You can probably picture a Yeti, even if you've never visited the Himalayas. From Scooby Doo to Doctor Who, Tintin and Monsters, Inc., the "Abominable Snowman" has popped up regularly in films, video games and television for decades.
In popular culture, a Yeti is an enormous, shaggy ape-man with huge feet and aggressive sabre-like teeth. Its fur is either grey or white. It is often depicted roaming the snowy mountains alone, a feral throwback to our violent evolutionary past.
Is there anything to this mythical figure, beyond tall tales and vivid imaginations? In the last few years, modern genetics has been brought to bear on the Himalayan Yeti. As a result, we may finally be able to put the mystery to bed.
The Yeti is one of several supposed "ape-men". Elsewhere in the world, people tell tales of Bigfoot or the Sasquatch, which are beyond the scope of this article.
The Yeti figure has its origins in folklore. The character is an ancient and important part of the legends and history of the Sherpa, the communities that live at an average altitude of 12,000 feet in eastern Nepal.
Shiva Dhakal collected 12 traditional stories in his book Folk Tales of Sherpa and Yeti. In the stories, the animal is always a figure of danger.
For example, "The Annihilation of the Yeti" is about Sherpas seeking revenge on a tormenting group of Yetis. They make a show of drinking alcohol and fighting to encourage the Yetis to follow suit and destroy each other. Instead, the surviving Yetis declare revenge and move up high into the mountains to continue their depredations.
In another story, a local girl is raped by a Yeti and loses her health soon after. In a third, the Yeti grows taller and bigger as the sun rises and the human that sees him loses consciousness and energy.
These tales fulfil one of the key purposes of folklore: to provide motivational or morality lessons. Specifically, they warn the Sherpa community about approaching dangerous wild animals.
"Perhaps, folktales of Yeti were used as a warning or, likely, for morality, so that kids wouldn't wander far away and they would be always close and safe within their community," says Dhakal.
"Some say that Yeti is just a fear that has been built inside the head of mountainous people to make them stronger and more fearless in the harsh weather."
But when Western mountaineers started travelling to the Himalayas, the myth evolved into something even more monstrous and sensational.
In 1921 the explorer and politician Charles Howard-Bury led a British expedition to Mount Everest. He spotted some large footprints and was told that they belonged to "metoh-kangmi". This means something like "man-bear snow-man".
When the expedition returned, a journalist interviewed some of the members. Unfortunately, Henry Newman was not the most accurate reporter. He first mistranslated "metoh" as "filthy", then decided that "abominable" was even better.
In that moment, a legend was born. Accounts of sightings by locals continued to be translated by Western visitors and the story of a mysterious ape-like snow-man took off.
By the 1950s, interest ran high. Various mountaineers launched expeditions to find the creature.
Even the Hollywood film star James Stewart supposedly got in on the act, by storing a Yeti finger in his luggage. In 2011, DNA testing revealed that the finger was human.
Ever since, there have been footprints in snow, DIY-style films, grainy photographs and eyewitness accounts from mountaineers. Putative Yeti skulls have been found, as well as bone fragments and hair samples. But on inspection, they've usually been attributed to other wildlife, such as bears, antelopes and monkeys.
Despite any concrete proof, people still go looking for Yetis in the Himalayas. Yetis are an example of cryptozoology: the search for creatures that cannot be said to exist because of a lack of evidence.
The mountaineer Reinhold Messner is perhaps the most famous Yeti-hunter of all. He claims to have seen one in the Himalayas in the 1980s, and returned dozens of times to get to the bottom of the mystery.
Messner has a simple theory to explain all the sightings: the Yeti is a bear.
Messner argues that the Yeti legend is a combination of a real bear species and Sherpa tales about the dangers of wild animals.
"All the Yeti footprints are all the same bear," says Messner. "The Yeti isn't a fantastic figure. The Yeti is reality."
He is contemptuous of the idea that the Yeti is some sort of ape-man, as promoted by Howard-Bury and Newman.
"People don't like reality, they like crazy stories," he says. "They like the Yeti as a Neanderthal, the Yeti as a mix between a human and an ape."
In 2014, Messner's point of view received some unlikely support: from genetics.
Bryan Sykes, formerly a professor of genetics at the University of Oxford in the UK, decided to test some supposed Yetis.
He and his team analysed hair samples from anomalous primates said to be Yetis, some of them supplied by Messner. They then compared the "Yeti" DNA with the genomes of other animals.
The team found that two Himalayan samples – one from Ladakh, India and the other from Bhutan – were most genetically similar to a polar bear that lived 40,000 years ago.
This suggested that the Himalayas is home to an as-yet-unknown bear, a hybrid of an ancient polar bear and a brown bear. "If these bears are widely distributed in the Himalayas, they may well contribute to the biological foundation of the Yeti legend," the team wrote.
