5 Things Humans Got Really Wrong About Our Bodies

♩ No matter where you go, you have to carry
around this meatsack 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. ♩

Comments 100

  • 0:57 That's… not his name. How do you get that wrong? His name is on the bloody page and you spell it wrong right next to it!

  • Wait..does this have anything to do with the phrase "humor me"??

  • 3:28 damn they got bubble butts!

  • Dr. John Snow looks like Tom Cruise's character in Tropic Thunder,… look at those sausage fingers

  • This is so humorous

  • Dr. John Snow? For real?

  • Who else learned about the floating rib growing up? A lie naturally to perpetuate the Adam and Eve myth, but I thought it was real even when I didn't believe in Adam and Eve.

  • Wtf bro you forgot about pillow pants

  • You know nothing dr John Snow

  • Don't give up on humoral theory yet! I am pretty sure Donald Trump is full of bile and phlegm…

  • I think another interesting topic is how much thought and energy went into old theories explaining the relationships of the celestial bodies—sun, moon, other planets, stars, etc.—when it was all completely bogus.

  • Trepanation can actually relieve pressure from fluid buildup.

  • "black bile causes melancholy", "chemical imbalances causes sadness".
    How far we've come.

  • I don't understand how extramission was ever taken seriously. It sounds like something someone drunk or high came up with that should be immediately shot down with "Then explain how come we can't see in the dark!" To learn there are people today who believe that… oy. I have more respect for flat earthers than I do for those people. (But it's a negative number either way.)

  • Talking about people believing in bad science…how about a video about ancient and modern day people believing the world is flat? It'll be a good laugh, I promise!

  • Come on! I am trying to take a break from GoT videos, and here you come with Jon Snow???

  • There actually is a condition that IS treated by bloodletting today, but it's a VERY specific condition that medieval medicine would not have even known about. It's hemochromatosis (iron overload). And it is treated by controlled bloodletting. But it's done in a very specific way, as a very specific medical procedure called phlebotomy. And the funny thing about miasma theory…that's one of those cases where they were kind of right, while also being wrong. They thought the bad smells carried disease. Bad smells actually do carry disease, just not for the reason people thought. Isolating homes from things like sewers because they smelled bad, was actually a good idea. They did the right thing for the wrong reason. Because the things that cause the bad smells (primarily bacteria) also cause disease, even though the concept of bacteria was unknown then.

  • one would think that one moment in darkness would make people realise that our eyes do not emit any light, but… nooooooooo

  • Worms in the teeth just thinking about it makes me shiver

  • Emissions Theory: I'll bet that's where the expression "…punch your lights out" came from. Also, I've heard people referring to eyes they're going to eat* as "lights."

    *Sorry, yes, you're reading that right. There's no accounting for taste is what they say.

  • On cavities, fun fact:
    In Chinese they are still called 蛀牙 or 虫牙, which literally translates to "worm-eaten tooth". Not that people think worms caused it, but the name stuck.

  • “I have too much phlegm, I do not want to do things”

  • Blood letting was the most inhuman idea ever. I don’t think the Greeks mean to have that level as the stupid Victorian quacks developed it. They were sadistic, blood thirsty, no wonder one of them became Jack the Ripper.

  • Favorite Sci-Show quote 11:31

  • I have to agree with you, Hank… When my snotbox is full of phlegm, I don't want to do diddly.

  • War is stored in the balls.


  • Plague Doctor outfits were actually really impressive. The thing is, miasma was only one of several guesses about how the plague might be caused or spread. They were worn with smoked glasses to ward against the evil eye, and the entire thing was waxed to keep out liquids in case the victim's humors were somehow contaminated. It was incredibly clever, but only helped by coincidence if at all.

  • I remember still being taught that recapitulation was a thing back when I was in primary school (which was late 90s/early 2000s). Comes to show how long it can take for these wrong ideas to leave the collective conscious even after they have been proven to be false by science.

  • I think in the future there will be a video like this but about mental illness. We know nothing and probably most of them are not illness.

  • ahhh science always being proven wrong by itself.

  • I think the weirdest thing we thought about our bodies, by far, is that they somehow contain "souls".

