Although cave bears became extinct 25,000 years ago, their DNA lives on in brown bears today, reports a paper published online this week in Nature Ecology & Evolution.

Axel Barlow and colleagues investigate the genomic sequences of four cave bears that lived between 71,000-34,000 years ago. They compare these with DNA and genome sequences from both ancient and modern brown bears, as well as American and Asiatic black bears, spectacled bears, pandas and polar bears - the last of which were previously found to have interbred with brown bears.

The authors find that all the brown bear genomes they sequence include a contribution of cave bear DNA - amounting to between 0.9-2.4% of the genome - and that, similarly, cave bears also harbour brown bear DNA, albeit in smaller proportions. The authors conclude that brown bears and cave bears must have interbred before the latter became extinct. Further DNA analysis reveals that there was gene flow between both species.

Although it had been known that modern-day, non-African humans share small amounts of DNA with extinct ancient hominin populations such as Neanderthals and Denisovans, this is the first time that DNA from an extinct Ice Age species has been reported in a living population outside of the human lineage.