Somewhere along the journey from ape to human, our ancestors developed hands that were better suited to holding tools than climbing trees. A new study suggests the transition took place as much as 3.2 million years ago — about 600,000 years earlier than previously thought.

Researchers studying the palm and thumb bones of early hominids in South Africa were surprised to find some distinctly human characteristics. Wear patterns in the bones suggested that the hominids had tightly gripped tools using an opposable thumb and fingers, much the way one would hold a hammer.

“It’s not just that they could oppose their fingers and thumbs, but that they could do it very forcefully and that they were doing it on a regular-enough basis to leave a signal inside the bone,” said Matthew Skinner, an anthropologist at the University of Kent and the lead author of the study. The result, he added, is similar to the wear seen in the hand bones of modern humans.

The earliest stone tools in the archaeological record are just 2.6 million years old. Those tools, unearthed in the 1950s, have long fueled the belief among scientists that hominids evolved the ability to use tools somewhere around that time. But that idea has been challenged in recent years by the discovery of 3.4-million-year-old animal bones with markings that seem to have been made by stone tools.