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Go to CuriosityStream.com/SciShow to learn more. [♩INTRO]. The highest permanent human settlement is a tiny mining village called La Rinconada, in Peru, which is about 5,100 meters above sea level.

Considering that some people start to get altitude sickness at around 2500 meters, that's pretty intense. Not everything on Earth is as restricted as we are, though. You might have heard of the hardcore microbes that can live at super high elevations or the deepest parts of the ocean.

But there are surprisingly complex forms of life that can survive in those places, too. They just need some extreme adaptations to do it. The most immediate thing you'll notice at higher elevations is that it's way harder to breathe.

The higher you go, the less air pressure there is. Your lungs struggle to take in oxygen, and you can develop altitude sickness, where the lack of oxygen causes symptoms like nausea, a rapid heart rate, and in severe cases, swelling in the brain. Spend enough time up there, though, and your body will start to adapt by producing more oxygen-carrying red blood cells.

Other complex animals, like mammals and birds, can have pretty similar problems. And yet, yaks regularly hang out at elevations of 6,100 meters. To cope with that, they've evolved larger chests, lungs, and hearts, as well as thick, shaggy coats to help deal with the bitter cold.

Researchers have also discovered a bunch of changes to their genetic code that help them survive up there, like by controlling their body's stress response when they're low on oxygen. Birds can go even higher. The bar-headed goose, for example, regularly migrates over the Himalayas at altitudes up to 7,000 meters.

And the highest known vertebrate ever recorded is Rüppell's griffon vulture. In 1973, two pilots flying at 11,000 meters, 2,500 meters higher than the top of Mount Everest, made a sudden and terrifying discovery when a bird got sucked into the plane's engine, forcing an emergency landing. Not a good day for anybody involved...

We don't fully understand how these birds do it, but both the bar-headed goose and Rüppell's griffon vulture have genetic mutations that allow the hemoglobin proteins in their blood to hold onto more oxygen. Researchers have also found that when they're low on oxygen, bar-headed geese can move more air in and out of their lungs than any other species we've ever studied. Any higher and you start to get into the realm of microbes.

Bacteria, for example, have been found at the tops of storm clouds and up to 15 kilometers above the surface of the Earth. We don't yet totally understand how they survive, either, but it's not too uncommon to find microbes in some of the most extreme environments on Earth. It doesn't take quite as many genetic changes to adapt when you're a super tiny simple organism.

Funnily enough, the biggest problem for survival at extreme depths is the same as at extreme heights, at least for mammals. You start running low on oxygen. The deepest a human has ever dived without taking oxygen with them is about 250 meters, but that's nothing compared to some other mammals.

Southern elephant seals can dive more than 2400 meters, and in 2014, scientists watched as a Cuvier's beaked whale dove 3000 meters below the surface. Since mammals have to breath air, they need special adaptations to dive that far. For example, some species have more red blood cells, can slow down their heart rate, or can temporarily shut down non-vital organs like their kidneys.

Some also have extra myoglobin, a protein that stores oxygen in muscle cells. But these divers are just visitors. There are species of animals that spend their entire lives in the deep ocean.

In 2017, scientists found the deepest known vertebrate: a pink, wiggly snailfish at over 8,000 meters down in the Mariana Trench, the deepest part of the ocean. The fish doesn't have that oxygen problem that whales and seals do, since it gets its oxygen directly from the water. But there is another problem: pressure.

Water is heavy, and the deeper you go in the ocean, the stronger the water pressure becomes. At 8,000 meters, the pressure is like having a cow sitting on every square centimeter of your body. And not a small cow either...

At that point, the pressure is so great it might actually destabilize the proteins the fish's body is made of. To compensate, biologists think the fish's tissues are full of a compound called trimethylamine N-oxide, or TMAO for short, which can help stabilize proteins. TMAO is common in fish, and the deeper you go, the more of it they tend to have.

But even with this adaptation, this may be the deepest fish we're ever going to see. Researchers have calculated that beyond 8200 meters, fish would need so much TMAO to withstand the pressure that their cells would be saltier than the seawater around them. Except then more water would rush into the cells through osmosis, and then they would explode, and you can see why that would be a problem.

And yet, we can go even deeper. In 2012, James Cameron, yeah, the guy who made Avatar and Titanic, visited the deepest part of the Mariana Trench in a submersible. And even there, over 10,000 meters below the surface, he was greeted by relatively complex life: giant, foot-long crustaceans known as amphipods, sea cucumbers, and weird, gooey, shelled things called foraminifera.

There's a lot we still don't know about the organisms that live in the deepest parts of the ocean. It's one of the biggest habitats on Earth, and yet it's also the hardest to explore. But we do know that life is super adaptable.

And with a collection of those amazing adaptations, complex life can exist at some incredible highs and some very low lows. If you want to learn more about science, and of course you do, because you're watching SciShow, as well as other topics like history and technology, we think you might enjoy the videos on offer over at CuriosityStream. Today's episode is brought to you by CuriosityStream, which is a subscription streaming service that offers over 2000 documentaries and non­fiction titles from some of the world's best filmmakers, including exclusive originals.

They have videos on nature, history, technology, even society and lifestyles, which is one of the reasons why we like them so much. Like, if you liked the extreme environments in this episode, there's a whole show called Underwater Wonders of the National Parks. As if national parks don't have enough to offer above ground, there's even more to discover beneath the surface.

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