To get around these issues, the researchers collected pumice samples from the Shala volcano more than 248 miles (400 kilometers) away, grinding them down until they were less than a millimeter in size. The fossils lacked nearby stone artifacts or fauna that could be dated, and the ash they were buried under was too fine-grained for radiometry - a method that quantifies the amounts of certain radioactive isotopes (versions of an element with a different number of neutrons in the nucleus) with known decay rates. Despite discovering the fossils more than 50 years ago, scientists have found it difficult to give the Omo I remains a conclusive age. The remains were found in the East African Rift valley, an active continental rift zone where the African tectonic plate is in the process of splitting into two smaller plates, the Somali Plate and the Nubian Plate. The Omo Kibish formation in southern Ethiopia has preserved ancient human remains inside its layered deposits.
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