Everest is the tallest mountain in the world, at 8,847 meters above sea level. Low oxygen at that altitude pushes human endurance to its limits. Pictured, Sherpa mountaineer Pemba Dorje Sherpa and others on Everest, 2009.
Having lived in the Himalayas for centuries, Sherpas have adapted to easily breathe the thin air.
In 2013, the Xtreme Everest research team conducted experiments on Sherpas and other volunteers at high altitudes to discover how Sherpas thrive at such heights.
Researchers set up their laboratory at the highest altitude possible -- Mount Everest base camp.
The secret behind this ability lies in their cells; Sherpas have differences in their mitochondria, which means they use oxygen very efficiently.
Nepalese climber Apa Sherpa is the joint world record for most successful climbs of Mount Everest with 21 ascents. Another Sherpa, Phurba Tashi, is the other joint record holder.