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Go to Brilliant.org/SciShow to learn more. ♪♪♪. Even the most seasoned mountain climbers can start to struggle at high altitudes.

That's because the higher you hike, the thinner the air gets. It's literally less dense because it's under less pressure, and that means you get fewer oxygen molecules from every breath. And since oxygen is kind of important for us, humans run into all sorts of problems at high elevations.

But there are millions of people around the world who spend their whole lives thousands of meters above sea level and seem to have little to no trouble with the oxygen levels. Their secret: their bodies are more efficient at operating in low-oxygen environments thanks to special tweaks to their physiology. And those differences are passed down from generation to generation—making them a great example of human evolution.

We need oxygen because it's one of the major building blocks in biological molecules. It's involved in countless important reactions in our bodies, including the ones that allow us to use sugars as cellular fuel. And because of that, being low on oxygen means struggling with physical activities that are normally easy.

It can also cause all sorts of uncomfortable side effects, like headaches, nausea, and sleeplessness. And, of course, if there's not enough oxygen, you can't survive at all. The technical term for having too little oxygen in your tissues is hypoxia.

And there's nothing special about the hypoxia people experience at high altitudes, it's just that when hypoxia comes from being at high elevations, we call it altitude sickness or mountain sickness. To combat it, experienced mountain climbers usually ascend slowly. That gives their bodies time to acclimatize to the new conditions.

You see, if you spend enough time at a high altitude, you'll find your body automatically adjusts to get more oxygen adjustments which disappear once you return to lower elevations. Your body will produce more red blood cells, for example, which carry oxygen around, allowing you to absorb more from the air in your lungs. You'll also breathe more rapidly so you take in more fresh air.

And there are less obvious changes that happen within your cells. But such changes aren't cheap. Ironically, to survive when there's less oxygen, your body increases your metabolism by as much as 27% for the first few weeks, meaning you need more oxygen than usual during that time.

Not every person at high altitudes has this increase in metabolic rate, though. Like, the people who are indigenous to the Andes mountains in South America, for example. And that's because their low oxygen-coping mechanisms aren't temporary.

They're long-term adaptations. People have lived in the Andes for at least 11,000 years, and in that time, they've evolved to survive in a low-oxygen environment. We're talking human populations that exist 3500 meters or more above sea level, where oxygen levels are only around 60% of what we're used to at lower elevations.

Andeans produce more red blood cells so they can pack more oxygen-carrying hemoglobin into their blood. They also have more of those hemoglobin molecules bound to oxygen at any given time—a measure known as oxygen saturation. That way, they can get enough oxygen to their tissues without needing to increase their metabolism or breathing rate.

It's an incredible example of how evolution has allowed humans to survive in extreme conditions, but it comes with risks. Cramming more blood cells into your veins and arteries can increase the chances of clotting. And clots can jam up the circulatory pipes to cause strokes and heart attacks.

So the people from the Tibetan Plateau in Asia have a different approach to the oxygen problem. Millions of Tibetans live and work at an average elevation of 4,000 meters above sea level, and there have been settlements there for an astounding 25,000 years. Like with Andeans, Tibetan ancestry comes with some physiological perks which help them survive and operate in the oxygen-poor air.

But they don't have more hemoglobin. In fact, they have special mutations that limit the production of hemoglobin to make sure of that! They do breathe more rapidly—much like people do when they acclimatize to high elevations.

But Tibetans have combined their high breathing rate with another trick. They also produce higher-than-normal levels of nitric oxide, a chemical which plays a part in vasodilation, the widening of blood vessels, so their blood can flow more easily. And that's because their strategy is all about boosting circulation.

Faster breaths and smoother blood flow allows Tibetans to deliver the oxygen from their lungs to their tissues quickly and efficiently. So they can get enough oxygen without needing more red blood cells. And just like Andean populations, these are heritable traits.

Tibetan babies come into the world better equipped to breathe thin air, even when they're born at low elevations. But there's a lot more to living in such high places than we currently understand. Genetic research has revealed that high-elevation populations have all sorts of unique mutations in their genomes that have little or nothing to do with oxygen.

Like, Tibetan people have mutations in genes associated with higher body weight and body fat. Life can be pretty tough high up on the plateau, with the harsh weather and a lot of seasonal variation in nutrition. So these mutations may help them store energy better during tough times.

Interestingly, they also show mutations to genes which boost the production of a vitamin called folate. Folate is broken down by ultraviolet radiation, which is more intense at higher elevations, so this might be important for maintaining normal vitamin levels up on the plateau. And some high-altitude adaptations remain mysterious!

The people of the Ethiopian plateau in Africa survive just fine without basically any of the physiological tweaks we see in Tibetans or Andeans. How they do that is still unclear. But, the fact that high-altitude adaptations have evolved at least three separate times in populations from around the world is a clear case of convergent evolution.

And it's not just seen in humans. Just like the people of Tibet, Tibetan mastiffs have tweaks in a hemoglobin-regulating gene called EPAS1, which helps keep the dogs' blood from getting too thick. These adaptations, arising over and over again, are part of the fascinating evolutionary story that has allowed us humans — and our pets — to thrive in some of the most extreme environments on Earth.

While we've known about high-elevation adaptations for decades, it wasn't until recently that scientists could peer into people's genomes to begin to figure out how those adaptations took place. And if you want to better understand how they did that, you might like the courses being offered by Brilliant.org. Brilliant has interactive courses that cover all sorts of science, math, engineering, and computer science topics.

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