Quirks of Human Anatomy
sdbonline.org138 points by gurjeet 2 days ago
138 points by gurjeet 2 days ago
The urethra is routed through the prostate specifically because it needs to occlude it during sex. The prostate swells during an erection to obstruct the urethra and significantly reduce the likelihood of a bacterial infection in the bladder. It's a very important function.
Amazing. I didn't know that. Prostate in Greek means Protector and took its name from it's role. Ages ago this function was known.
> f. Nipples are useless in human males (cf. Ch. 5).
When I was a kid, I got my tonsils removed "because they were useless and a source of illness".
I've recently heard that tonsil removal is now more disputed: it may collect filth, sure, but it may also prevent it from going deeper into the body, which may cause more serious illnesses.
Given its vast complexity, and the timeline of its creation/evolution, I remain skeptical over bold claims about the human body. It's really missing an "as far as we know." The ability to go beyond what is known is paramount to the progress of science, and historically attested with some intensity (e.g. Earth's shape, relativity with time/space & axiomatic geometry). Humility thus feels like a better posture.
Who would let a junior dev trim bits, or boldly modify a decades old codebase?
There's also a difference between having no immediate use, and having no reason to exist. From what I understand, sexual differentiation works by having the Y chromosome act as a switch, and both sexes have to share the same blueprint with hormones guided the development of their organs.
For males not to have nipples, they'd need to be actively destroyed, which poses a risk for females to also not have nipples, which is much worse than males having harmless, inactive nipples.
It doesn't seem like eliminating nipples should be any harder than eliminating the uterus...
That's true, but inactive nipples don't cost anything, which certainly isn't the case for an inactive uterus. I don't know how it works, but I assume that such developments follow some kind of cost-benefit function.
Aren't nipples pretty recent? The egg part has been there for a very long time, nipples haven't evolved as long, maybe in a few hundred million years we no longer have nipples.
male and female sexual organs are the same thing inside out of each others, to some extent.
The actual switch (in humans and I believe most mammals) is a gene called SRY. The Y chromosome is just the (usual) container for the switch.
>Given its vast complexity, and the timeline of its creation/evolution
I will just say, the human body in particular has only been around for a vanishingly short period of time in evolutionary terms. A lot of the quirks and arguable flaws identified in this piece (painful childbirth through the pelvis, back pain) and others (varicoceles in the left internal spermatic vein, hernias, other pelvic floor disorders) can be attributed to our very recent move to full bipedalism.
If we’re talking about features we share with other mammals or even other primates, sure, they’ve probably stood the test of time for a reason. But for features that have only really been in existence for a couple million years, those I don’t think we should treat with the same kind of reverence.
> human body [...] has only been around for a vanishingly short period of time in evolutionary terms.
"as far as we know." Every few years, I see in the headlines stuff like "oldest 'human' ever found in X." The theory of evolution itself has morphed since Darwin [0], and is probably far from being in its definitive form.
The timeline remains astronomical w.r.t. a human life, and the perception of a single human. A few centuries ago, we may have burnt people for proposing something like the theory of evolution.
> [...] can be attributed to our very recent move to full bipedalism
Admittedly. But it's still not contradictory with this still having unknown roles. Actually, multi-causality feels like a good way to ensure the stability and solidity of a design: "don't put all your eggs in the same baskets", portfolio diversification, etc.
Thinking about painful pregnancies and birth, [1] hints at the "need" for pain/discomfort. If it's indeed some sort of a necessity, then it may be more of a feature than a bug for us to experience pain directly, through the womb, etc.
[0]: https://en.wikipedia.org/wiki/History_of_evolutionary_though...
> as far as we know." Every few years, I see in the headlines stuff like "oldest 'human' ever found in X."
The discovery of older humans does not respond to the point you’re arguing against — in evolutionary time scales, humans are recent.
> The theory of evolution itself has morphed since Darwin [0], and is probably far from being in its definitive form.
Fields continue to change as they grow older, but the magnitude of changes tend to get smaller. Of course evolution will change, but it would be very surprising to have large changes in the fundamental elements
> Thinking about painful pregnancies and birth, [1] hints at the "need" for pain/discomfort.
Evolution only optimizes for what results in dna being passed on. It doesn’t care about ancillary details. I think painful childbirth pretty much shouldn’t matter much to evolution, because the parents have no control over the birth at that point — it’s happening one way or another. Perhaps it promotes bonding with the child, or something like that? But in general, I think it’s wrong to say “evolution provided X, so X must be needed”. If X has no significant effect on the passage of dna, then it could just be random noise.
