The technique is demanding. A capuchin weighing three to four kilograms lifts a quartzite hammer that can approach a kilogram, places the nut in a pit worn into a sandstone anvil, rises on its hind legs and brings the stone down with both hands. Individuals select hammers by weight rather than size alone, and carry them to productive anvils.
At Serra da Capivara National Park, researchers excavated below active anvil sites and recovered damaged stones in dated stratigraphic layers. The oldest reached roughly three thousand years, and the size of the hammers changed across the sequence, hinting at shifts in what was being processed.
The finding matters beyond monkeys. It shows that a non-hominin primate can leave a stone-tool record deep enough to excavate, which complicates the assumption that flaked and battered stone at an old site necessarily indicates a human ancestor.