However, this claim quickly ran into trouble.
"Polar bears in the Himalayas sounded like a really cool thing," says Ross Barnett of the University of Copenhagen in Denmark. Working with Ceiridwen Edwards, then at the University of Oxford, he decided to double-check.
Sykes and his colleagues had put all their DNA data onto a public database called GenBank. "It was really easy to download it," says Barnett.
They found a big mistake. "There wasn't an exact match to the Pleistocene polar bears like they said," says Barnett. "The match was to modern polar bears, and the actual matches were really slight."
This suggested a less exciting interpretation. Instead of a secret population of polar bears living in the Himalayas, Barnett and Edwards concluded that the DNA from the hairs had been damaged.
This does happen. Hair is a good source of ancient DNA, because the keratin keeps harmful water away from the DNA, but it can degrade.
"I think I was disappointed," says Barnett. "These out-of-the-blue discoveries are the things that everyone likes to hear about. The fact we were shooting that down was slightly sad, but at the end of the day it's important to get to the bottom of these sorts of things."
The study has since been replicated again, by Eliécer Gutiérrez of the Smithsonian Institution in Washington, D.C. and Ronald Pine of the University of Kansas in Lawrence. After comparing the DNA sequences, they found "no reason to believe" that the two "Yeti" samples came from anything other than brown bears.
Sykes and his team released a statement acknowledging their error. However they also pointed out that "the conclusion that these Himalayan 'Yeti' samples were certainly not from a hitherto unknown primate is unaffected." In other words, the samples don't look anything like an ape-man.
Still, the idea of ape-like creatures living in the mountains is more believable now than it was a few decades ago. We now know that hominid populations can go unnoticed for a long time.
Take the Denisovans, an extinct species of human known from a few fragmentary remains from a cave in Siberia. The remains were only discovered in 2008, yet genetic analysis suggests they survived for hundreds of thousands of years, only dying out around 40,000 years ago.
Another lost species endured until even more recently. The diminutive "hobbits" Homo floresiensis may have survived in Indonesia until just 12,000 years ago. That suggests there might be other populations to learn about.
Writing in the journal Nature in 2004, in the wake of the hobbit discovery, Henry Gee wrote that: "The discovery that Homo floresiensis survived until so very recently, in geological terms, makes it more likely that stories of other mythical, human-like creatures such as Yetis are founded on grains of truth."
There is clearly logic to that. But the trouble is, there is still no hard evidence, and if a population of unknown ape-men did exist, there are certain things we ought to see.
If you can get into their habitat, primates and other big animals are hard to miss, even if they're uncommon. "When you look at primate species that are really rare, like bonobos and orang-utans, the evidence is all around and easy to spot," says Barnett.
"There are places in the Himalayas where a population of large apes could theoretically survive," says Vladimir Dinets of the University of Tennessee, Knoxville, who has worked in the Himalayas.
"But all these places have lots of people living off the land, so all local species of mammals larger than a rat are regularly hunted by various means," he says.
The animals would need to roam widely to find enough food, which means they would struggle to stay hidden.
The climate is also an issue. Primates would struggle in the harsh Himalayan weather.
"Even if they were as hardy as Japanese macaques, the most cold-tolerant primates known, they would have to descend into subtropical forests in winter," says Dinets.
Again, that would surely mean discovery. "These forests exist as small patches," says Dinets. "Most of them have been cleared for agriculture a long time ago."
Yet the Yetis stubbornly refuse to show up.
In 2011, a Russian-led expedition and conference claimed to have "irrefutable evidence" of the Yeti's existence, including a bed.
However, Russian-born Dinets says it was a publicity stunt with no substantial evidence whatsoever found. Instead, it's part of a long tradition of hoodwinking outsiders.
"For about twenty years, summer trips to the mountains to look for Yetis were a popular pastime among city intelligentsia," says Dinets.
"The only result was that every village in the mountains of Tajikistan and Kyrgyzstan got a designated 'Yeti witness', whose job was to tell visitors tall tales, guide them to remote valleys where sightings were supposedly taking place, and charge them a lot of money for the service."
In summary, there is no hard evidence of the existence of an unknown primate in the Himalayas, and plenty of reason to suspect that it can't possibly exist.
It also seems that the evidence for polar bears in the Himalayas doesn't hold up. Bears might well be involved in the legend, but they are probably brown bears, which are common in Asia anyway.
Barnett believes the most likely explanation of the Yeti legend is misinterpretations of animals like brown bears, combined with the human tendency to tell tall tales about unknown animals.
That probably won't mean the end of the search, though. "The fact there has never been any evidence hasn't stopped people from searching," says Barnett. As long as we enjoy legends and fairy tales, we won't forget the Yeti.