It's quite odd, really, that we have discovered (or possibly imagined) so much about these people with no fossil record beyond a toe bone and some fragments. This one jaw increases our knowledge of Denisovan bones tenfold. It was found in Tibet back in 1980 and has been sitting on a museum shelf ever since, paleontologists occasionally wondering what it might be but not coming to any conclusions. It dates to about 160,000 years ago.
As to why Chinese paleontologists think this jawbone is Denisovan, well, that is a tale. Pondering this ancient jaw, which doesn't seem to belong to any known species but isn't all that old as these things go, they at first tried to extract DNA. No luck. So they tried again with proteins, eventually extracting and isolating collagen proteins that could be sequenced. The sequence is clearly not from a modern human, and it matches more closely to those from Denisovan bones than those from Neanderthals.
It's impressive work but this is a new technology and far from certain. Still, that jawbone has to be something, and if it isn't Denisovan then there was yet another human species wondering around out there.
And then there's this:
The altitude of the new Denisovan’s home — 3,280 metres above sea level — surprised researchers, and helps to solve a mystery about Denisovans’ genetic contribution to modern Tibetans. Some Tibetans have a variant of a gene called EPAS1 that reduces the amount of the oxygen-carrying protein haemoglobin in their blood, enabling them to live at high altitudes with low oxygen levels. Researchers had thought that this adaptation came from Denisovans, but this was difficult to reconcile with Denisova Cave’s relatively low altitude of 700 metres. The latest study suggests that Denisovans evolved the adaptation on the Tibetan Plateau and passed it to Homo sapiens when the species arrived around 30,000–40,000 years ago.Don't ask my why having less haemoglobin helps you survive with less oxygen, but wikipedia's article about EPAS1 says the same thing, so there it is.