Modern brown bears are carrying a bit of genetic material passed down from the cave bear, in a study that suggests extinction does not always vanquish a species’ genes.
ImageThe skull of a cave bear, which went extinct 25,000 years ago. The discovery is the first time species of mammals so far apart have been shown to carry each other’s DNA.CreditCreditAndrei Posmoanu
About 25,000 years ago in Western Europe, the last cave bear drew its final breath and the species went extinct.
But a study published on Monday in Nature Ecology & Evolution finds that some cave bear DNA lives on in modern brown bears, much like humans carry around a bit of Neanderthal.
The finding challenges our view of extinction, said Ludovic Orlando, a professor at the Centre for GeoGenetics at the Natural History Museum of Denmark, who was not involved in the research.
“Extinction does not imply that genetic material is gone forever,” he said via email.
In the new study, researchers initially set out to learn more about the cave bear by studying its genetics. Almost as a lark they compared the genetic sequence they’d derived for the cave bear with an existing sequence of a brown bear, said Axel Barlow, the lead author and a postdoctoral researcher at the University of Potsdam in Germany. The results surprised them.
“There was a really quite obvious signal of hybridization between these species,” Dr. Barlow said.
Wondering whether their results were a fluke, the team then compared the cave bear genome to that of seven other brown bears — one ancient and six modern — and found the same genetic mixing. Cave bears contributed from just under one percent to 2.4 percent of the genomes of brown bears, the study showed.
“We did not expect to find this at all because they’re really quite diverse in terms of their evolution,” Dr. Barlow said.
The team was also able to determine that the genes flowed both ways between species, with the cave bears also carrying some brown bear DNA. The most recent transfer of genes came from the cave bear to the brown, the study found.
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Brown bears are more closely related to polar bears than they were to cave bears from whom they diverged more than a million years ago, he said. Cave bears were largely herbivores, while brown bears are meat-eaters and about 20 percent smaller than cave bears, with more delicate bones. A brown bear would probably have looked “wimpy” next to a cave bear, he said.
They both shared roughly the same territory until about 25,000 years ago in Europe, Dr. Barlow said. Two Alaskan brown bears also showed the presence of cave bear DNA, suggesting an unexpected diffusion of genes that needs to be investigated further, he said.
ImageA new study indicates brown bears may have interbred with ancient cave bears, and now carry some of the DNA of the extinct species.CreditLajos Berde
It’s not clear why cave bears went extinct, Dr. Barlow said, although the timing roughly followed the migration pattern of anatomically modern humans from Africa into Europe. The last cave bears lived in western Europe, and at least one skeleton has been found with fragments of a human-made spear embedded in its vertebrae.
Also unknown is whether the inbreeding between bear species provided any evolutionary advantage to the brown bears, something Dr. Barlow said he hoped to investigate. In people, it’s been found that a gene from another early hominin species called the Denisovans helped Tibetans adapt to the high altitudes in which they live.
Pontus Skoglund of the Francis Crick Institute’s Ancient Genomics Laboratory said in some ways it was not surprising that similar creatures who shared the same geography would intermingle and have overlapping genetics. But it does force scientists to change their definition of a “species,” as animals that don’t interbreed, he said.
“It confronts this idea of species that many people have in biology and emphasizes how we really need to take in this empirical data that gene flow does occur,” Dr. Skoglund said.
Dr. Orlando said he’s also interested in learning more about the genes that were preserved and those that may have led to the cave bear’s extinction. “Those cave bear genes that are never found in brown bears (as incompatible) might prove essential to understand the process of speciation,” he said.
The genetic mixing work has implications for how today’s endangered animals should be protected, Dr. Barlow said. He wondered whether animals that have DNA from other species should be preferentially conserved because they are likely to be hardier, or whether biologists should preferentially protect “purer” animals with less genetic mixing. A better understanding of species that have already gone extinct could also help conservationists provide optimal conditions for preserving today’s endangered species, he said.