DNA from bone scraps too small to identify turns up an extinct yak relative in Siberia

The screen was set up to answer a narrower question. Dalén's group started on the Denisova Cave fragments hoping to learn about bison evolution, New Scientist reported ^[10]. "But it turned out that nearly all bone fragments in the cave had DNA that didn't match any living bovine species, nor did it match the extinct steppe bison or aurochs," Dalén said ^[11]. An earlier study of trace amounts of DNA had already hinted that some of the fragments were unusual ^[13].

The design works because a fragment too small to assign by shape can still yield a genome ^[3]. Comparison with other bovine genomes then marked these as a previously unknown species, known so far only as yak X ^{[1] [12]}. "This is a remarkable discovery of a large extinct species based on small nondiagnostic bones," Dalén said ^[2].

The denominator is harder to pin down. Gilardet told New Scientist the team had access to hundreds of tiny fragments from the cave ^[4], and the full set, other sites included, produced genomes from 20 individuals ^[3]. The reports do not say how many fragments were screened or how many yielded no usable DNA. Another lab would need that yield before budgeting its own screen.

The species call rests on divergence time. The lineage began splitting from yaks about 400,000 years ago and was fully separate by about 250,000 years ago ^[5], so the separation took roughly 150,000 years ^[19]. "A useful comparison for when this lineage originated is the divergence between brown bears and polar bears, which has also been estimated to have occurred around 400,000 years ago. This age overlaps with a particularly strong warm climatic period, which could have prompted population isolation and, ultimately, speciation," Gilardet said ^[6]. The team itself says genetic evidence alone is not enough to formally classify and name a new species ^[7].

That caveat bears on how much bone shape actually missed. Dalén said the team suspects yak X might actually correspond to yak-like remains that Russian palaeontologists previously described as a subspecies of the Himalayan yak ^[8]. The team plans to compare its genomes with those described remains ^[7]. Until that work is done, it has no idea what traits set yak X apart, according to New Scientist ^[9].

Two of the more vivid claims are inferences. Every bone found so far comes from well below the Tibetan Plateau range of modern yaks, and Oppenheimer suggested yak X may have been less adapted to extreme mountain environments ^[15]. Find-spot elevation records where bones were recovered; it does not measure how the animal handled altitude. Dalén said Denisovans, Neanderthals, modern humans, hyenas and lions likely hunted it, though there is no direct evidence of that yet ^[22].

The coverage also disagrees on the end date. New Scientist puts the extinction by around 30,000 years ago ^[21], earlier than the youngest dated specimen in the phys.org and Popular Science accounts ^[20].

I think the evidence supports a narrower claim than the idea that DNA found what shapes missed. DNA established a species-depth split in an animal that may already be catalogued under another name, and it did so from fragments no one could identify by eye ^{[2] [8]}. The other gain is time depth: the 20 genomes span 200,000 years ^[3]. "The long time span of our data set is really rare in ancient DNA and allows us to look across whole glacial cycles to see how they might have impacted biodiversity," Oppenheimer said ^[16].