Expeditions to mountain summits as treacherous as K2 and Mount Everest require more than experienced guides, equipment and good weather. They also rely on the durability of mountain-climbing Sherpas.
Sherpas are the indigenous high-altitude guides that help haul equipment and coax westerners up the face of some of the world's most dangerous cliffs.
The risk Sherpas face while mountain guiding was highlighted last week when a massive avalanche at Everest killed 16 people – all of them Sherpa guides – marking the deadliest event in the storied mountain's history.
On Monday, some Sherpa groups in Nepal threatened a boycott until the government met their demands, including larger insurance payouts for the families of those killed on Everest and tighter climbing regulations. A Sherpa boycott could paralyze the region's lucrative mountaineering industry.
The Sherpas are a well-known cultural force in their native land. Sherpas guiding foreign mountaineers up Everest and other peaks could earn more than 10 times their country's annual salary in a three-month climbing season and are often revered by their fellow countrymen.
"They're seen as sort of rock stars of their community and their whole society," said Freddie Wilkinson, a New Hampshire-based climber and guide whose book, One Mountain Thousand Summits, details the Sherpas' role in helping climbers reach the summit of K2 mountain on the Pakistan-China border.
But risks loom every season. Before the first paying client leaves Base Camp, teams of Sherpas have criss-crossed the route up Everest several times, bringing up ropes, tents, oxygen bottles and other supplies to camps further up the mountain, Wilsinson says.
In a particularly treacherous stretch of the route known as the Khumbu Icefall, where the climbers were killed in Friday's avalanche, a western climber may cross that way two to four times, while an accompanying Sherpa may cross it 15 to 20 times, Wilkinson said.
"Everyone knows it's dangerous," he said. "Sending the Sherpa through time and time again is outsourcing the risk."
Not all guides on Everest belong to the ethnic Sherpa community and a few who died in Friday's tragedy were actually from other mountain-dwelling ethnic groups. But, collectively, they're often called Sherpa guides because most do come from that community.
"The whole system rests on their hard work," Wilkinson says.
Sherpas – meaning "eastern people" in Tibetan – migrated from the eastern edge of what is today known as the Tibetan Plateau more than 30,000 years ago. They settled in the highlands around Nepal and Tibet, said Cynthia Beall, a Case Western Reserve University anthropologist who has studied the Sherpas.
They were farmers and traders who adapted well to the high altitudes. But it wasn't until British exploratory teams visited the area in the early 1900s that Sherpas began climbing the summits, she said. The western mountaineers recruited Sherpas for their earliest attempts to summit Everest and used them for years to follow. When New Zealander Edmund Hillary made the first ascent on the mountain in 1953, he was famously accompanied by a Sherpa, Tenzing Norgay.
"They have a tradition of doing this and making a good living at it," Beall said. "It's a very dangerous living and they recognize that."
But it's more than just tradition. Sherpas are so adept at working hard at high altitudes because of a mysterious physiological trick their bodies pull off unseen anywhere else on Earth, said Rasmus Nielsen, a biology professor at University of California, Berkley, who has studied the Sherpas' genealogical makeup.
Unlike mountain dwellers in the Andes Mountains in South America and other highlanders, Sherpas produce less oxygen-carrying red blood cells at high altitudes, rather than more, as is the norm, he said. This prevents them from developing long-term illnesses and allows them to work more.
"They seem to function well in high altitude without producing as many red blood cells," Nielsen said. "No one knows for sure why."