For a decade, people who study Europe's bison population have been baffled by a genetic mystery. The animals, which are a protected species, seemed to have appeared out of thin air about 11,000 years ago.

"There's something very fishy in the history of European bovids," says Alan Cooper of the University of Adelaide, one of the lead authors of a paper published Tuesday in the journal Nature Communications.

Before 11,000 years ago, all the bison in Europe were thought to be of a variety called steppe bison, which ranged all the way across what is now Russia into Alaska and the North American mainland during the last Ice Age.

But shortly after the steppe bison disappeared from the fossil record, the European bison suddenly appears. And even stranger, its mitochondrial DNA (which is only inherited from the maternal side) shows the new species is very closely related to cows.

"That was misleading for a long time," Cooper says in a video about the new research. "There were these two different species that did not overlap." The key to how the cow genes ended up in modern bison was contained inside bone fragments from ancient bison found in caves across Europe, including the famous Lascaux cave in France.

The team ground up bone fragments and sequenced the full DNA, only to find a surprising answer to the mystery.

About 120,000 years ago, ancient cows and steppe bison had created a hybrid species that survived for thousands of years, which Cooper says is rare.

Cooper and his colleagues jokingly called the new hybrid animal the "Higgs bison," a reference to the famed particle physics discovery.

But having discovered the missing link in European bison ancestry, the researchers had another question: What did the animal look like?

Without any skulls to help reconstruct the animal's body shape, Cooper's co-author, Julien Soubrier, turned to experts on European cave art, hoping they could help.

"We called them and explained the results we had," says Soubrier. "Their reaction was, 'Oh finally!' The cave men could not have painted all these drawings with only one [bison] model. ... There were clearly two morphologies in the drawings."

The two morphologies, or body shapes, represented the steppe bison and the newly discovered ancestor of modern European bison.

Soubrier, Cooper and other researchers teamed up with multiple cave art experts from across Europe, and included examples of cave drawings of both the older steppe bison and the newer cow-like bison in their paper.

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