EPAS1 is a rare gene mutation sometimes called the "super athlete gene," since it allows carriers to perform under great physical strain without causing the body to break a sweat. Received from an ancient extinct species of human called the Denisovans, the Tibetan strand of EPAS1 is so efficient at transporting oxygen it causes them to effectively shrug off their climate's paltry 60% oxygen level. It basically means Tibetans are the only humans whose body isn't forced to adapt living so high even birds get dizzy, resulting in incredible physique and strength compared to the oxygen-starved climbers they have to schlep around.
What's even more impressive is that this genetic overhaul, now incorporated in 87% of the Tibetan population, only started happening in the last 2,750 years, which is when their population split with the local Han Chinese, of whom almost none have the mutation. This makes EPAS1 "the fastest change in the frequency of a mutation described in humans" ever. Show-offs.