On one of Thailand's coastal islands, a group of wild macaques have learnt to use stone tools to eat shellfish and nuts. They have truly entered the Stone Age.

You can see their behaviour in the video above.

It is not clear just how long they have been using stone tools for. They may have been doing so for thousands of years, ever since the island Piak Nam Yai was separated from the mainland during the last ice age.

The macaques that live on the mainland do not use stone tools, as they have access to a large variety of fruit and leaves.

"The fact we have the only animals on islands off the coast using these tools suggests they were trapped on those islands when the sea levels rose at the end of the last glacial period," says Michael Haslam at the University of Oxford in the UK, and leader of the Primate Archaeology (Primarch) project.

Now for the first time, archaeologists have excavated tools they used about 50 years ago.

"We are particularly interested in the comparison between what the monkeys are doing and what's has been argued for modern humans outside of South Africa," says Haslam.

The monkeys forage for shellfish in unpredictable tidal situations, in the same way that early humans may have done. Eating shellfish could also have accelerated cognitive development, says Haslam. "There's been a lot of focus for decades on the kind of nutrients that seafood can provide for the brain."

Finding old stone tools means researchers can now identify and reconstruct past behaviour. They hope to find many older stone tools, both from this group and from other tool-using monkeys elsewhere.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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