A monk's discovery
In 1980, a Buddhist monk meditating in Baishiya Karst Cave, a sacred site high on the Tibetan Plateau, found a human jawbone. Recognizing its potential importance, he gave it to a local religious leader, who then passed it to Lanzhou University. For decades, the fossil remained an enigma. Its robust, primitive shape and large molars were unlike modern humans, but its identity was unclear. The jawbone sat unclassified until a new generation of scientific techniques could finally reveal its identity. The cave itself is located at an altitude of 3,280 meters (10,760 feet), an environment with dangerously low oxygen levels for most people.
The protein puzzle
The fossil's high altitude and great age made extracting DNA impossible. Instead, scientists turned to a newer method: paleoproteomics. They drilled into the dentin of one of the molars and managed to extract fragments of ancient, degraded proteins. By sequencing these proteins, they could reconstruct their genetic code. The analysis, published in 2019, showed matched the Denisovans, a mysterious group of archaic humans previously known only from a few tiny bone fragments and teeth found in Denisova Cave in Siberia, over 2,200 kilometers away. This was the first time an ancient hominin was identified using protein analysis alone.
Dating of the heavy carbonate crust attached to the jawbone provided a minimum age. Using uranium-series dating, scientists determined the Xiahe mandible is at least 160,000 years old. This makes it the earliest evidence of a human presence on the Tibetan Plateau by a staggering 120,000 years.
The high-altitude gene
The Xiahe mandible provides a physical link to a piece of modern human genetics. Many present-day Tibetans and Sherpas possess a special variant of a gene called EPAS1, which allows their bodies to thrive in low-oxygen environments without the dangerous side effects most people experience. This "superathlete" gene prevents the overproduction of red blood cells at high altitudes, which can lead to thick blood and cardiovascular problems.
Genetic studies have shown that this high-altitude adaptation was not developed by Homo sapiens. Instead, it was acquired through interbreeding with Denisovans tens of thousands of years ago. The Xiahe fossil is direct evidence that Denisovans lived in this exact high-altitude environment where such an adaptation would have been essential for survival. They occupied the "roof of the world" long before modern humans arrived, passing on a genetic gift that allows people to live there today.