Stone tools discovered in Kenya are the oldest ever found, and at 3.3 million years old were made before the earliest humans of our genus Homo evolved, researchers say.

The tools found on the shores of Kenya's Lake Turkana are 700,000 years older than any tools ever discovered before, suggesting that some earlier ancestor species of hominids may have been more sophisticated than previously believed, they say.

The discovery is leading scientists to rethink a commonly accepted assumption that tool-making ability is what separated the earliest humans from earlier hominid lineages and allowed us to evolve and dominate.

"This discovery challenges the idea that the main characters that make us human  — making stone tools, eating more meat, maybe using language — all evolved at once in a punctuated way, near the origins of the genus Homo," says Jason Lewis, a paleoanthropologist at Rutgers University and co-author of a study appearing in the journal Nature.

It had previously been believed our earliest ancestor in the Homo genus to use tools was Homo habilis — "handy man" — but fossils of Homo habilis only go back to around 2.4 million years.

Earlier, more primitive species such as Australopithecus afarensis — known by the famous fossil dubbed "Lucy" — and Kenyanthropus platyops, with their mix of ape-like and human features, had been considered not very intelligent.

However, since they date to the time of the Kenya tools, they may have been smarter than they've been given credit for, making the 3.3 milion-year-old tools "a game-changing find," says Ignacio de la Torre from University College London's Institute of Archaeology.

He calls it the most important discovery in paleontology in the last 50 years.

"It suggests that species like Australopithecus might have been intelligent enough to make stone tools — that they had the cognitive and manipulative abilities to carry tasks like this out."

The tools found in Kenya are crude but show definite signs of having been worked in a process called knapping, where a rock is struck by another to create small, sharp flakes than can be used as cutting tools.

"There is no doubt these were intentionally knapped," says study co-leader Sonia Harmand, a research associate professor from Stony Brook University in New York.

 "And that makes these rocks very special, because it means they were probably intentionally crafted tools," she says.

The Kenya tools shed light on a period of hominid behavior that was unexpected and previously unknown, she adds.

"They tell us a lot about cognitive development in our ancestors that we can't understand from fossils alone."