The human thumb is a nimble wonder, allowing us to make tools, sew clothing, and open pickle jars. But just how and when this unique digit evolved has long been a mystery. Now, a new study modeling muscle in fossilized thumbs suggests about 2 million years ago, our ancient ancestors evolved a uniquely dexterous appendage while our other close relatives remained … all thumbs.

It’s a “thorough, robust analysis,” says Tracy Kivell, a paleoanthropologist at the University of Kent who was not involved with the work. But she and others caution that the research is too preliminary to provide a true smoking gun.

Figuring out how ancient thumbs worked isn’t easy. Fossils don’t preserve muscles, so most previous attempts to estimate ancient dexterity relied on how closely our ancient relatives’ hand bones resembled our own. Hand bones are also small and relatively rare in the fossil record. But resemblance can be deceiving: Depending on how the muscles are connected, some species with similar bone anatomy might have very different grip strengths, and vice versa.

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