WASHINGTON – Neanderthal fossils from a cave in Belgium believed to belong to the last survivors of their species ever discovered in Europe are thousands of years older than once thought, a new study said on Monday.

Previous radiocarbon dating of the remains from the Spy Cave yielded ages as recent as approximately 24,000 years ago, but the new testing pushes the clock back to between 40,600 to 44,200 years ago.

The research appeared in the Proceedings of the National Academy of Sciences and was carried out by a team from Belgium, Britain and Germany.

Co-lead author Thibaut Deviese from the University of Oxford and Aix-Marseille University said he and colleagues had developed a more robust method to prepare samples, which was better able to exclude contaminants.

Having a firm idea of when our closest human relatives disappeared is considered a key first step toward understanding more about their nature and capabilities, as well as why they eventually went extinct while our own ancestors prospered.

The new method still relies on radiocarbon dating, long considered the gold standard of archaeological dating, but refines the way specimens are collected.

All living things absorb carbon from the atmosphere and their food, including the radioactive form carbon-14, which decays over time.

When it comes to bones, scientists extract the part made up of collagen because it is organic.

“What we have done is to go one step further,” said Deviese, since contamination from the burial environment or through glues used for museum work can spoil the sample.

The team looked for the building blocks of collagen, molecules called amino acids, and particularly selected specific single amino acids they could be sure were part of the collagen.

The authors also dated Neanderthal specimens from two additional Belgian sites, Fonds-de-Foret and Engis, finding comparable ages.

“Dating all these Belgian specimens was very exciting as they played a major role in the understanding and the definition of Neanderthals,” co-lead author Gregory Abrams, said of the Scladina Cave Archaeological Centre in Belgium. “Almost two centuries after the discovery of the Neanderthal child of Engis, we were able to provide a reliable age.”

Certain stone tool use has been attributed to Neanderthals and was interpreted as a sign of their cognitive evolution, said Deviese.