> I've recently heard that tonsil removal is now more disputed: it may collect filth, sure, but it may also prevent it from going deeper into the body, which may cause more serious illnesses.
Hasn't it been settled for a while that they're part of the immune system? Wiki is clear [0] on the subject; they're there to repel bacteria. They're quite important and removing them, unless there is no other choice, seems like a terrible idea.
Happened to me in the 90s (France). Seems to still be practiced regularly [0].
A 2022 article [1] quotes an ENT advocating it for kids with frequent (3 to 4) bacterial throat infections in winter.
It's often difficult for new ideas to get through people who have upheld the same point of view for decades though. Especially for "selective" fields like medicine (ego issues are probably more developed than in less selective fields). Let alone in fields strongly impacted by money or politics.
[0]: https://www.idref.fr/234378662
[1]: https://www.santemagazine.fr/sante/maladies/maladies-infanti...
> Who would let a junior dev trim bits, or boldly modify a decades old codebase?
"Chesterton's Fence": you shouldn't tear down a fence (or a piece of code) until you understand exactly why it was put there in the first place.
Chesterton’s Fence is a thing because a fence is something that was definitely placed with intent. It’s possible the intent is no longer applicable or was just plain stupid, but it didn’t happen by chance, so you need to make sure the original reason no longer applies.
Nothing in the human body was placed with intent. It’s still important to understand what it does before you go messing with it, but it’s a very different sort of thing.
Can you explain the fundamental difference between our intent and the intent of, say, a molecule in our chain of evolution? Is it free will? Because that debate hasn't been sorted out yet.
The difference is intelligence, which might be a better word than “intent.” A fence was built by someone with some measure of intelligence. An evolved feature is randomness combined with selection pressures, and sometimes it’s only there because it’s not sufficiently detrimental to be selected out.
Ok, intelligence is better than intent yes. But intelligence doesn't have a good definition and most people would say that it exists on a continuum. And when thinking of human-designed products, you can also say that there is selection pressure, and features can be selected out, etc. So I don't think there's a fundamental difference here.
My doctor has (with case by case exceptions as needed) a general rule to encourage a conservative approach: if at all possible, attempt to leave with everything you arrived with
Just like clergymen used to omit "we believe x, y, or z" when making some mystical claims, "scientists" of today omit "as far as we know from their communication with the layman...
>Who would let a junior dev trim bits, or boldly modify a decades old codebase?
Ones who does not understand the idea of the "Chesterson's Fence"
similar story with the appendix.
Initially considered a useless vestige, now thought to be involved with maintaining gut bacteria.
And yet has no discernible positive effect on modern human survival rates but one of the most acutely lethal negative ones.
Pre-medicine you got appendicitis and just died painfully.
Let me defend ear twitching. It's a subtle way of communication for those of us so blessed with the ability.
Part of this reads like a shortlist of things that doctors and scientists don’t know enough about yet. If you’re looking for a PhD topic, here are some ideas.
I strongly disagree. Literally every item in there makes perfect sense if you have an understanding of how evolution works.
E.g. many of these items are simply vestigial in some sense, where their presence doesn't actively harm the species and it doesn't impose any substantial energy budget. E.g. the current top comment here is about male nipples. Male nipples may be "useless", but they're not actively harmful (and they can certainly be pleasurable during sex), so there is no evolutionary pressure to get rid of them. The perineal raphe (i.e. the "male taint stitch") also has no purpose but is simply a byproduct of how the male forms in utero.
As the article points out, most of the other "quirks" are simply what evolution had to deal with. You may say the eye is "weird" because the photoreceptors lie behind the ganglion cells, but it certainly works quite well, generally. And it doesn't "have to" be this way. Octopus eyes are completely the opposite and a great example of convergent evolution.
Other examples are simply tradeoffs. There is pretty obvious survival advantage for humans having a large brain, but this then adds complexity for how the head gets out of the relatively small pelvic canal.
I honestly didn't see any examples in this list that aren't well understood and well explained by scientists. If anything most of these provide excellent examples of how evolution works.
It also reads like a collection of evidence against intelligent design.
Akshully…
These flaws were left in so there would be no proof of a creator. Because with proof, there is no faith. And faith is a requirement of winning.
Imagine how hard it must have been to design “flaws” like these but still allow humans to survive. I can’t. Therefore, you can’t. Only an omnipotent god could do it.
Checkmate atheists!
If we must have faith, then why does He need constant advertising?
I'd say kill that advertising for a few generations, then see what is left.
