Earliest stone artifacts found in remote Kenya

Tools discovered during wrong turn may push back human evolution trace by 500,000 years

By taking a wrong turn in a dry riverbed in Kenya, scientists discovered a trove of stone tools far older than any ever found before. Nobody knows who made them - or why.

At 3.3 million years, they push back the record of stone tools by about 700,000 years, and they are half-a-million years older than any known trace of the human evolutionary tree.

Scientists have long thought that sharp-edged stone tools were made only by members the branch designated "Homo", such as Homo sapiens. The discovery questions that thinking and boosts the idea that toolmaking may have begun with smaller-brained forerunners instead.

The discovery was made by Sonia Harmand and Jason Lewis of Stony Brook University in New York, the co-authors of a paper published on Wednesday in the journal Nature.

The paper describes 149 stones and stone flakes found west of Lake Turkana in a remote area of northern Kenya.

Most objects are "cores", which are stones that have been struck to break off sharp-edged flakes. Other stones appear to have been used as hammers or anvils.

The site was discovered in July 2011, when Harmand, Lewis and a crew set out to survey one area and by accident ended up in another one.

As stone tools go, the artifacts are remarkably big. On average, the cores are approximately 15 centimeters square and weigh more than 3 kilograms, while the flakes are up to 20 centimeters long.

Compared to the next-oldest-known tool artifacts, "these things are enormous", which adds to the mystery of what they were used for, said David Braun, a tool expert at George Washington University.

Harmand said she thinks the overall purpose was to make sharp-edged flakes for cutting, but exactly how they were used is not known. Researchers are examining them with a microscope to look for clues.

Then there's the question of who made them. "The jury is out on that," Lewis said.

One candidate would be some Homo species not yet known to science, he said. Other possibilities come from outside the Homo branch, such as Australopithecus afarensis or a creature called Kenyanthropus platyops, known from remains found not far from the site of the stone tools.