They’re calling it “Yak X”:
The genetic data are described by Jonas Oppenheimer and a team led by Love Dalén, from bovid bone fragments of Denisova Cave and other nearby sites in the Altai and southern Siberia. Among these faunal collections are small pieces of twenty individuals of an unknown branch of yaks.
Based on comparisons with known yaks and other cattle relatives, Oppenheimer and coworkers suggest that Yak X is a different species from present-day wild or domesticated yaks (Bos mutus and Bos grunniens, respectively). That conclusion is based upon the extent of mitochondrial lineage separation and the lack of evidence of introgression or mixture in the nuclear genomes. None of the bone fragments in the study themselves provide enough anatomical information to assess any differences from living yaks.
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Even though bison and cattle separated from common ancestors more than 2.5 million years ago, interbreeding between these species is possible, with some hybrid sterility of males. Breeders over time have bred bison genes into many breeds of cattle, including the American Beefalo breed.
Almost all herds of living American bison (Bison bison) have some cattle genes, which were introduced into wild bison to varying degrees during the late nineteenth century as bison populations were extirpated in much of the continent. These cattle genes are usually responsible for the light pigmentation that leads to the occasional birth of a “white bison”. Sometimes portrayed as a supernatural sign, the pigmentation really does indicate something new introduced by European influence on this classic North American mammal.
Cattle themselves were domesticated from six or more wild populations that are usually classified as different species by zoologists. Taurine cattle (Bos taurus) are most common in western Eurasia, China, and Mediterranean Africa and the zebu (Bos indicus) in much of South Asia, with varying mixture of zebu into taurine cattle across most of sub-Saharan Africa. Zebu have been mixed with gaur (Bos gaurus), kouprey (Bos sauveli), and gayal (Bos frontalis) in cattle breeds of southeast Asia. In Indonesia, varying mixtures with banteng (Bos javanicus) occur in domesticated breeds. The common ancestry of taurine and zebu cattle is Middle Pleistocene, around 500,000 to 250,000 years ago, with the common ancestor generally identified as the Pleistocene aurochs (Bos primigenius).
All these cattle bred from different wild populations are interfertile. Their male offspring are fertile, making them different from bison-cattle hybrids, where males usually are not fertile. Still, livestock breeders pay a lot of attention to fertility, and hybrids have some fertility depression compared to purebred animals
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There are no real reproductive boundaries; only gradients and local breeds maintained by human selection. For many, the solution to the species problem is that domestication makes them an exception to the rules of speciation. If you’re like me, you likely aren’t happy with an answer that looks like an asterisk.
Yaks diverged from bison during the Early Pleistocene, around 2 million years ago. Yak and North American bison can produce hybrids, sometimes called yakalo. Males are generally infertile, as for bison-cattle hybrids, but breeders have rarely but breeders have crossed them and there have been few studies of hybrids.
Yak populations may have been in contact with Eurasian bison during the Pleistocene but there is no genomic evidence of historic interbreeding between them.
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Altitude tolerance in yaks has been brought into Tibetan cattle breeds, along with disease resistance, coat length, and other traits. Domestic yaks carry introgressed cattle genes, adding up to five percent of their genome in some breeds. That includes a number of selected traits, notably pigmentation.
In terms of classification, yaks, bison, and cattle present a genus problem. All kinds of cattle are classified within Bos, and yaks generally are also. Both Eurasian and North American bison have historically been classified within their own genus, Bison. This classification makes the genus Bos straddle the Bison branch, what systematists call paraphyly. That’s a no-no in modern classification, and researchers have tried to solve it in two different ways. One is to put yaks into their own genus, Poephagus. The other is to lump all the species into Bos, including the bison species, which would then be Bos bison and Bos bonasus. I point out this taxonomic tangle because paraphyly is another classification challenge that bovines share with hominins.