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 old, they push back the record of stone tools by about 700,000 years.
More significantly, they are half-a-million years older than any known trace of our own branch of the evolutionary tree.
Scientists have long thought that sharp-edged stone tools were made only by members of our branch, whose members are designated “Homo”, like our own species, Homo sapiens.
That idea has been questioned, and the new finding is a big boost to the argument that tool-making may have begun with smaller-brained forerunners instead.
The discovery was reported by Sonia Harmand and Jason Lewis of Stony Brook University in New York and co-authors in a paper released on Wednesday by the journal Nature.
The Nature 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.
Africa is where our own species first appeared, and it has long been a hotbed for finding fossils of our forerunners.
The Kenyan site was discovered one day in July 2011, when Harmand, Lewis and a crew set out to survey one area and by accident ended up in another one.
There were gullies and hillsides that seemed promising, so they looked around, Lewis recalled.
Just before tea time, a team member spotted a stone tool on the ground. More quickly appeared. Excavations followed.
As stone tools go, the artifacts are remarkably big.
On average, the cores stretch about 15 centimetres long and wide and weigh nearly three kilograms, for example, while the flakes are up to 20 centimetres long.
Generally, ancient human relatives are thought to have used stone tools for hammering, such as for cracking nuts, and for their sharp edges, useful for butchering and skinning animal carcasses as well as cutting up tough plant material.
Experts said they were stumped about the purpose of the Kenyan tools.
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.
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, best known for the skeleton nicknamed Lucy.
Still another candidate is a creature called Kenyanthropus platyops, known from remains found not far from the site of the stone tools.