Animals

Humans, not glaciers, likely doomed Ice Age cave bears

Analysis of genetic material from dozens of prehistoric bears shows that their decline neatly matches the arrival of Homo sapiens in Europe.Thursday, 15 August 2019

By Tim Vernimmen
<p>A cave bear skull found in Italy represents one of the last traces of this prehistoric animal. Scientists have long debated whether the onset of the last glacial maximum drove the giant species extinct. But analysis of ancient cave bear DNA suggests it's more likely the arrival of modern humans in Europe played a crucial role in their demise.</p>

Neanderthals lived in Europe for thousands of centuries, and during that time, they had to watch their step. Mammoths, woolly rhinos, and sabre-tooth cats were common in the region, and the caves these human relatives would sometimes enter for shelter were often already occupied by cave bears, the heaviest adults of which may have weighed over 2,000 pounds.

Today, controversy swirls around the question of why all these large animals eventually disappeared. Some scientists think they were victims of the last glacial maximum, which peaked around 26,500 years ago. Other experts have argued that the appearance of a new human species with a knack for hunting, Homo sapiens, could have driven the unfortunate beasts to extinction.

Now, research presented in the journal Scientific Reports suggests that in the case of cave bears, humans most likely played a crucial role.

“If not for our arrival in Europe, I don’t see any reason why cave bears should not be around today,” says study coauthor Hervé Bocherens, a paleobiologist at the University of Tübingen in Germany who has studied cave bear remains for 30 years.

In some ways, the result may foreshadow the situation of today’s brown bear, which currently has a stable population but may soon be at risk due to conflicts with humans in an increasingly crowded and warming world.

Clan of the cave bear

Bocherens and a team of researchers led by Verena Schuenemann at the University of Zürich in Switzerland collected the remains of 59 cave bears found across Europe to extract what’s known as mitochondrial DNA, or mtDNA. These small bits of genetic material are only inherited from an animal’s mother and can reveal genetic relationships between animals found at different locations. Crucially, however, mtDNA can also provide clues to past population sizes.

“Models of the genetics of populations tell us that the more diverse the mtDNA found in fossils from the same period, the larger the population must have been, allowing us to estimate the number of bears at any point in time,” Bocherens says.

Grizzly Bears and Their Insatiable Appetites, Explained
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A number of the grizzlies’ physical characteristics centre around finding and eating their meals.

When the scientists ran their analysis, the data suggested that the cave bear decline started some 40,000 years ago—long before the last ice age set in. This also means that cave bears thrived throughout a number of earlier periods when temperatures significantly decreased. Instead, their downward trend starts right about the same time that our species began to spread across Europe.

“There is some evidence suggesting some modern humans may have set foot in Europe even earlier,” Bocherens says. “But as far as we know, they only really populated the continent around the time the cave bears start declining.”

Though Neanderthals were probably killing cave bears as well, modern humans may have used more advanced hunting techniques and were probably more likely to venture into caves, Bocherens argues. Soon, anatomically modern humans became much more numerous than Neanderthals had ever been, sealing the cave bear’s fate.

The work “represents the maximum amount of information we can get from mtDNA data,” says Michael Knapp, a paleobiologist now based at the University of Otago in New Zealand. Knapp was not involved in the present study, but he published an earlier paper based on a more limited dataset that found similar results.

Bear necessities

Humans may have killed cave bears not just for their meat, but also for their fur or even simply because they were perceived as a threat. And as more humans settled in Europe, cave bears may have had a harder time moving into milder climates when it became cold, or finding the abundant plant foods required to sustain their large bodies. Remnant populations survived only in remote corners of Europe, such as the Italian Alps, where the most recent remains appear to be about 24,000 years old.

“Yet as these populations grew more and more isolated, they became genetically impoverished, as it was increasingly difficult for animals to travel between populations to find a mate,” Bocherens says. This may have weakened their offspring and could have made the bears more vulnerable to disease.

Meanwhile, brown bears survived into the modern era, perhaps because they were smaller and had more flexible diets that included meat they probably scavenged from large predators. Still, the decline of the cave bears carries a warning for brown bears, Bocherens says.

“First of all,” he says, “it shows that the most isolated populations are at risk, and that we should do whatever we can to allow some exchange of individuals between them, even if that means moving animals around ourselves.”

Perhaps even more importantly, he adds, the climate is again changing drastically, this time due to the actions of Homo sapiens, and that means it is not enough to have nature reserves where the animals are left alone. In a world increasingly cluttered with roads, railways, fences, and buildings, we must also preserve the bears’ ability to travel around and keep their populations healthy and diverse.

“Species may survive a changing climate if they can track the changing temperatures,” Bocherens says. “But as the example of the cave bear shows us, climate change can be a very big problem if you cannot move.”

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