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Researchers identified a previously unknown yak-like mitochondrial lineage, called yak X, in ancient bovine remains from Denisova Cave and other mid-latitude Asian sites. The 2025 finding establishes a distinct branch in the mitochondrial family tree—not a confirmed new species.
What is yak X?
Yak X is the name given by the study authors to a previously unknown mitochondrial lineage. In their mitochondrial family tree, it is sister to the known diversity of modern and ancient yaks. The finding comes from ancient bovine bones, including material from Denisova Cave.
Mitochondrial DNA represents one inherited genetic compartment. It can identify a lineage, but by itself cannot establish that the animals belonged to a separate biological species or show how their nuclear genomes related to other bovines. The study, published in Genome Biology and Evolution in 2025, is titled “Paleogenomics Reveals a Loss of Bovine Lineages in Mid-latitude Asia Over the Last 200,000 Years”.
When was yak X present in the region?
Direct radiocarbon dating and archaeological stratigraphy suggest yak X was present in the region from about 203,000 years ago to about 18,000 years ago. The earlier evidence is from Denisova Cave layer 15; the later evidence is from layer 11. These dates describe an approximate regional timespan, not proof that one continuous population persisted throughout it.
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The authors also discuss limitations in molecular tip dating, so the chronology should be understood through the direct dating and stratigraphic evidence rather than as a precisely established start and end for the lineage.
How many bovine samples did the study identify?
The authors taxonomically identified 63 bovine samples using mitochondrial genomes. Their assignments were:
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| Assignment | Samples |
|---|---|
| Yak (Bos mutus) | 31 |
| Clade X (Bb1) bison mitochondrial lineage | 20 |
| Steppe bison | 9 |
| Aurochs | 3 |
These are the study authors’ 2025 assignments, not equally certain identifications for every bone. They caution that some low-coverage samples may have been misassigned; several yak X samples, for example, produced steppe-bison top hits in a downsampled analysis.
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Could yak X be the Baikal yak?
The authors propose that yak X may correspond to the fossil form known as the Baikal yak (Bos mutus baikalensis), which had previously been identified morphologically in the Altai. They do not claim the two have been conclusively matched. Further sampling of bones that can be identified reliably is needed to test the hypothesis.
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What does the finding tell us about Denisova Cave?
Yak X adds to evidence for lost bovine diversity in the Altai. The study also reports a divergent bison mitochondrial lineage and rarer aurochs. Its authors conclude that “The Altai mountains therefore appear to have been a hotspot of both bovine and hominin diversity.” That comparison is about the region’s animal and archaic-hominin records: yak X is an animal lineage, not a human relative.
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