Ancient DNA Reveals a Lost Yak Species That Roamed Siberia 400,000 Years Ago

A shaggy wild cattle species nobody knew existed, likely larger and sturdier than today’s yaks, roamed Siberia and Central Asia for more than 200,000 years before disappearing completely, leaving no living descendants. Scientists only learned it existed by pulling ancient DNA from bone fragments that, on their own, revealed little about the animal they came from.

Researchers are calling it “yak X.” Its discovery adds a new twist to the story of Siberia’s Altai-Sayan mountains, the same region that produced the Denisovans, the extinct human relatives first identified through DNA rather than recognizable bones. Humans weren’t the only ones hiding a lost branch of the family tree there. Yaks had one too.

Reporting in the journal Current Biology, researchers sequenced genetic material from 20 ancient bone fragments, some dating back nearly a quarter of a million years. The DNA showed that this mystery animal split off from the ancestors of today’s yaks somewhere between 400,000 and 250,000 years ago. That is a long time to go unnoticed. Despite living in the same place and era as other wild cattle such as steppe bison and the aurochs, the wild ancestor of modern cattle, yak X mostly kept to itself, rarely mixing genes with its neighbors even when their paths crossed.

How Ancient DNA Revealed This Hidden Ice Age Species

This story began with an earlier study that had already spotted something odd: a pattern in mitochondrial DNA, the small set of genes passed down only through mothers, that didn’t match any known yak. Researchers nicknamed this odd genetic signature “yak X.” But mitochondrial DNA alone couldn’t settle the question of whether this was a truly separate species, an unusual yak variant, or the result of interbreeding between species.

To settle the question, the team analyzed the full genetic picture: the nuclear genome, which carries DNA from both parents, not just the mother’s side. They examined 20 bone specimens: 14 fragments from the famous Denisova Cave and six more from other sites across the broader Altai-Sayan region, an area that stretches from the mountains out toward the Eurasian steppe. The bones varied enormously in age, from roughly 250,000 years old down to one dated to about 27,200 years ago. That means this single lineage of animals survived for more than 200,000 years, enduring multiple swings between brutal ice ages and warmer stretches.

To figure out where yak X belonged on the family tree, the team compared its ancient genomes against DNA from 112 other cattle-family genomes, both modern and ancient, covering yaks and other species like bison, cattle, and water buffalo. Every method they used pointed to the same conclusion: yak X formed its own distinct branch, clearly related to yaks but separated by an old, deep split. The genetic gap between yak X and modern wild yaks was about as wide as the gap between other, already-recognized wild cattle species. That’s a strong sign yak X deserves to be classified as its own species, not just an unusual-looking yak.

Genetic data also let researchers track the population’s health over time. Yak X went through a long, slow decline for much of its existence, then saw its numbers rebound somewhat around 130,000 years ago. That was followed by a possible population shake-up tied to a warm spell called the Last Interglacial, which lasted roughly from 130,000 to 115,000 years ago. Genetic diversity appeared to drop between older and younger members of the species, hinting at a population crash or turnover during that warm stretch, perhaps because rising temperatures and spreading forests shrank the habitat yak X needed to survive.

One of the most surprising results concerned mixing with neighbors. Despite sharing the same caves, valleys, and time periods as steppe bison and aurochs, yak X showed almost no sign of interbreeding with either species. That sharply contrasts with what scientists have found among ancient human relatives in that same cave system, where Denisovans, Neanderthals, and modern humans interbred in overlapping ways. Yak X, it seems, kept its bloodline separate right up until it disappeared.

Exactly when yak X disappeared remains unclear. The youngest confirmed sample, dated to about 27,200 years ago, falls in the early part of the last glacial maximum, the coldest peak of the most recent ice age. Researchers aren’t ready to say the species died out right then. Yak X could have held on much longer, only to vanish as part of the larger wave of big-animal extinctions that swept the globe toward the end of the Ice Age and beyond.

Why This Ancient DNA Discovery Matters

Yak X shows how much the fossil record can hide without genetic tools. A large, distinctive animal endured one of the planet’s harshest climates through repeated ice ages, yet confirming it as more than an odd yak still required careful DNA work. Researchers even think yak X might match a fossil species that paleontologists described decades ago, based on unusually large, sturdy yak-like bones nicknamed the “Baikal yak,” though confirming that link will take more work.

A bigger takeaway reaches beyond one lost species of wild cattle. Across the Altai-Sayan region, scientists keep turning up evidence of hidden diversity, from ancient human relatives to rhinos to horses, and now an entire line of cattle nobody knew to look for. Each time researchers look more closely at this stretch of mountains, they find another branch of the evolutionary tree that lived and died out unnoticed, recognized only when ancient DNA brought it to light.