If you see an organ-grinder monkey smashing a rock against another, it might not be just monkeying around. The capuchin monkey may actually be hammering deliberately.

These primates use stone tools to crack open cashew nuts, and they've used this technique for centuries, according to new research.

Stone tools matching those used by the monkeys in Brazil today appear in the archaeological record as far back as 600 to 700 years ago, scientists report in a paper published Monday in the journal Current Biology.

"We thought that this behavior could be happening for a long time, but this is the first time that we have hard evidence for that," study co-author Tiago Falótico, a postdoctoral research fellow at the University of São Paulo, tells The Christian Science Monitor in a phone interview.

"This is really a first. This is the first time that non-great-ape tool use has been documented in the archaeological record," Robert Shumaker, an evolutionary biologist and vice president of conservation and live sciences at the Indianapolis Zoo who was not part of this study, tells the Monitor. "It's pretty astounding."

Cashew nuts are surrounded by a thick, irritating shell that contains toxins to discourage animals from munching on the tree's seeds. But capuchins found a way to the nutritious nut inside.

The little primates set up a sort of hammer and anvil near the tree. A flatter rock, or even a log or branch, serves as the anvil, while another rock is the hammer. The monkey puts a cashew on the anvil rock and then hammers at it until it cracks open to reveal the nut.

"From a human perspective, capuchin stone tool use appears quite simple," study first author Michael Haslam, a primate archaeologist at the University of Oxford, tells the Monitor in an email. But the monkeys actually put a good deal of care into their work.

"The capuchins are carefully selecting their hammer and anvil stones, and adjusting their striking behaviour, to efficiently open the nuts," he says. "It takes the monkeys years to learn how to do this properly, so even though it appears simple, there are actually a lot of parts to the process that need to be learned and practiced."

These monkeys aren't quite the innovators that humans are, though. The stone tools from the archaeological record don't seem to have changed much over centuries.

But capuchins do use tools in a variety of ways, including using stick tools as a probe to gather food in other ways.

"The capuchins are the only, so far that we know, neotropical primates that use tools regularly," Dr. Falótico says.

But primates in other parts of the world, such as wild chimpanzees in West Africa and wild macaques in southeast Asia, also use tools to crack open nuts, says Dr. Haslam. "This situation suggests that primates have independently re-invented the use of stone tools for opening hard foods on three different continents."

So what evolutionary pressures pushed these primates to start processing previously inaccessible foods?

A food shortage may have stimulated the monkeys to become stone tool innovators, Falótico says. The monkeys that figured out how to crack open nuts encased in hard or toxic shells would have had an easier time surviving.

Dr. Shumaker hopes to see other scientists following this team's lead by scouring the archaeological record for clues into the evolution of tool use among non-humans.

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"One of the real struggles when we talk about non-human tool use is that we really have no record of the ontogeny associated with some of these behaviors," he says. In other words, there isn't much of a record of how this behavior has evolved over time, particularly because tools are often sticks or other organic material that rots and degrades quickly. So, Shumaker says, it is exciting to find these ancient stone tools.

"Humans are the greatest tool users that the world has ever known," Haslam says. "However, a greater understanding of how our primate relatives also use tools in their daily lives gives us a better appreciation both of the way we currently fit into the natural world, and some idea of how our own ancestors may have reached the extraordinary position that we find ourselves in today."