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By Alan Boyle

They don't look like much, but scores of crude stone tools discovered in Kenya are special: Scientists say they date back 3.3 million years, which makes them the oldest such artifacts ever found by 700,000 years.

The finding suggests that members of the genus Homo — the biological grouping that includes our own species, Homo sapiens — were not the first to shape stones for their own purposes. But that raises deep questions.

"When we first discovered the tools, we had to start re-examining who the potential makers were, and why they might have started making such tools at this new time," Jason Lewis, a paleoanthropologist at Stony Brook University's Turkana Basin Institute, said in a podcast provided by the journal Nature.

The discovery, detailed in this week's issue of Nature, was made in 2011 during an excavation conducted by the West Turkana Archaeological Project in Kenya. The project's directors are Lewis and Sonia Harmand, the lead author of the Nature study (and Lewis' wife).

Harmand told Nature that the find was a case of serendipity: "We were driving in a dry riverbed, and took the left branch instead of the right, and ended up in a new area that looked promising. So we were very excited."

She said the team came across a set of large stones that were chipped, or knapped, in a pattern that's characteristic of primitive tools. Eventually, more than 100 artifacts and 22 fossil remains were collected from the site, which was designated Lomekwi 3. The researchers' estimate of the age of the tools — 3.3 million years — is based on an analysis of the sedimentary layer where they were found.

Suspicions confirmed

Before Lomekwi, the oldest known stone tools came from other sites in Ethiopia. They're thought to have been made 2.6 million years ago, most likely by early members of the genus Homo. But scientists had suspected that tool-making went significantly farther back: Five years ago, paleoanthropologists reported finding animal bones in Ethiopia that bore the signs of being cut 3.3 million to 3.4 million years ago.

That evidence of tool use was associated with Australopithecus afarensis, the species best-known for the fossil dubbed "Lucy." In an email, Lucy's discoverer told NBC News that the latest find could represent "a major step back in time" for the history of stone tool use among hominins — that is, the prehistoric ancestors of modern humans.

"It is not yet possible to ascertain what the hominins were doing with these fashioned stones, but if chimps are a guide, perhaps they were using them to open hard-shelled fruits or nuts," said Donald Johanson, who came across Lucy's skeleton in 1974 and now heads the Institute of Human Origins at Arizona State University. "Some may argue that the flakes [of rock] were not necessarily the desired outcome of the activity, but an accidental product of the behavior of using rocks and anvils to open hard-to-get-at food."

An enigmatic discovery

David Braun, a paleoanthropologist at George Washington University, said the Lomekwi tools were an enigma.

"They are extremely different from anything found previously, and are extremely different from event those tools made by chimpanzees," Braun wrote in an email to NBC News. The biggest difference has to do with their size: The average weight is 3 kilograms (6.6 pounds), which is more than 10 times heavier than the stone tools previously regarded as the world's oldest.

"The enormous sizes of these tools does require pause as to what these tools would have been used for," Braun said.

Stony Brook University researchers Sonia Harmand and Jason Lewis examine the stone tools found at the Lomekwi 3 excavation site on the western shore of Lake Turkana in northern Kenya.MPK-WTAP

Braun also would like to see additional confirmation for the dates assigned to the tools. "It's an amazing find, but I think we don't exactly know just yet what it means for hominin behavioral evolution," he said.

In light of the Lomekwi find, Johanson said researchers will probably "refocus their energies on this time period in attempts to locate more instances of such activity." Among the candidates for the tool-makers are Australopithecus afarensis, another extinct species known as Kenyanthropus platyops, or perhaps some unknown species that could conceivably fit in the genus Homo.

"It now appears that early humans possessed the seeds of early tool-making that ultimately led to the culture-bound creature we call Homo sapiens," Johanson said.

In addition to Harmand and Lewis, the authors of "3.3-Million-Year-Old Stone Tools From Lomekwi 3, West Turkana, Kenya" include Craig Feibel, Christopher Lepre, Sandrine Prat, Arnaud Lenoble, Xavier Boes, Rhonda Quinn, Michel Brenet, Adrian Arroyo, Nicholas Taylor, Sophie Clement, Guillaume Daver, Jean-Philip Brugal, Louise Leakey, Richard Mortlock, James Wright, Sammy Lokorodi, Christopher Kirwa, Dennis Kent and Helene Roche.