Analyzing ancient DNA has revealed that mystery species nicknamed "Higgs Bison" by researchers is actually an unknown hybrid of bison and cattle.

For a study published in Nature Communications, an international team of researchers identified the unknown species from cave paintings done by Ice Age artists over 15,000 years ago. The unknown species was affectionately referred to as the Higgs Bison in reference to the subatomic particle Higgs Boson whose existence was only proven in 2012 despite being suspected of existing since the 1960s.

The mysterious bison species was first spotted by Beth Shapiro, a professor from the University of California, Santa Cruz, as part of her PhD research at the University of Oxford with Alan Cooper in 2001. Today, Cooper is the director of the Australian Center for Ancient DNA, which led the current study.

"Fifteen years later, it's great to finally get the full story out. It's certainly been a long road, with a surprising number of twists," said Shapiro.

According to the researchers, the mystery species is a hybrid of the Steppe Bison, an Ice Age bison that thrived in the cold grasslands from Mexico to Europe, and the Aurochs, which is the extinct ancestor of modern-day cattle. Originating more than 120,000 years ago, the hybrid eventually became ancestor to today's European bison, or the wisent, which lives in protected reserves like the Białowieża forest between Belarus and Poland.

The researchers were surprised to discover a hybridization event because this doesn't often occur in mammals. Based on radiocarbon dating data, the mystery species enjoyed dominating Europe at several points for thousands of years and was the largest of species in Europe to survive through megafaunal extinctions. The modern European bison nearly became extinct and the resulting genetic bottleneck involving just 12 individuals back in the 1920s led to the species looking a lot like its ancient form.

According to French cave researchers Cooper and colleagues talked to for the study, there were two bison forms distinguishable from art in caves during the Ice Age. When the periods the bison species appeared in cave paintings were compared to the time they were in dominance, there was a match.

"We'd never have guessed the cave artists had helpfully painted pictures of both species for us," said Julien Soubrier, the study's lead author.

Based on cave paintings, the mystery bison species had small humps and shorter horns. And once it was formed, the hybrid species kept to itself genetically, successfully carving out a niche on Europe's landscape and dominating during colder periods.

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