They wrote that they "could feel that something was special about this particular place." They fanned out and surveyed a nearby patch of craggy outcrops.

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But researchers have been uncovering tantalizing clues that some other, earlier species of hominin, might have figured it out.

The researchers do not know who made these oldest of tools.

But earlier finds suggest a possible answer: The skull of a 3.3-million-year-old hominin, Kenyanthropus platytops, was found in 1999 about a kilometer from the tool site. platyops tooth and a bone from a skull were discovered a few hundred meters away, and an as-yet unidentified tooth has been found about 100 meters away.

The discovery is the first evidence that an even earlier group of proto-humans may have had the thinking abilities needed to figure out how to make sharp-edged tools.

The stone tools mark "a new beginning to the known archaeological record," say the authors of a new paper.

"The whole site's surprising, it just rewrites the book on a lot of things that we thought were true," said geologist Chris Lepre of the Lamont-Doherty Earth Observatory and Rutgers University, a co-author of the paper who precisely dated the artifacts.

The new find came about almost by accident: Harmand and Lewis said that on the morning of July 9, 2011, they had wandered off on the wrong path, and climbed a hill to scout a fresh route back to their intended track.

In an accidental discovery that will upturn many existing theories, scientists have found stone tools dating back 3.3 million years in northwestern Kenya, the oldest such artifacts yet discovered.

This age is long before the advent of modern humans.