent scientific models of human evolution placing the emergence of Homo sapiens between 350,000 and 200,000 years ago, said University of Cambridge volcanologist Celine Vidal, author principal of the study published in the journal Nature.

Research published in 2017 showed that bones and teeth discovered at a site called Jebel Irhoud in Morocco were over 300,000 years old, representing the earliest fossils attributed to Homo sapiens. Some scientists have wondered if these fossils really belong to our species.

The Jebel Irhoud remains “lack some of the key morphological features that define our species. They particularly lack a high, globular cranial vault and a chin on the lower jaw, which can be seen on Omo I,” said paleoanthropologist Aurélien Mounier of CNRS and the Musée de l’Homme in Paris, co. – author of the new study.

“Omo I is the oldest Homo sapiens with unmistakable modern human traits,” added University of Cambridge volcanologist and study co-author Clive Oppenheimer.

The volcanic ash layer defied previous efforts to calculate its age because its grains were too fine for scientific dating methods.

The researchers determined the geochemical composition of the ash and compared it with other volcanic remains in the area. They found it to correspond to a light, porous volcanic rock called pumice created when the Shala volcano erupted about 230 miles (370 km) away. They were then able to date the pumice stone to determine when the eruption occurred.

“I think it’s important to keep in mind that the study of human evolution is always on the move: boundaries and timelines change as our understanding improves,” Vidal said. “But these fossils show how resilient humans are: that we have survived, thrived and migrated in a region so prone to natural disasters.

While the study resolved the minimum age of the fossils, their maximum age remains a mystery. There is also a layer of ash under the sediment containing the fossils which has not yet been dated. This date would fix the maximum age of the fossils.

“It is probably no coincidence that our earliest ancestors lived in such a geologically active Rift Valley – it collected precipitation in lakes, providing fresh water and attracting animals, and served as a corridor of natural migration spanning thousands of kilometres,” Vidal said. “Volcanoes provided fantastic materials for making stone tools, and from time to time we had to develop our cognitive abilities when large eruptions transformed the landscape.”

(Reporting by Will Dunham in Washington; Editing by Lisa Shumaker)

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