What about testicles outside the body? Every man has a painful story why it’s not a good idea..
I read this does have a functional reason: Sperm cells have to be kept at slightly lower temperature than the body temperature, so if the testicles were inside the body, the sperm wouldn't survive.
Of course you could ask why sperm is so temperature sensitive in the first place...
If elephants did it, I’m sure we can too.
They have a complex system to mantain the specific temperature. Huaman and other smaller animal do not have enough space to it inside.
Yeah, it makes more sense to put all the fragile body parts in one place. For instance, the face.
This is how future codebases will be analysed. Don't let perfect be the enemy of good. Evolution been doing Agile for aeons. Responding to change over following a plan ...
Yes, don't give a crap about doing it right so all your costs instead move into support and maintenance (which in this analogy is healthcare)
This is kind of funny. This is what produces one-dimensional humans.
The desire to put it this way... the underlying assumptions like "pain is not good", and so on... is ideological, not scientific.
There are occasional cases of male lactation reported in humans. Very rare though.
In the guinea pig, the large head at birth is provided for by the carteliginous symphysis joint in the hips detaching. However unless the animal gives birth early enough (which always happens in the wild), they lose this capability and die if impregnated later. Some doctors thought it a good idea to try to emulate this in humans by cutting the cartilage there instead of doing a cesarian section, but this causes permanent problems, as in humans the joint does not reattach. Notoriously, for religious reasons some doctors decided to do so anyway, since cesarian section reduces the number of pregnancies a woman can have, which they regarded as more important than being able to walk easily and being continent.
seems to be heavily affected by hormones as many trans women end up experiencing some level of lactation while undergoing hormone replacement therapy.
It would be nice to refactor some of these. Vas deferens and laryngeal nerve look like easy pickings. Leave me my ear-wiggling. Any last bit of expression matters.
I'm dreading the horror of genetic manipulation it would open. The gene editing craze feels like it is right around the corner.
I cannot wait for the second pair of arms. :D
it has been suggested that a human genetically-engineered for a life in zero G would be better of with no legs and extra arms.
>The hole in the retina is sizeable (~9 full moons in the sky), but we don’t notice it because [...] (2) our brain automatically fills in gaps in our visual field by interpolation
I still remember this bit from school and various pop-sci book, but is it actually true? Is there really some group of neurons in the brain somewhere that actively tries to restore the "raw" visual information that was blocked by the blind spot?
Thinking of ANNs, I felt it was more realistic that higher layers in the visual cortex are mostly only using the visual information to find patterns anyway, and that they're robust enough they can still find those patterns without the data from the blind spot locations. (As long as a pattern isn't fully contained within the blind spot regions of course)
An analogy would be a QR code reader that can still parse the encoded information if a part of the QR code is missing - but it won't actually "reconstruct" the missing sections to do this.
But I don't know if it really works like this.
I've read a number of consciousness books and this idea that the brain is inpainting the blind spot. I know Dennett in his "Consciousness Explained" book of 30+ years ago did his best to debunk that idea.
The problem with inpainting is that it suggests there is a generator that knows how to fill in that spot before the "witnessing" part of the visual system then gets to work. This is perhaps best thought about from a more extreme example of dreamining: witnessing visuals with the eyes complete closed. In short, the visual system isn't like a projector screen with the finish fixed-up image that some interior witness views.
I believe that most researchers have a very different model, that of the controlled hallucination. What we experience isn't the photons hitting the array of rods and cones like a 2D array of pixels. Instead, we have an internal model of what we are looking at and visual input is there to provide feedback to keep the model of the world updated. The blind spot isn't experienced because we aren't looking at the 2D grid of pixels -- our model is coherent, and the presence of the blind spot simply means that no corrective feedback comes from that area of that one eye.
One compelling bit of information is there while there are neurons feeding processed visual information forward into the brain, there are more feeding back from that area to the visual system. That is, the the visual system is providing error signals and not the image we are experiencing. When something not predicted appears, the visual system sends forward information guiding the internal model to be updated.
Have you ever had the experience where you have been asked to close your eyes and put into a novel environment before then being allowed to open your eyes? It takes less than a second, but you can feel a moment of disorientation while your brain builds that world model. Another way is via some optical illusions (Necker Cube) or Escher drawings. You can look at a part of the drawing and everything is fine, but then as you change your focus there is a transitory feeling of unease as that world model is in flux as it tries to resolve the new visual input with the model it had been using.
I was going to say the same thing. When we look at a book, we see the book in our minds, not a picture of a book. The processing that happens in our brains works to create the representation, it’s not transforming one image into another.