  • Hmm, seems like John Snow knew something…

  • Funny how he used a common fallacious believe himself, in his while talking about Washington’s death. Getting wet and catching your death is not in anyway true. How many times have you got caught in the rain and snow without catching a cold? Now if you became hypothermic then your weakened health condition could make you susceptible to viral infection and catch your death.

  • I think Hank needs more body butter.

  • 9:04 Source? And when you say people still believe in extramission, do you mean someone straight-up asked if they believed it and they said yes, or (more likely) they worked through questions about light in a way that assumed extramission?
    I don’t know about what was going on, but if it was the latter, they may be jumping the gun by thinking people assumed extramission; it is perfectly legitimate form of working backwards to figure out the path of a light beam into the eye by imagining a beam coming out of the eye and seeing where it goes.

    Edit: Found the source; in addition to some supernatural beliefs such as the “evil eye”, there was a study that consistently found a good proportion of people who did agree to some extramission ideas, even when given reading about perception immediately before being questioned.
    They decided it was part of our “intuitive physics”, but aren’t sure why people who should know better than their intuition are so willing to explicitly support it, considering we have experiences contradicting it more often than most intuitive physics. Maybe they should have asked the subjects afterwards about common anti-extramission experiences (like being unable to see when it is dark) and see their reactions and if it changed their answers.

  • Embryonic development was described in the Holy Qur'an in the early 7th century

  • You are one weird dude! I've watched a lot of your videos and you usually say something dumb. But saying Light emanating from our bodies is more gross than tooth worms takes the cake. I'm officially out of here.

  • My housecleaner still believes in the humours theory, despite being a fanatical Protestant who dispells anything that vaguely smells like supersition =P

  • John Snow saves the day again.

  • When I was little I thought my balls was where my pee was stored for when I had to pee.

  • I always wondered about that transparent-as-glass watery substance in our eyes … yeah … the VITREOUS HUMOR … what's so funny about that?

    can someone humor me by telling me the answer?

  • "I'm so glad I'm alive now." – literally what I was thinking as he said that

  • man I love that old-timey science is just people accidentally stumbling into the truth while trying to prove something ridiculous

  • Some people have no humors at all

  • Jon Snow: improving latrine management in the North since 1929.

  • why does every wrong theory go back to hypocrities

  • U know nothing 4:44

  • Whats up Scishow

  • You lost me at tooth worms.

  • When your name is Galen as well…


  • The tubes through the tooth are through the enamel.too many citrus drinks, clear these tubes out and cold drinks will hurt. Its that easy.

  • Why would you say BCE and CE instead of Before Common Era and Common Era? It makes sense to abbreviate it in writing but in speech it's really jarring

  • In Polish you still say that if you have cavities it's worms that dug them int he teeth lol but everyone beyond elementary school understands that these worms are actually bacteria and not literal worms and it's just a way of calling it that way for uneducated people to make them understand it more easily when they can visualise for instance worms in fruit. I never thought it had an origin like what you described though lol

  • Heykle, I know you are dead man, but that is not science!🙄😆

  • C.E.? What is that, some measure of time? What do they use to determine what was 1 C.E.? I'm very curious.

  • Huh turns out jon snow did know something after all

  • You know nothin snow.

  • “What do we have?” “TOO MICH PHLEGM!”
    “What are we going to do about it?” “Meh. Nothing really.”

  • John Snow, coming to save the day, both medically (4:45) and to show proof that the Army of the Dead are real

  • After 4:45 I was expecting many Game of Throne comments!

  • Lights do shoot from your eyes because we reflect light from the sun but if you have sensory or are sensing yo me you are essentially bouncing everything off not just the sun but the moon and any light bigger than say a leaf or a speckle on the screen.

    Feeling the breeze from a fan against my legs. I am seeing this now. I am not arguing that it is one or the other. I am saying that these light beams are just us reflecting lights from all sources creating what a spiritual man or guru would refer to as visions. The Rock is a Guru. Robin Williams is a guru and I say this because he's still in my thoughts mind and memory. These are people anyone is able to be. Go outside and sit or stand on the ground and look at the sky. This will improve the whole of your existence. If you listen and see through your eyes.

