Our species, Homo sapiens, is the most geographically diverse of all primate species, permanently inhabiting every continent except Antarctica. We have achieved this through our unprecedented ability to develop adaptations that increase our chances of surviving and producing in a variety of environments.
Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University, previously told Live Science that highly localized adaptations such as those that allow humans to survive at high altitudes occur when environmental pressures persist that drive the need to create new biological solutions.
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The sophistication and control of the heart and lungs makes this system a jewel of evolutionary perfection. But evolution is a tinkerer, a junkyard mechanic who solves problems using the materials at hand. Trade-offs and limitations are inevitable. Just ask Jimi Hendrix.
Hendrix was an otherworldly talented guitarist who revolutionized rock music in the 1960s. He was also an avid participant in the recreational chemistry of the time, and was deeply indulged in a variety of legal and illegal pharmaceuticals. Hendrix died on September 18, 1970 in a London hotel after drinking and taking approximately 18 times the recommended dose of sleeping pills. But while the drugs were certainly the cause of his death, it wasn’t the chemicals themselves that killed him. Instead, Hendrix, who passed out and vomited from a massive overdose, became the victim of a more conventional killer. he choked.
Humans are special beings who are easily suffocated. More than 5,000 people die each year in the United States alone. Other species don’t have this problem, but this is basically a plumbing problem. The larynx (also called the voice box) is the gateway to the lungs. It is a cylinder of hard cartilage closed at the top by two fleshy lips called the vocal cords and a flapping lid called the epiglottis. The human larynx is in a precarious position low in the throat and feels like it’s going to get stuck every time you take a bite of food or drink a sip of water. All other animals (including our ape relatives) consciously tuck their larynxes high and out of the way behind their noses, so why has evolution given them such a dangerous position, threatening our breathing and access to oxygen?
It turns out that the stupid position of our larynx is the result of evolutionary tinkering with our respiratory system to produce language. The voice is produced by pressing the vocal cords together and forcing air into the larynx. This is similar to a trumpet player saying “ptbtptpbptptp!” It makes a sound by forcing air through pursed lips (I call it the raspberry sound, but the kids claim it’s a fart sound). The puff-puff-puff air that escapes travels through the air as pressure waves, which our ears perceive as sound. High and low notes are achieved by pulling the vocal cords tight or relaxing. (Testosterone thickens the vocal cords, which tends to make men’s voices deeper.)
Manipulate the shape of your mouth and throat to shape the sounds into vowels, then use your teeth, tongue, and lips to cut them into consonants. The low position of the larynx makes this possible. If they are at the same height as the nostrils, as seen in other great apes, they can make noises, but their ability to form those sounds into words is severely limited. This makes it nearly impossible to get dogs, chimpanzees, and other mammals to form phonetic words. Of course, they can still communicate with barks and growls, but the rich soundscape of human language is out of reach.
Our ancestors were so social and cooperative that the evolutionary benefits of better communication outweighed the increased risk of suffocation. Choking is the price we pay for the ability to speak.
Other adaptations to our respiratory and circulatory systems are costly as well. Traveling to the mountains faces the challenge of extracting enough oxygen from the high-altitude air. The evolved solution is to produce more red blood cells. When the liver and kidneys sense low oxygen levels in the blood, they produce the hormone EPO. [erythropoietin]stimulates the bone marrow to produce more red blood cells. (This is why some endurance athletes cheat with EPO injections; they give you extra red blood cells and oxygen-carrying capacity.) While this is a good solution, it increases the cell-to-water ratio in your blood, making your blood slightly thicker. The result can be altitude sickness, which usually involves headaches and nausea, but can progress to dangerous and even fatal fluid buildup in the lungs and brain.
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Natives of the Andes Mountains, South America’s highest mountains, live with increased red blood cell counts throughout their lives. They also have larger lungs and thoraxes, likely due to a combination of genetic adaptations to increased air exchange and environmental pressures from growing at high altitudes. However, although many genetic adaptations to altitude have been identified in Andean populations, they still suffer from altitude sickness. Approximately 15% of adults experience chronic altitude sickness. Physiological solutions to low oxygen levels come at a high cost for many people.
Interestingly, altitude sickness is not a big problem for high-altitude communities living in the Himalayas of Asia. The peoples of the Himalayas and Andes are descendants of different lowland populations separated by thousands of miles and millennia. Their migration into the mountains was completely independent, with evolved adaptations that solved the same set of challenges in different ways.
