WASHINGTON — Some of our tree-swinging prehuman ancestors may have been a bit more like us than previously thought, thanks to a tiny section of their thumbs.

One key attribute that separates humans from other animals is our opposable thumb, and the way parts of the thumb are structured to allow for a strong yet precise grip that fostered advanced use of tools. It’s what allows us to throw items more precisely, pick guitars and turn a key.

And now, thanks to high-tech tools of our own, scientists have determined that a couple million years ago one of our prehuman ancestors had the same human-defining precision grip, even though researchers think of them as little more than upright walking apes, according to a study published Thursday in the journal Science. That supports earlier but controversial evidence that the small-brained Australopithecus africanus fashioned early tools.

“It forces us to revisit how we think (the entire prehuman genus) made a living,” said study lead author Matthew Skinner of the University of Kent in the United Kingdom. “It could be evidence of our greater reliance on tools.”

This is the oldest evidence of prehumans using hands to manipulate items, said Brian Richmond, human origins curator at the American Museum of Natural History in New York. He wasn’t part of the study but praised it as important.

This species, not technically part of the Homo family, roamed South Africa between 2 and 3 million years ago. A similar prehuman species of hominids, typified by the famed Lucy fossil, lived in East Africa.

“These are some very primitive creatures overall,” Richmond said. “Basically they would have more or less been like upright walking great apes. We wouldn’t think of them as very human, but this makes them a little more human than we thought.”