    Hey I love you SciShow but Ibn al-Haytham was correct. I love science. I want science to learn also. Because science taught me to this point to share my knowledge just as this show does and shows everyone our confusing but ultimately understandable planet.

    Ibn al-Haytham was the first to explain that vision occurs when light reflects from an object and then passes to one's eyes. And he was the first to point out that vision occurs in the brain, rather than in the eyes.

    He was also an early proponent of the concept that a hypothesis must be proved by experiments based on confirm able procedures or mathematical evidence—hence understanding the scientific method five centuries before Renaissance scientists.

  • You forgot the part where we thought we were created by a god.

  • My wife is pregnant bot somehow a fish swam out

  • You know nothing john snow

  • hank, can you put 'Haekel, i know you're dead, but that's not science man' on a t-shirt? 😛

  • John Snow. Woah!

  • I just understood the French expression “avoir la flemme” wow

  • I see/hear miasma and can only think of the song from the metal band The Black Dahlia Murder

  • Most of everything before the 20th century

  • Contemporaries: "You know nothing, John Snow…"

  • However, there is some recapitulation in human embryology, such as the development of the genitourinary system. Haeckle's most egregious theory was that acquired characteristics of individuals were transmitted genetically

  • John Snow…you do know something

  • The bad smell hypothesis had some evolutionary evidence. Things that smell bad to us are generally bad for us. Granted they had no idea about that but it was definitely better than blaming it on sin.

  • 2:12
    ddaeng the lady cutting the other woman's arm reminds me of Shwaz (idk how to spell his name,sorry) from Henry Danger👀😂

  • re: Miasma. the Italian term for "bad air" is "MAL ARIA"!!!

  • Talk about your low hanging fruit. x stupid things that humans have done. You could just make videos about this and you'll be able to retire in no time.

  • We had an heirloom dictionary from the 1700s in my family. It defined the nose as "an emunctory for the brain." They thought the snot was waste matter from the working of the brain and the nose was the channel through which it was ejected.

  • Omg! The black death bird outfit should be the image on febreeze!!! Lol!

  • Bloodletting is still the best treatment for having too much iron in your blood but now we give transfusions of it to anaemic patients (because they lack iron)

  • Pee is stored in the balls.


  • 2:10 we were curing ourselves all along

  • Life of the flesh is in the Blood !!! Obviously without blood our flesh dies the Bible obviously told us this long ago.

  • I would like to add the hymen to this list. It is NOT a freshness seal it is not suppose to rip the first time you have sex. You’re suppose to take care of your partner and enjoy the extra tightness the hymen provides. Seriously it won’t rip if all you do is make sure your partner is enjoying themselves.

  • hypocrites really finessed like 2000 years of medical science

  • 11:20 Is it just me or the last picture looks like something out of anime?

  • 9:49 You COMPLETELY misunderstand the mythology of our European ancestors. Not surprising. None of what you said was true. The original idea is based off of looking at the placenta and what animals it looked like.

  • physical health science wasn't the only thing that had a misunderstanding in early history mental health studies were also loose many mistook people mental illness to demonic possession so instead of contacting a doctor a exorcist is called

  • Scientific racism and racialism (the idea that human races exist) should really have been on here.

  • i dont know bout yall but i love my tooth worms

  • So basically they thought that people evolve as they grow in a embryo

  • Omg number 3!! Yes I totally get it!!! I had a cavity that freaked me out because it felt like a tiny worm moving around! It really was odd and horrible. It was just nerve pain of course but omg I thought the worst things!!!

  • It's interesting because I can totally understand why they thought certain things. You can imagine them debating about it. We just make it sound like far dumber way to think, but they were very curious and didn't just except everything.

  • …that moment when you sit 3 minutes away from j. kepler's birthhouse, don't think of him, watch a vid about medical science – und suddenly he's mentioned having done a small but important part.

  • I believe that black bile is excrement, probably due to stomach problems and diet it would have appeared more often as a liquid. Black back then being linked to stuff that was poor (like black soap). Yellow bile would be urine, phlegm is mucus and blood is blood. They also believed that the brain was used to regulate body temperature and believed that the heart was responsible for thought.

  • I love how we went from embryos having gills to seeing inside the human body without any surgery.

  • I base my human world understanding on TooL! Sue me!

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