Most of these fragments have no effect on our body’s functions. They are just reminders of the wild events of our ancestors, like misspelled tattoos from a Paleolithic spring break.
Himalayan populations have specific alleles [version] A gene called EPAS1 that is involved in the production of red blood cells. This Himalayan allele has the effect of keeping EPO levels and red blood cell counts low, allowing people to withstand the chronic stress of high altitude without developing altitude sickness. This solution has its own drawbacks, as it also means that the ability to carry oxygen is limited, but other adaptations to blood vessels and respiratory rate maintain the supply of oxygen throughout the body.
Even more remarkable than the Himalayan EPAS1 allele is the story of how they acquired it. As our ancestors spread across Africa and then Eurasia over the past 200,000 years or so, they encountered other closely related human-like species, such as Neanderthals in the Near East and Europe. And, like humans everywhere throughout history, some of our ancestors were not particularly picky and slept with them.
Our species is genetically very similar, and these couplings have produced fertile children, hybrids of our species, and more. (Some may argue that Neanderthals and other groups should be considered humans because of this ability to interbreed. Semantic debates are fun to have over drinks with anthropologists.) Today, we can find genetic evidence of these events scattered around our genome, and we can, for example, find DNA fragments from other species that allow retail genetics companies to calculate how much Neanderthal DNA you have. Genomically speaking, I’m just under 2% Neanderthal.
Most of these fragments have no effect on our body’s functions. They’re just reminders of the wild events of our ancestors, like misspelled tattoos from a Paleolithic spring break, and reminders that humans will sleep on just about anything. Using the distinction described in the previous chapter, these alleles are considered neutral.
The Himalayan EPAS1 allele is a clear exception. This allele likely entered the human gene pool about 50,000 years ago through a secret Paleolithic meeting with a group called the Denisovans somewhere in Asia. For tens of thousands of years, it was just a neutral allele that existed in the mix, with no strong effects on survival or reproduction. But about 9,000 years ago, as some of those populations began moving deeper and deeper into the mountains, that allele proved advantageous. People with the Denisova variant of EPAS1 did not suffer from altitude sickness and were able to grow better and support their families at high altitudes. This went from being neutral to being local and becoming the predominant allele in the Himalayan people. Today, it is an adaptive EPAS1 allele found in almost everyone living in the Himalayas.
Another notable example of local cardiovascular adaptation was recently discovered in a group known as the Sama (also known as the Bajau). The Sama live in houseboats in the oceans around the Philippines, Indonesia, and Malaysia, spending almost their entire lives at sea. Their lifestyle is hunter-gatherer, but in the ocean. They swim and use weights to support themselves while spearfishing and gathering food as they walk along the ocean floor, sometimes more than 200 feet below the surface. Like many indigenous peoples, their lifestyle is rapidly changing, but traditionally they could spend four to five hours a day foraging in the water. It’s a lifestyle they seem to have maintained for thousands of years.
Living partially underwater presents the same oxygen supply challenges as living in the mountains. Evolutionarily, one ancient response to diving that is common among mammals is to constrict the spleen. The spleen is a child’s slipper-shaped organ tucked into the left side of the abdomen, high next to the stomach. The spleen is the immune system’s monitoring station, a sponge-like organ that checks the blood for bacteria and other harmful substances. It is normally filled with blood, so it is essentially a reserve tank for red blood cells. When you jump into cold water, your spleen contracts, expelling red blood cells from your body and oxygenating the rest of your body. As you practice breath-holding, your spleen grows and becomes more effective at this task. People in high-altitude areas like the Himalayas have larger spleens than people in low-lying areas, apparently due to a combination of genetic adaptation and a life spent at high altitudes.
Natural selection favors an allele of the PDE10A gene that increases spleen size in Samas, with individuals with two copies of the allele having nearly twice the average volume compared to individuals with none. Other diving response genes appear to be similarly selected in this population. Environment still matters. Holding your breath also helps enlarge your spleen. However, this is a clear case of genetic adaptation, where natural selection responded to a consistent, strong, and localized challenge in the Therma population.
Excerpt from “Adaptable: How Your Unique Body Really Works and Why Our Biology Unites Us (Penguin Random House, 2025)”
Adaptable was shortlisted for the 2026 PEN/EO Wilson Literary and Scientific Writing Award.
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