Around 25,000 years ago, the hulking cave bear went extinct following a long period of decline. But the ancient creatures’ genes did not die out with the species. As Michael Greshko reports for National Geographic, a new study has found that segments of cave bear DNA persist in the genomes of modern brown bears.
Cave bears, so called for their preferred habitat, looked similar to brown bears, but were larger and herbivorous. The two species coexisted in Europe and, according to a new paper in Nature Ecology and Evolution, appear to have mated with one another. A team of scientists led by Axel Barlow, a paleogeneticist at the University of Potsdam in Germany, stumbled upon this discovery after comparing cave bear DNA—which was extracted from the bones of four animals that died more than 35,000 years ago—to a previously sequenced brown bear genome.
The scientists weren’t expecting to glean particularly significant results; the focus of their research was learning more about the cave bear, and they decided to expand their study to include brown bear DNA “almost as a lark,” writes Karen Weintraub of the New York Times. But the team was surprised to find “a really quite obvious signal of hybridization between these species,” Barlow tells Weintraub.
The researchers then compared the cave bear DNA to the genomes of six other modern brown bears and one ancient brown bear, and discovered further evidence that the two species had been interbreeding. In fact, the team’s results indicated that cave bears contributed between 0.9 to 2.4 percent of the brown bear genomes that were analyzed in the study, according to the paper’s authors.
This DNA swap wasn’t one-sided; the researchers also found that the cave bears carried brown bear DNA, though the most recent exchange of genes was from the cave bear to the brown bear.
The results of the new study are significant for several reasons. Firstly, the team’s findings add further complexity to our idea of what a “species” is. Traditionally, scientists viewed distinct species as animals that don’t interbreed, but more recent research has shown that this is not necessarily the case. We know, for instance, that ancient hominins interbred; just last week, a report in the journal Nature documented the remains of the first known hominin hybrid—a young girl with a Neanderthal mother and Denisovan father. Modern humans also carry Neanderthal DNA, indicating that our ancient ancestors were mating with the bygone species.
The study also raises intriguing questions about what it means to go extinct. Cave bears died off thousands of years ago, possibly due to ice age-induced food shortages and competition with humans for habitats. “By any standard definition, [cave bears] are extinct,” Barlow tells Greshko of National Geographic. But, Barlow adds, that doesn’t mean the animals are gone completely; their DNA lives on in the genomes of modern bears.