A cave bear skull from a natural history museum in Serbia. (R. Kowalczyk)

Cave bears, Ursus spelaeus, must have made magnificent impressions on the Neanderthals and modern humans who encountered them. The animals, burly and broad-shouldered, lumbered all over Europe. True to their name, they slept and wintered in caves, emerging each spring to blink awake in the Pleistocene sunlight. The biggest cave bears grew to 2,000 pounds, hundreds of pounds larger than the largest brown bears alive today.

Now the bears’ bones are scattered in the caves they once occupied. They went extinct around 20,000 years ago. What killed off the bears is a question that’s gotten a lot of attention from scientists. For one thing, they have lots of bear fossils to examine. What’s more, the cave bears were closely related to brown bears — yet they met very different fates.

A study published Thursday in the journal Scientific Reports will not put the question totally to rest. But the genetic data wrung from cave bear bones make a compelling case that humans played a large role in their extinction.

“It’s a very nice study” that shows something “quite profound happened to the global cave bear population around 40,000 years ago,” said Axel Barlow, a biologist at the University of Potsdam, in Germany, who was not involved with this research. Last year, Barlow and his colleagues showed that living brown bears contain DNA from cave bears, which means the two species mated, just as humans and Neanderthals did. Cave bears split from the brown bear lineage around 1.2 million years ago.

The authors of the new study examined mitochondrial DNA from 130 bears. Mitochondria provide a cell with energy; in bears as in humans, mitochondria and their DNA are inherited only from mom through the egg cell. Subtle differences in this genetic material allowed the scientists to calculate the numbers of female bears in during the species’s final years on the planet.

“Based on the diversity and the radiocarbon dates of the mitochondrial genomes, you can model the effective female population size through time,” said study author Verena Schünemann, a professor of paleogenetics at the University of Zurich. This bear population remained stable from 200,000 to 50,000 years ago. At around 40,000 years, it began to crash. Twenty thousand years later, cave bears vanished.

Some researchers have proposed that climate change felled the bears. Ice sheets advanced and shrank during the Pleistocene Epoch, the period from 2.6 million to 11,700 years ago. Wear on the bears’ teeth suggests the animals mostly ate plants. (Though more recent discoveries of tooth marks on bear bones suggests they ate the bodies of other cave bears.) If vegetation shifted with the ice cycles, perhaps the bears struggled with smaller food supplies.

But, the study authors note, in Europe between 200,000 to 50,000 years ago, the climate cooled and warmed twice without any apparent impact on the bears. Or, as study author and University of Tubingen paleobiologist Hervé Bocherens put it, “Cave bear populations did not show significant fluctuations during climatic oscillations before 50,000 years ago.” Instead, the drastic population decline began at the same time as the arrival of modern humans in Europe.

“We can’t rewind the clock, taking humans out of the picture, and see whether the cave bears survived or not,” Barlow said. “The emerging picture for cave bears, shown by not only this study but also decades of careful research, is that the influx of anatomically modern humans in Europe matches very well — both in timing and geographically — with the decline in cave bear.”

Physical evidence shows humans killed the bears. More than a decade ago, paleontologists discovered the tip of a flint weapon embedded in a cave bear’s spine. More recently, a group of scientists in Italy, studying butcher marks on bear bones, suggested that Neanderthals hunted the animals when they were most vulnerable — as the animals awoke from spring hibernation.

“They probably killed cave bears for the same reasons that they killed other large mammals,” Bocherens said, meaning for their skins and meat.

Cave bears returned to the caves where they were born, Schünemann said. As human populations grew, requiring more shelter for longer periods of time, humans may have ejected the bears from their birth caves.

Schünemann would not totally absolve climate change. There “might be still a synergistic effect of both factors: humans and climate,” she said. It may be that humans applied pressure to the bears as climate caused their population to fragment and food supplies diminish.

Cave bears, mammoths, giant sloths and other megaherbivores of ages past were, in the lingo of biologists, ecosystem engineers. As they chowed, pooped and stomped, these animals spread plants and recycled nutrients. The grassy steppes woolly mammoths once tended have collapsed into tundra and Arctic desert. Giant sloths once spread avocado seeds across South America. Cave bears brought great amounts of organic material into caverns, something no living European creature can do.

If humans helped destroy the bears, then the animals join a list of creatures we deemed too tasty, too annoying, too docile — too anything — to live. Only in the last sliver of human history have we begun defending rare species through efforts like the Endangered Species Act, which the Trump administration recently moved to weaken.

“Looking at the impact of climate change and human impact on large mammals can tell us a lot about the general process and consequences of large mammal extinction,” Bocherens said. “Studying the past to save the future: This is what we try to do.”

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