The Insane Biology of: Humans

Here’s a sanitized version of the provided YouTube transcript:

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Our planet is full of animals with incredible anatomical adaptations. Cheetahs can run up to 75 miles per hour (120 kilometers per hour), frogs can survive being frozen alive, cows have four stomachs, and octopuses have nine brains. Compared to these animals, humans might seem like we’re lacking. We’re not the best jumpers, climbers, or sprinters, and we definitely can’t hibernate or regrow limbs if they get cut off. Yet, humans occupy a very special part of the evolutionary tree, so much so that we barely even consider ourselves animals. When you take a step back, humans are remarkable and you might even say we are quite unique.

There is no other animal quite like us. We have many adaptations that are completely unique to us, including those we use daily and often take for granted: our nimble fingers, large brains, and the ability to walk on two legs. These traits have allowed us to create megacities and occupy nearly every habitat across the world, as explored in our Nebula original series, “Becoming Human.”

Then there are adaptations that we don’t see every day—some of us have them but never use them, or they are only found in select groups of people. From throwing to diving to high-altitude climbing, when properly trained, humans are among the most incredible athletes in the animal kingdom.

If you’re a fan of baseball, you know that one of the most valuable players on any team is the pitcher. The very best can accurately throw balls at tiny targets and reach speeds over 105 miles (169 kilometers) per hour. These throws are so intense that since 1862, nearly 200 batters have died after being hit in the head or chest. While this is a tragic example of risk in sports typically considered safe, it also highlights a trait that no other species possesses: the ability to throw projectiles accurately.

It’s not only professional pitchers who have such strong arms; young boys between the ages of 8 and 14 with moderate levels of little league training can throw twice as fast as a chimpanzee, even though chimps are about four times as strong as young men when controlling for size. You might think that given the way chimps and other apes move through trees, they would have better throwing arms than we do. However, several physiological changes during evolution have set us apart from other primates.

First, the length of our waist expanded, allowing our torso to rotate independently from our hips. Next, our shoulders have developed a lower position on the torso, and the glenoid joint—where the collarbone meets the shoulder socket—points out to the side rather than upward, as in chimps. This means we can turn our arms into levers. Finally, our humerus (the bone in the upper arm) has a slight twist, allowing us to rotate the arm even further back.

Combining these three elements creates an incredible throwing mechanism. Twisting the arm back stores energy in the tendons and ligaments crossing the shoulder, which then catapults the arm forward, creating the fastest motion the human body can produce. All three of these factors appeared in the fossil record almost 2 million years ago with our hominid ancestor, Homo erectus. At the same time, we start to see evidence of more intense hunting in the fossil record, suggesting that throwing played a major role in how we obtained food and possibly how our brains grew so large.

Unfortunately, not all spears would have fossilized, but some wooden spear points as old as 300,000 years have been found in archaeological digs in Africa, Germany, and England. While some argue that these spears were too heavy to be thrown far, a recent study with trained javelin athletes found that these spears can be thrown over 50 meters at speeds ranging from 12 to 21 meters per second. At such high speeds and distances, it was certainly possible for ancient humans to take down large prey from a relatively safe distance. This throwing ability may have also been used for fighting between humans, which we can still observe today.

Our early ancestors were not only expanding their abilities in hunting on land; they were also exploring territories no other primate had ever ventured into. One of our most fascinating adaptations arose from the need to avoid drowning. This adaptation is not unique to humans, but we have learned to manipulate it for incredible athletic purposes. In 2007, a freediver reached a depth of 214 meters while holding his breath for over three minutes. The record for stationary breath-holding is even more astonishing: a full 24 and a half minutes without any air.

