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It is now possible to determine when an ancient human was alive by examining their DNA.

The new technique could reveal the ages of bones that can’t be carbon dated. “This is a huge problem in the field that is either being ignored or just solved incorrectly,” says Eran Elhaik of Lund University in Sweden.

Carbon dating is used to estimate the age of bones and other remains. It works by measuring how much of a radioactive form of carbon in the sample has decayed. However, carbon-dating only works on samples less than 50,000 years old, and only if the sample contains a lot of organic material.

“Over half the samples in the literature are not carbon dated,” says Elhaik. Instead, archaeologists have dated them using other clues, such as the age of the sediments in which they were buried. This can go badly wrong: a bone from a cave in Afghanistan was thought to be 30,000 years old, but .

It’s in the genes

When carbon dating isn’t an option, Elhaik says it is now possible to use ancient DNA.

His team turned to ancient DNA from 961 people who lived in Eurasia between 14,000 and 1000 years ago, 602 of whom had also had their remains carbon dated. They identified points in the genome that varied predictably with time: just as some genetic variants are more common in particular places, some were more common in specific periods.

By reading these markers of time in the genome of a fossil, the team could estimate its age to within 500 years. For the 602 cases where it was possible to compare the DNA-estimated age with the carbon dating age, there was generally close agreement.

Confident that their method worked, the team has used it to date the 359 individuals who couldn’t be carbon dated. In some cases, this has resolved apparent inconsistencies.

For example, 10 of the individuals were the subject of a : a “barbarian” tribe that attacked the Roman Empire in the 6th century. However, archaeological finds uncovered with one of the skeletons seemed to be several centuries too old to date from the Longobard era. The DNA suggested this skeleton did, in fact, belong to this era despite what the archaeological finds indicated.

The method isn’t a replacement for carbon dating, Elhaik says. It only works for regions where many people, from different periods, have had their DNA sequenced. That means it can’t yet be used to date ancient people from Africa, where little ancient DNA has been recovered. Furthermore, the reference genomes needed to calibrate the DNA method must be carbon dated.

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