We know the stories and the photos of suspect-looking footprints, and the folklore of Himalayan people. There are also the slightly suspect artefacts of fur, bones, and preserved body parts Tibetan and Nepalese villagers have waved in front of Western cameras for decades. So clearly there is something going on. The mythology of the Yeti is based on something, but what? Mark advanced two hypotheses: that a species of Himalayan bear is the not-so-fantastical basis of many sightings for mountain climbers and local people, or that there is a prehistoric species of human rattling around the roof of the world. Which is it to be? Well, a bit of both.
Much of the programme is spent driving to remote villages, wallowing in breathtaking panoramas, interrogating Tibetans about their local legends, and hanging around labs explaining DNA analysis. For added padding, Mark meets the celebrated mountaineer Reinhold Messner, who happened to have a brush with our elusive friend on an expedition 30 years ago. Having done a great deal of research, he came to conclusion his Yeti could be a hitherto undiscovered species of bear. Au contraire thinks Steve Berry, another professional climber. Presenting a photograph of a set of footprints in the snow, he was adamant only a bipedal creature could have produced them - though when Mark asks a villager, he straight out says they were snow leopard tracks. Darn the cats!
Getting to the nitty gritty, we get back DNA results for fur, bone, and a bit of a dessicated paw and, alas, genetics say no. The Yeti relics were either lowland black bears or highland brown bears. No new species of human then. And yet the alternative hypothesis isn't entirely dead. In a bit of a left field twist, Mark makes mention of recent findings in Denisova cave in southern Siberia. Between 2000 and 2014 archaeologists unearthed fossils pointing to a new species of prehistoric human, who apparently lived alongside Neanderthals and overlapped with modern humans. Mark hypothesised that tales of the Yeti might be folk memory stretching back to these encounters. This begs the obvious question: if that's the case, then why are the legends confined to the Himalaya and not the surrounding lowlands?
In previous analysis of Denisovan fossil DNA, Mark argued that they shared a specific mutation with modern humans living in the highlands: the EPAS1 gene. He explains how non-Himalayans adapt to altitude my producing more red blood cells to carry oxygen throughout the body. However, over a long period this can lead to blood becoming more viscous and result in a number of potentially serious health problems. EPAS1 prevents blood from coagulating at altitude, which allows modern day Himalayans to live without any ill-effects. Mark suggests that this mutation could have been passed into the local population back in the dim and distant through interbreeding with Denisovans, and explains why the legends are particular to Himalayan peoples. This isn't entirely fanciful: there is evidence our ancestors did with Neanderthals. Taking DNA samples from Tibetan volunteers from a previous programme to a Californian lab, Mark was keen to pin down the date when such interbreeding could have taken place - and the result was stunning. The date range was between 40,000 and 7,000 years ago. Amazing.
Case closed then? Bears are responsible for contemporary sightings, and the interaction/interbreeding between our ancestors and Denisovans underpin the mythology. Seems pretty airtight. Yes, but one problem with Mark's "science bit". According to the work underpinning the identification of the EPAS1 gene, its emergence is far more recent in time. 3,000 years to be exact, and it's an independent evolutionary adaptation to a previously unhealthy environment - not the result of interbreeding. That the Denisovans had this too shouldn't be a surprise - nature is littered with examples of convergent and coincident evolution. For instance, combination of different genes had the same effects in the Andean and Ethiopian highlands too without the need of an exotic intervention from another species of human.
While systematic genetic analysis of these populations are ongoing, it's fair to conclude that bigfoot is dead, and has been for tens of thousands of years. Another mystery has been peeled back, and once more what was previously strange phenomena is something interesting but entirely explicable. As unreason spreads its wings and infects too many people with magical thinking, conspiratorial nonsense, and a desire to believe the weird and fantastical, Mark and Channel Four deserve praise for reminding its audience that we have the wherewithal to explain the world without the need for modern day pixies.