AP — It looked like a white splotch on the underside of a Neanderthal stone tool, but a microscope showed it was a bunch of fibers twisted around each other.

Further examination revealed it was the first direct evidence that Neanderthals could make string, and the oldest known direct evidence for string-making overall, researchers say.

The find implies our evolutionary cousins had some understanding of numbers and the trees that furnished the raw material, they say. It’s the latest discovery to show Neanderthals were smarter than modern-day people often assume.

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Bruce Hardy, of Kenyon College in Gambier, Ohio, and colleagues reported the discovery in a paper released Thursday by the journal Scientific Reports. The string hints at the possibility of other Neanderthal abilities, like making bags, mats, nets and fabric, they said.

The string came from an archaeological site in the Rhone River valley of southeastern France, and is roughly 40,000 to 50,000 years old. Researchers don’t know how Neanderthals used the string or even whether it had been originally attached to the stone cutting tool.

Maybe the tool happened to fall on top of the string, preserving the quarter-inch (6.2 mm) segment while the rest disintegrated over time, Hardy said. The string is about one-fiftieth of an inch (0.55 mm) wide.

It was made of fiber from the inner bark of trees. Neanderthals twisted three bundles of fibers together counterclockwise, and then twisted these bundles together clockwise to make the string. That assembly process shows some sense of numbers, Hardy said.

Paola Villa, a Neanderthal expert at the University of Colorado Museum who was not involved in the new study, noted that Hardy had previously found “tantalizing evidence” for string-making by Neanderthals. The new work now illustrates the ability directly, she said.