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No matter where you go, you have to carry around this meat sack of a body. So of course, over the years, humans have tried to figure out how our bodies work — and how things can go wrong, like with diseases.

And yes, sometimes we’ve been completely off base. But that's with the benefit of hindsight, and the forward march of science. So here are 5 strange ideas humans used to have about our bodies, from how we get sick to how our eyes work, that ended up being really wrong.

One classic misunderstanding is the Four Humors — the idea that the human body is filled with 4 different fluids: black bile, yellow bile, blood, and phlegm. A few cultures had similar ideas, but the humoral theory we’re most familiar with first showed up in the 5th century BCE, in a document attributed to a student of Hippocrates. Hundreds of years later in the 2nd century CE, the Roman physician Galen reintroduced it.

And it was developed even more by Arabic writers in the 9th century, and by Europeans in the 11th. In 1921, a Swedish physician suggested the four humors came from people observing how blood clots and settles outside the human body. A dark clot of deoxygenated red blood cells forms at the bottom — that’s probably what inspired black bile.

Above that is a layer of oxygenated red blood cells — the blood — followed by a clot of mostly white blood cells — the phlegm. So this phlegm might not have anything to do with the fae phlegm. And the top layer is clear yellowish serum — the yellow bile.

Supposedly, because each human is unique, we have our own ideal balance of humors. And if that gets out of balance, it will cause diseases, like the plague or acne, or an abnormal mental state. Like, depression was blamed on having too much black bile, aggression on too much yellow bile, and apathy on too much phlegm.

I actually can totally agree with that. When I have too much phlegm, I do not want to do things. The humors supposedly varied over time, though — both from hour to hour, and over the of a person’s life.course Each one was also linked with one of the four seasons.

So keeping them balanced was a constant struggle. Bloodletting became a popular “cure,” along with purging methods like vomiting or enemas. These were all terrible ideas, of course.

They didn’t work, patients were getting severely dehydrated...and, well, you kind of really need blood to be alive. It transports oxygen, sugar, waste, and lots of other chemicals around your body. In fact, it’s a common hypothesis that George Washington was accidentally killed by his physicians, who bled him 4 times, gave him an enema and made him vomit, and like blistered his throat… all ‘cause he got wet and got a cold.

Starting in the 16th and 17th centuries, publications started to challenge humoral theory and question just how helpful bloodletting actually was when treating diseases. But both physicians and the public stuck with the humors until around 1858, when Rudolf. Virchow’s Cellular Pathology was published, which laid the groundwork for modern medical science.

Once we better understood the inner workings of the human body, germ theory, and pharmacology, the four humors became obsolete. Speaking of germ theory… in the Middle Ages and the Renaissance, there was another big idea about how diseases spread: miasma, or bad air. While the term miasma wasn’t popular until the early 18th century, it comes from the.

Greek word for pollution, and the idea began around the time of Hippocrates. This bad air supposedly came from lots of sources: decaying organic matter, so-called “exhalations” from swamps or stagnant water, or even poisonous gases released from the ground during earthquakes. It was blamed for the Black Death and other plagues, malaria, and cholera outbreaks.

That kind of makes sense, because many of these epidemics happened during hot summer months, when city air was humid and smelled like garbage, dead animals, and poop. And those things do often carry disease because they’re part of a lot of pathogen life cycles — either as a source of food or a way to get picked up by another organism. So to improve health, physicians tried to eliminate bad odors or replace them with good ones.

Like, you know those creepy bird masks that plague doctors wore? The noses were stuffed with nice-smelling flowers and spices to protect them while tending to sick patients. Which, like, worked better than wearing no mask at all, I guess!

Even city engineers got behind the idea of miasma. During the mid-1800s, there was a cholera outbreak in London, and they basically changed their entire sanitation system to carry stinky sewage outside the metropolitan area. These things improved public health, but not because bad smells were the cause of illness.

So miasma theory held on longer than some scientists would have hoped. For one, the English physician John Snow, who made the connection between cholera and typhoid epidemics and contaminated water sources. During the cholera epidemic of 1854, he traced high mortality rates in Soho to a specific water pump.

After the local government removed the pump handle, the death rate went down. Snow also used statistics to show that people who got water from upstream sources were much less likely to develop cholera. Unfortunately, his findings were kind of ignored at the time.

But combined with other work — like German scientist Robert Koch’s discovery of the microbes responsible for diseases like anthrax — miasma faded from medical texts in the late 19th century. But what if you weren’t whole-body sick, and just had toothaches, cavities, or gum infections like periodontitis? Turns out, we thought those were caused by tiny worms that lived in your teeth.

Because what else could it be? That might be because the non-mineralized living tissue part of the tooth, called the pulp, kinda looks like a worm. You’d only see pulp, though, if the tooth was super damaged or decayed.

It’s tucked below the enamel — the hardest natural substance in the human body — and the mineralized living tissue layer called dentin. The earliest references to tooth worms we’ve found are in a Babylonian cuneiform tablet entitled “The Legend of the Worm.” But the concept stuck around for thousands of years. For example, there was a text from a Roman physician in the 1st century CE that described a cure for toothaches.

