Four decades ago, a monk came to pray at a cave two miles above sea level in the Tibetan Plateau. There, the monk found half of a mandible, which his village gave to scientists.

In 2010, archaeologists made a remarkable discovery: This high-altitude jaw is not like yours or mine. Proteins pried out of its ancient teeth revealed it belonged to a Denisovan, an extinct human species related to Neanderthals.

Denisovans were a “very mysterious group,” said anthropologist Jean-Jacques Hublin, whose international team published a study of the mandible in the journal Nature.

This Denisovan had a big chin. “This mandible is larger than my mandible, and very likely your mandible,” he said.

The half jawbone is the largest piece of a Denisovan discovered, the authors said. The world’s collection of Denisovan fossils — several teeth, a bit of skull and bone splinters — could fit in a cereal bowl.

Their genes still echo through the human lineage. Modern humans, some of whom mated with Neanderthals, must also have mated with Denisovans. The proportion of Deniosvan DNA in genomes of people native to Melanesia, for instance, reaches 6%.

The find supports evidence that Denisovan DNA helped modern people thrive in the thin air of high altitudes. Certain Denisovan variants, such as a gene that allows the blood’s proteins to use oxygen more efficiently, are found in Sherpas and others living in the highest climates of Asia.

No remains of another human species have been found at such a high altitude. Before this study, anthropologists assumed the tallest mountains were the domain solely of Homo sapiens, Hublin said.

The site, Baishiya Karst Cave, is at an altitude of 3,280 meters, or 10,800 feet. (Denver is about 1,600 meters high.) To live there is to survive on cold, low-oxygen air. The bone is at least 160,000 years old — more than 100,000 years before the first signs of modern humans living in the Tibetan Plateau.

This “suggests that genetic adaptation to life at high altitudes is very old,” said Svante Pääbo, a geneticist at Welker’s institute. Denisovans and Neanderthals adapted to many environments in Eurasia, he said, “then modern humans from Africa came along and mixed a bit with them.” Our ancestors picked up their genetic variants in the process, the blender of human evolution pulsing once again.