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Madjedbebe rock shelter in Australia's Northern Territory, for instance, has recorded single-grain OSL dates of between 50,000 and 60,000 years, seemingly making it Australia's oldest site of human occupation.

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But once it dies, no more fresh radiocarbon is absorbed, and what’s left starts to decay.

Once samples are older than around 40,000 years, though, amounts of radiocarbon remaining are very small and difficult to measure.

“I think there is a strong compelling argument to re-date these key sites using single-grain OSL,” he says.

And sometimes the dating techniques are fine, but the stability of the sedimentary layers throws things into question.

This fits with the 49,000-year-old radiocarbon date, given that it takes a few hundred years before amassed sand is firmly trampled into the floor and no longer exposed to sunlight.

Previous multi-grain OSL dating at a number of ancient sites have suggested humans arrived in Australia well over 50,000 years ago, but Spooner is sceptical of many of these dates.

Scratching around in a cave in the middle of nowhere, you find a bone.

How do you find out if it’s the remains of an ancient animal that stomped the land tens of thousands of years ago or a discarded scrap from a cooking fire only a few hundred years back?

While a crystalline grain such as quartz – found in desert sand – is buried and tucked away from sunlight, natural radiation from surrounding soil and rocks knocks electrons in the crystal out of position.

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