The crab-eating macaques of Thailand and Burma are notoriously clever. Using various stone tools, they are able to pry open nuts, oysters and sea snails along the Andaman coast, and around the shores of smaller islands, with brutal efficiency. Some of them have even managed to work out that they can use human hair as dental floss for their teeth – all this, despite having far smaller brains than us humans.

Writing in the Journal of Human Evolution, a team of researchers have pointed out that tool use in Old World nonhuman primates, including these macaques, has only been documented with living animals; until now, no-one has looked for any archaeological evidence of this type of activity. Although it’s reasonable to assume that tool use in macaques has been occurring for multiple generations, there is currently no evidence that shows this is demonstrably true.

“What we don't have at the moment is a body of archaeological evidence to compare the evolutionary behavior of other primates with our own,” the study’s lead author Michael Haslam, from the School of Archaeology at the University of Oxford and leader of the Primate Archaeology (Primarch) project, said in a statement. New excavations reveal that they have been in the Stone Age for at least 50 years.

^{Macaque versus oyster shell. University of Oxford via YouTube}

For this study, groups of Macaca fascicularis aurea (crab-eating macaques) were carefully observed on the Thai island of Piak Nam Yai. These opportunistic omnivores eat whatever may be available to them at the time, but unlike many other animals, they have the capability to use stone tools – mainly ones used to fracture or crush things – to aid their foraging.

After spending some time observing these macaques using their various stone implements, the researchers noted that they often left their tools near piles of discarded oyster shells. One remarkably ravenous macaque ate 63 oysters in a row, and left his stone tool beside his tower of shells after he decided that he couldn’t possibly stomach another. Scratch marks on the boulders, specific fracture marks on shells, and indent marks on the tools were also common indicators of tool-use activities.

Shortly after the macaques had filled their bellies with seafood and vacated the area, the team then dug into the ground, looking for these tell-tale signs of tool use. Identifying a range of stone tools, they excavated them. Radiocarbon dating of the nearby oyster shell debris found the tools to be around 10 to 50 years old, which at the very least shows that macaques have used tools here for half a century.

“Uncovering the history of the macaques' foraging behavior is a first step,” Haslam added. “As we build up a fuller picture of their evolutionary history, we will start to identify the similarities and differences in human behavior and that of other primates.”

Tool use was once thought to make humans unique, but it’s now abundantly clear that this is no longer the case. These macaques are one of the most prolific groups of tool users known to science, and their oyster-prizing, nut-smashing activity has been documented in incredible detail for some time, but they are far from alone in their ingenuity.

^{Macaques often search for the best stones to use for different tasks. University of Oxford via YouTube}

Chimpanzees can fashion hunting spears out of branches, bonobos can fish for termites using sticks and reeds, and orangutans use sticks, leaves, and rocks of various sizes to aid in eating difficult-to-reach food. Even non-primates can use tools: Birds belonging to the Corvidae family regularly fashion tools out of sticks, octopuses make shields out of shells, and elephants can make sure water reserves don’t leak out of a tree by plugging the gap with plants, mud, and sand.