There’s a significant portion of the population that doesn’t “see” anything in their mind’s eye when reading.
Clickety_clack propably wasn't referring to people with aphantasia not seeing the story unfolding while reading the book, but the book itself: We predict that the thing we hold in our hands is a book entity, and then we use our sensory perception to affirm or change that prediction. We don't continuously parse a 2D array of pixels and interpret that as as a book every frame.
>(As long as a pattern isn't fully contained within the blind spot regions of course)
There are dedicated optical illusion/explainers that give you the experience of the brain patching over the space with neutral background, even if there's something there, like a symbol or a star.
So if it's something featureless or continuous, like a wall of your room that's a solid color, or a sheet of college ruled paper, the pattern can just be continued.
That said I would stress there's limits to how much of that you can do just by pattern extrapolation as opposed to deriving images from distinct and specific information in a given region of the visual field. You have to know enough about a stretch of visual space to know that it's appropriate to spread a pattern over it, and that's the thing the blind spot doesn't know.
What’s interesting about that is that brain doesn’t actually give you much access to the sensor information directly, but gives an interpretation instead. There is a thing called Saccadic Suppression that blocks visual data processing for 50ms when eyes are moving, and the brain just backfills that missing data from the “next frame”. https://en.wikipedia.org/wiki/Saccadic_masking
Thanks, I had not heard of that one. As a recovering philosophy bro I love cataloging all of these peculiar artifacts of our visual experience. They turn out to matter quite a bit in some of the endless mind versus brain and mind versus matter debates. Off the top of my head:
- Blind spot where the optic nerve exits the eye
- Saccadic Suppression (new to me!)
- Panum's fusional area (how close the overlapping images of your eyes have to be to each other to get merged into a unified image)
- The wagon wheel effect
- trichromatic vision (obvious but important because it easily could have been different)
- The foveial field, the central part of vision that's extremely precise, while things increasingly further away from it are blurry
- specialization in peripheral vision, (eg better sensitivity to starlight, as well as better sensitivity to flickers and motion)
Add those all up and you get a bunch of specific but contingent properties of visual experience. Some people of a certain philosophical frame of mind like to imagine that we inhabit a kind of pure mental experience detached from the physical world, but even if you think you're making no assumptions about the empirical world, all of these contingent facts show up, which make a lot more sense as being the products of biological structure.
The eyes of squids are right side out, unlike ours. I wonder what other animals have the "correct" version of these features.
Full-featured eyes have developed a few times independently [0], so it could be that mammals and squids have evolved on different branches that split before the common ancestors had eyes as a feature.
[0] https://en.wikipedia.org/wiki/Evolution_of_the_eye#Origins_o...
The page claims that babies can suckle and breathe at the same time, but upon first-page-of-google research, it seems not to be true https://pubmed.ncbi.nlm.nih.gov/34636089/
Fun and educating article. Reading that we once had three eyes, lead me to this paper: https://www.sciencedirect.com/science/article/pii/S096098221.... Fascinating that many vertebrates had a third light-sensing organ (called the pineal gland) on top of their heads millions of years ago. If our eyes are now the main mode of light transmission to the pineal gland, should we be wearing sunglasses all of the time?
Anatomy is the latest image snapshot of Evolution. Evolutionary quirks are more general, interesting and important. One is the debated and funny Haeckel’s law: ontogeny recapitulates phylogeny.
Not humans specifically but one of my favorite quirks of vertebrate evolution is the recurrent laryngeal nerve that loops around the aorta and goes back up to the larynx[1].
In giraffes that nerve is several meters long.
How about feeling the hand even after amputation. What do you think, why is it so?
You’ve probed Chesterton’s Fence; now let’s turn the page to Chesterton’s Appendix!
I hear that visual cortex being on the rear of the brain when the eyes are on the front is another example.
I assume the intelligent designer was fired for this.
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>The common crowding of human teeth—especially "wisdom" teeth, which erupt last >is traceable to the evolutionary shortening of our jaw.
Complete baloney.
Teeth are easily lost, so wisdom teeth are nature's backup plan, or so I heard.
Could be the case.
But the explanation of crowded tooth being ascribed to evolution is completely false. If one dug around literature even a teeny bit one realizes that many hunter gather groups even today have a a full set of tooth perfectly aligned in the jaw. Dentists, Weston A. Price about a century back and more recently John and Mike Mew (father/son so the same last name) come to mind who tell a different story from the mainstream evolutionary explanation for crowded tooth.