Freedivers utilize the mammalian dive reflex, a set of physiological changes that occur when a mammal is submerged in water and holds its breath. This reflex begins when you submerge your face in cold water while holding your breath. The cold water triggers the trigeminal nerve to send signals to the brain stem to initiate the survival response. First, the heart almost immediately enters bradycardia, a dramatically slowed rhythm. Then, the capillaries in the fingers, toes, hands, and feet constrict, directing blood flow to the brain and heart. As the oxygen level in the lungs drops, more carotid bodies send signals to the brain stem to further constrict blood flow in the extremities to preserve it for vital organs in a life-or-death situation.

This response can keep enough oxygen flowing to the organs that need it most to prevent death. Additionally, the spleen, which helps filter blood, controls the levels of red blood cells, white blood cells, and platelets circulating in our bodies. When humans go underwater, the spleen contracts to release a burst of oxygen-carrying red blood cells, another important survival mechanism.

However, when comparing spleen sizes across different populations, we see that humans have taken this survival mechanism and turned it into an extraordinary oceanic ability. For example, the Bajau people, who live around Indonesia, the Philippines, and Malaysia, are known for their remarkable ability to hold their breath. They dive repeatedly for about eight hours a day to depths of over 70 meters for several minutes at a time. Compared to nearby non-diving populations, the Bajau have spleens that are 50 percent larger. This anatomical quirk isn’t just a consequence of repeated diving; even Bajau who don’t dive as much still have larger spleens, indicating an evolutionary response to their diving lifestyle.

Researchers believe it’s not just the Bajau who have leveraged the mammalian dive reflex for hunting; perhaps our entire species has been exploring aquatic environments for a long time. In contrast, our primate relatives, such as gorillas and chimps, have the same diving reflex but are not nearly as comfortable or proficient in the water.

In 1960, marine biologist Alister Hardy proposed the Aquatic Ape Theory, suggesting that early humans foraged for aquatic foods and spent significant time in the water. He pointed to traits like our lack of fur and the layer of body fat under our skin, which other primates do not have. However, this theory became somewhat of a catch-all, used to explain many aspects of the human body, and was largely dismissed by the archaeological and anthropological communities.

There is evidence in the fossil record of our close relationship with aquatic environments. A Homo erectus site in Northern Kenya contains butchered remains of turtles, crocodiles, and fish dating back nearly 2 million years. Additionally, several Neanderthal skulls have been found with exostosis, bony growths in the ear canal that form with repeated exposure to cold and wet conditions. In modern humans, these growths are often found in surfers, divers, and swimmers. Even if free diving didn’t shape our evolution from the beginning, there’s no denying that people today have learned to push the limits of our aquatic adaptations.

For those who can’t imagine diving three meters down or holding their breath for more than a minute, it’s worth noting that all of these skills can be improved with training. Leading researchers suggest that we may all be better suited to the aquatic world than it initially seems.

While we might all have some ability to throw a spear or hold our breath and swim, there are certain human adaptations reserved for only a select few. These adaptations are found at high altitudes, where the atmosphere is filled with a comfortable 21 percent oxygen at sea level. However, as altitude increases, the effective oxygen level steadily drops. Above 1,500 meters, effective oxygen dips to 17.3 percent, and above 2,000 meters, it drops below 16 percent, making physical activity more difficult and causing many to feel more out of breath than usual.

At altitudes above 2,500 meters, effective oxygen drops to 15 percent, and without acclimatization, about 50 percent of people will experience altitude sickness, which can develop quickly and be life-threatening. Symptoms include headaches, nausea, and shortness of breath, and in severe cases, fluid can build up in the lungs or the brain, which can be fatal. Above 3,000 meters, effective oxygen drops to 14.3 percent, and chronic altitude sickness affects 5 to 10 percent of people who spend months or years at high altitudes.

This condition results from long-term hypoxia, leading to an excess of red blood cells, which can thicken the blood and increase the risk of blood clots, heart attacks, and strokes. The problem can even result in heart failure over time. At altitudes above 4,000 meters, effective oxygen drops below 13 percent, and for most people without acclimatization, acute hypoxia is inevitable. This can rapidly progress to severe and potentially fatal high-altitude cerebral edema and high-altitude pulmonary edema.

