UNIVERSITY PARK, Pa. — The origin of flaked-stone tool production is older than 2.58 million years ago, according to an international team of scientists working at the Bokol Dora 1 archaeological site in the Afar region of Ethiopia. Previously the oldest evidence of flaked-stone tools was younger than 2.58 million years ago.

"At first we found several artifacts lying on the surface, but we didn't know what sediments they were coming from," said Christopher Campisano, associate professor, School of Human Evolution and Social Change, Arizona State University. "But when I peered over the edge of a small cliff, I saw rocks sticking out from the mudstone face. I scaled up from the bottom using my rock hammer and found two nice stone tools starting to weather out."

Bokol Dora 1 is near Lee Adoyta, where in 2013 archaeologists found the fossil jaw bone of a human ancestor dating to about 2.78 million years ago.

Recently, stone tools for hammering, dating to 3.3 million years ago, were found in Kenya, but these are not flaked-stone tools. The process used to make flaked-stone tools, flint knapping, systematically chips off smaller sharp-edged tools from larger nodules of stone, creating tools suitable for scrapping, cutting and piercing. Earlier stone tools like those found in Kenya or those sometimes used by chimpanzees and monkeys are used to hammer and bash foods such as nuts and shellfish.

The archaeologists working at the Bokol Dora 1 site wondered how these flaked tools fit into the increasingly complex picture of stone tools production. These oldest artifacts, ascribed to the "Oldowan," were distinct from the tools made by chimpanzees, monkeys and even earlier human ancestors.

"We expected to see some indication of an evolution from the Lomekwian (Kenya) to these earliest Oldowan tools," said Will Archer, Max Planck Institute for Evolutionary Anthropology, Leipzig and the University of Cape Town. "Yet when we looked closely at the patterns, there was little connection to what is known from older archaeological sites or to the tools modern primates are making."

Recovery of the flaked-stone tools took several years because the archaeologists needed to carefully excavate through many layers of sediment to reach the layer that contained animal bones and hundreds of pieces of chipped stone.

"These tools were dropped by early humans at the edge of a water source and then quickly buried," said Vera Aldeias, interdisciplinary Center for Archaeology and Behavioral Evolution, University of Algarve, Portugal. "The site then stayed that way for millions of years."

Kaye Reed, director of the Ledi-Geraru Research Project and research associate in the Institute of Human Origins, Arizona State, noted that the animal bones found with these tools are similar to those found a few kilometers away at Ledi-Geraru with the 2.78 million-year-old jaw bone.

The researchers used two methods to date the layer where they found the flaked tools. Because they found the fossil layer above a layer of consolidated volcanic ash, they dated the ash layer using Argon40/Argon39 dating to about 2.58 million years ago.

"We found and mapped a gray ash layer meters below the archaeological site," said Erin DiMaggio, assistant research professor in geoscience, Penn State. "We were really fortunate that it contained feldspar minerals and we successfully dated them to constrain the age of the archaeological layer."

They also used magnetostratigraphic dating, a method useing the status of the Earth's magnetic field at the time the sediment was laid down, to constrain the date. Throughout the Earth's history, the magnetic poles have flipped many times. The researchers found that the magnetic signature at the site indicates that it is older than 2.58 million years ago and therefore older than all previously known sites in the area. They report their findings today (June 3) in the Proceedings of the National Academy of Sciences.

The researchers found that the flaked-stone tool technology at Bokol Dora 1 was different than the stone tool technology at an older site in Kenya. They suggest that while many of our human ancestors had the ability to make stone tools, the change in technology to producing sharp-edged flakes would have increased the amount and variety of things to eat and that this change in diet may have been important in the evolution of our genus Homo.

"Given that primate species throughout the world routinely use stone hammers to forage for new resources, it seems very possible that throughout Africa many different human ancestors found new ways of using stone artifacts to extract resources from their environment," said David Braun, associate professor of anthropology, George Washington University. "If our hypothesis is correct, then we would expect to find some type of continuity in artifact form after 2.6 million years ago, but not prior to this time period. We need to find more sites."

The U.S. National Science Foundation and the John Templeton Foundation supported this research.