You were supposed to treat the tooth with smoke from burning a plant called henbane, and then rinse it with lukewarm water — after which “there may occur sometimes tiny worms.” This henbane fumigation did work, because it has chemicals called alkaloids that act as a pain-deadening narcotic. But it was a temporary fix... and had nothing to do with imaginary worms. Another fix was just to remove the tooth and the worm.

Of course, that so-called worm was probably the nerve sticking out. I am so glad that I’m alive now. In the 18th century, tooth worms finally had some serious scientific competition.

Pierre Fauchard, known today as the father of modern dentistry, was able to link tooth decay to sugar consumption. And in the 1890s, American dentist W. D.

Miller showed that mouth bacteria produced enamel-dissolving acids from the fermentable sugars and stuff from food. Microscopes also let us examine tooth pulp more closely. Scientists found hollow tubes in dentin, which conduct information about heat or cold from the surface of the tooth to the nerve.

Normally these tubes are protected by the enamel, and when they’re exposed to air, they can cause pain — no wiggling worms involved. Alright, here’s something a little, okay, a lot less gross that supposedly came out of our bodies at one point: light. I mean, it is true that we emit electromagnetic radiation.

That’s what thermal cameras pick up. But I’m talking about the light we use to see the world around us. Emission theory, or extramission, was the idea that we can see because our eyes shoot out beams of light.

Like a lot of out-of-date theories, the ancient Greeks were all over this debate. Some, like Pythagoras, Empedocles, and Plato, were on team extramission. Meanwhile, Epicurus and Aristotle thought light from a source like the Sun bounced off objects and into our eyes.

This idea was called intromission. Our old friend Galen thought we had eye beams too. But, after seeing lots of dissections, he was maybe the first person to connect sight from the eyes to the brain.

He thought a fiery air-like substance called optical pneuma flowed from the brain, through hollow optic nerves, to the eyes. And he argued the lens was the main part of the eye involved in vision — because cataracts, or clumps of opaque proteins in the lens, messed vision up. Galen’s work influenced Islamic scholars who finally shined a light on intromission.

In the 10th century, al-Hasan Ibn al-Haytham wrote the Book of Optics. In it, he used Ptolemaic optics, Galenic anatomy, and his own experiments to explain vision in a way that pretty much settled the debate. He understood how light enters the eye, but got one key part wrong: He still thought that the lens received visual information to send to the brain, not the retina.

You can thank Johannes Kepler for the final piece of the puzzle — yes, the guy who has a telescope hunting exoplanets named after him. At least, Kepler offered the first idea of a retinal image. But other scientists, like a Swiss physician in the 1500s, really helped solidify the idea that light hits the retina and gets transmitted through the optic nerve.

Even with all this vision knowledge, though, studies have shown that people still think our eyes send out rays or beams to help us see. According to researchers who reviewed over 20 studies about this trend, “the source and apparent strength of extramission beliefs...is somewhat of a mystery.” For a long time, scientists used to think that developing human embryos looked like other adult animals. The idea that species could descend from other species really started taking hold by the end of the 18th century.

And the first evolutionary model was published by Jean-Baptiste Lamarck in 1809. Just a couple years later, German scientist Johann Friedrich Meckel published the first recapitulation hypothesis. He thought the stages of development in a human embryo were like a slideshow of the adult stages of our evolutionary ancestors.

The French physician Étienne Serres expanded on this idea, and thought our developing brains progressed from fish, to reptile, to bird, to a generic mammalian brain, and then finally to a human one. In the 1820s, their work was summed up in the Meckel-Serres Conception of Recapitulation. And… there was almost immediate pushback.

In 1828, Karl Ernst von Baer proposed that early embryonic stages look similar between species, but they diverge as development goes on. None of this “representing adult forms” stuff. His research into embryology dealt a serious blow to recapitulation, which fell out of favor in the late 1830s.

Until the German biologist Ernst Haeckel came along, with his biogenic law and the infamous 1866 quote "ontogeny recapitulates phylogeny.” In other words, development mimics the evolutionary relationship between species. The biogenic law was based on three assumptions:. First, the law of correspondence.

Each developmental stage in higher animals, like humans, corresponds to adult stages of lower animals. Like, structures that look like gill slits in human embryos correspond to the gill slits in adult fish. Second, phylogenesis — or the diversification of a species — happens by tacking on extra adult forms to the end of development.

And third, the concept of truncation. Early stages of embryonic development must go faster in higher organisms, so it doesn’t take super long to go through more forms. Haeckel’s work was so popular that some of his embryo drawings still make it into high school textbooks… even though he admitted that he drew exaggerated versions of human embryos to prove his point.

Haeckel, I know you’re dead, but that’s not science, man. Long story short, other scientists weren’t able to observe what Haeckel claimed. And, instead, von Baer’s work led scientists toward our modern understanding of embryology.

His ideas weren’t 100 percent accurate either, but they were a step in the right direction. And sometimes that’s all science needs. If you want to learn more about the steps — and missteps — that led us to our modern understanding of science and us and the world, you can check out the History of Science series.

I’m hosting over on Crash Course at youtube.com/crashcourse. It has been so much fun. I’ve learned a great deal and I’m very excited to be sharing it with the world. ♩.