However, these risks almost completely disappear for a certain subset of people who have historically lived in three high-altitude locations around the world: the Tibetan Plateau in Asia, the Andean Altiplano in South America, and the Semien Plateau in Ethiopia. These locations range from 2,500 to over 4,000 meters, and in each of these places, the human body has adapted remarkably well to harsh conditions, but not always in the same way.

In the Andes and some parts of high-altitude Ethiopia, Highlanders have more oxygen-carrying hemoglobin in their blood than those living closer to sea level. This increased hemoglobin helps counterbalance the effects of hypoxia. In contrast, Tibetans and some high-altitude Ethiopians have far less hemoglobin than Andeans, at levels more similar to those at sea level. Instead, they have genetic changes that allow them to use smaller amounts of oxygen more efficiently. For instance, they naturally take more breaths per minute than people at sea level, and their lungs produce larger amounts of nitric oxide from the air they breathe. This nitric oxide helps increase the diameter of their blood vessels, allowing for more effective oxygen delivery throughout their bodies.

Among Tibetans, the Sherpa climbing community stands out. The Sherpa are one of the Tibetan ethnic groups native to the mountainous regions of Nepal, Tibet, and the Himalayas. Those who have dedicated their lives to mountaineering are regarded as some of the most elite climbers in history. Sherpa Tenzing Norgay was the first person to summit Mount Everest alongside Edmund Hillary in 1953, and since then, Sherpa records on Everest have continued to impress. Sherpas hold the records for the most descents to the summit (26), the longest stay on the summit (21 hours without bottled oxygen), and the fastest ascent to the summit (10 hours and 56 minutes).

This last record is particularly remarkable considering the total distance from the South Base Camp to the summit is 20 kilometers, with an altitude change of 3,500 meters. Most climbers take 6 to 10 weeks to complete this climb, making multiple shorter acclimatization ascents before the final push to the summit. In contrast, this climber completed it in just 11 hours.

For many foreigners who reach the top, numerous Sherpas are involved in laying ropes, ladders, and carrying equipment to facilitate the climb—a physical burden far beyond just climbing the mountain. The Sherpa record holders are trained professionals with years of experience, enhanced by unique high-altitude physiology. Some people even attempt to mimic this enhanced ability for exercise by training at high altitudes, but the benefits they gain from this strategy are always short-lived. Only those populations with a long history of living in high-altitude environments have genetic changes that allow their bodies to thrive where most of us struggle.

How the human body will continue to evolve in the future is an open question because evolution isn’t finished with us yet. Just as we can’t fathom what abilities future humans might possess, it would have been impossible for our early ancestors to imagine how far we’ve come and how strange the journey has been. Would our ape-like ancestors have envisioned their descendants abandoning quadrupedal movement for an upright stance? Would they have imagined those liberated hands crafting complex tools or communities creating languages and systems so intricate that they would one day travel among the stars?

The evolutionary journey of humans has been a remarkable departure from anything else that has occurred on this planet. Our story is utterly unique, even among primates. What makes us so special? How did the wheel of evolution land on us to become the dominant species on Earth? This topic has fascinated me for years, but I have hesitated to explore it on this channel since it’s a bit more philosophical and anthropological than our usual content. However, thanks to Nebula, we’ve been able to delve into the subject of human evolution in depth with our new original series, “Becoming Human.”

With Nebula, we’ve crafted a new 3D museum that features actual fossils and skeletons of early human ancestors, enhancing each episode with stunning imagery. The series takes you through the steps of how we became human. Every sign-up makes us happy because it means we can continue bringing you more and better content. If you’re looking for something else to watch right now, you can check out our latest video about the surprising reason some sharks live inside underwater volcanoes, or watch Real Engineering’s latest video about a new way to achieve nuclear fusion.

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This version maintains the original content’s essence while ensuring clarity and coherence.