Cave art reveals modern-day bison is a hybrid

One of the world’s two remaining bison species is a hybrid of Ice Age Steppe bison and an ancient cow ancestor, according to new research.

The surprising origins of the European bison were uncovered by DNA testing on ancient bones, and then checked against cave art from the Ice Age, which tracked bison appearance over time.

The work was published in Nature Communications.

The European bison (Bison bonasus) was nearly hunted to extinction in the 1920s, before being reintroduced in several countries across Europe.

Bison are among the few land-dwelling animals that survived the late Pleistocene extinctions, but the fossil record for European bison begins very suddenly – around 11,700 years ago.

To investigate its mysterious appearance, an international research team sequenced the genomes of 64 ancient bison using bones and teeth from as early as 50,000 years ago.

Their findings have unveiled a previously unknown species that preceded the European bison. Surprisingly, the species represents a hybrid between Ice Age Steppe bison (Bison priscus) and aurochs (Bos primigenius), the extinct ancestors of modern-day cattle.

“The genetic signals from the ancient bison bones were very odd, but we weren’t quite sure a species really existed – so we referred to it as the ‘Higgs Bison’,” explains Alan Cooper from the University of Adelaide and lead author of the study.

“Finding that a hybridisation event led to a completely new species was a real surprise – as this isn’t really meant to happen in mammals.”

Both aurochs and bison feature heavily in Palaeolithic cave art. Interestingly, their appearances change over time, allowing the researchers to double-check their hypothesis.

“The dated bones revealed that our new species and the steppe bison swapped dominance in Europe several times, in concert with major environmental changes caused by climate change,” explains co-author Julien Soubrier, also from the University of Adelaide.

“When we asked, French cave researchers told us that there were indeed two distinct forms of bison art in Ice Age caves, and it turns out their ages match those of the different species.

“We’d never have guessed the cave artists had helpfully painted pictures of both species for us.”

Cave paintings more than 18,000 years old showed bison with long horns and large forequarters similar to the modern-day American species, which is descended from the Steppe bison.

In more recent images, the paintings show bison with shorter horns and back-humps, closer in appearance to European bison.

Although the hybrid bison survived some of the toughest climate changes known to science, hunting during the 20th century may have had a significant effect on its appearance.

“Once formed, the new hybrid species seems to have successfully carved out a niche on the landscape, and kept to itself genetically,” explains Cooper.

“It dominated during colder tundra-like periods, without warm summers, and was the largest European species to survive the megafaunal extinctions.

“However, the modern European bison looks genetically quite different as it went through a genetic bottleneck of only 12 individuals in the 1920s, when it almost became extinct. That’s why the ancient form looked so much like a new species.”

According to the researchers, this finding adds to mounting evidence of the importance of hybridisation for animals – as well as plants – to adapt to their changing environment.