Oct 19 : A Siberian cave already famous for yielding the first remains of an extinct human lineage called the Denisovans, who once roamed widely in Eurasia, has now produced evidence of a previously unknown large Ice Age mammal closely related to today's yak.
Researchers said they discovered small bone fragments of the animal in Denisova Cave in the Altai Mountains of southern Siberia in Russia, and obtained DNA that showed these remains belonged to a bovine species distinct from others already known, living and extinct.
In all, the researchers studied 20 bone fragments from this species — 14 from Denisova Cave and 6 from other sites in the broader region. The oldest specimen dated to about 250,000 to 200,000 years ago, and the youngest to about 27,000 years ago.
The bones themselves were so fragmentary that their shape alone did not reveal that they belonged to a previously unknown large mammal. But the researchers were able to generate 20 ancient genomes from the bones, and these showed that this was a unique species whose evolutionary lineage diverged from the ancestors of the modern-day yak approximately 400,000 years ago.
"It is unusual, especially to recognize a large mammal species from small, unidentifiable fragments," said evolutionary geneticist Love Dalén of the Centre for Palaeogenetics, a collaboration between Stockholm University and the Swedish Museum of Natural History, senior author of the research published in the journal Current Biology.
"I'd say that this is related to the advances in the field of ancient DNA, as genetics is a powerful new layer of information applied to ancient materials," said Alexandre Gilardet, a doctoral student at the Centre for Palaeogenetics and co-lead author of the study, along with Centre for Palaeogenetics postdoctoral researcher Jonas Oppenheimer.
Genetic evidence alone is not sufficient to allow for giving a scientific name to a new species, so the researchers are calling this one "Yak X."
THE ICE AGE LANDSCAPE
Dalén said Yak X lived alongside other bovines including steppe bison and the aurochs, the extinct wild ancestor of today's domesticated cattle. It also shared the Ice Age landscape with woolly mammoths, woolly rhinoceroses, horses and large predators such as cave hyenas and brown bears, Dalén said.
"I'd expect it to have looked more like a yak than a bison, given its close relationship to modern yaks," Dalén said.
Yaks are sturdily built animals with horns, long fur and a hump at the shoulders, looking a bit like a hairy ox. The largest males can weigh as much as a ton. Wild yaks are restricted to higher elevations of the Tibetan Plateau.
The Yak X discovery deepens the understanding of the diversity of large Ice Age mammals and adds another species to the long list that disappeared at around the end of that epoch.
"Using the long, 200,000-year time span of our sampling, we can see that the species may have undergone a population decline during a period of warming around 120,000 years ago, and went extinct some time after our youngest sample, about 27,000 years ago," Oppenheimer said.
The vast Siberian expanse was a grassland ecosystem called the mammoth steppe, known as such for its largest denizens. Most of the region was left uncovered by the immense ice sheets that had blanketed large swathes of North America and Europe.
The Yak X lineage diverged from the ancestors of modern yaks around the same time as the evolutionary split that occurred between the ursine lineages that led to today's brown bears and polar bears.
THE DENISOVANS
It was in Denisova Cave in 2008 that scientists unearthed a finger bone that offered the first evidence of the existence of Denisovans, extinct cousins of Neanderthals and our own species, Homo sapiens. Since then, more remains of Denisovans have been discovered in the cave and in several far-flung parts of Asia.
Some modern populations of people, especially those in Melanesia and Papua New Guinea, carry a small percentage of DNA from Denisovans, indicating interbreeding with Homo sapiens before the Denisovans became extinct under circumstances that are not well understood.
Denisovans were hunter-gatherers who survived in many different types of environments. The Yak X species might have been among the prey that they hunted.
"It seems reasonable that Yak X was on the Denisovan menu," Dalén said.