Pretty sure you guys are onto it, believe it is theorised to be related to Denisovan genes

"Denisovans, an extinct hominin species, possessed genetic adaptations that helped them thrive in high-altitude environments, particularly in Siberia and potentially the Tibetan Plateau. A key example is their version of the EPAS1 gene, which is crucial for oxygen regulation in low-oxygen conditions and is also found in modern Tibetans, enabling them to live comfortably at high elevations without the negative side effects experienced by other populations according to a study published in Nature. This suggests that Denisovans interbred with humans, and their genetic contributions facilitated the adaptation of some human populations, like Tibetans, to high altitudes."