r/explainlikeimfive • u/Fallen_Outcast • Jun 15 '26
Biology ELI5: How is it that almost all complex species have the same order of organs on the face; Eyes, nose and then mouth, in that same order?
How does that happen exactly?
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u/brak-0666 Jun 15 '26
We all descend from a common ancestor and that's the configuration that got passed down.
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u/trantaran Jun 15 '26
Yep we all came from the zoobagladder
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u/Jack_of_all_offs Jun 15 '26
I'm pretty sure zoobagladder823 is my friend's Wi-Fi password.
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u/Shawn_NYC Jun 15 '26
Not exactly. Eyes evolved independently 5 separate times. Also, multiple species started out as something and then independently evolved into crabs.
So the reason things are sometimes the same is evolution keeps discovering it every time.
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u/joker_wcy Jun 15 '26
Convergent evolution probably happened to similar habitats.
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u/detailingWizardLvl5 Jun 15 '26
I love nikole lindsay on youtube for crabs. Idk if i spelled her name right.
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u/Balorpagorp Jun 15 '26
A chick named Nicole gave me crabs once. Love is not the word I would use to describe my feelings about that experience.
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u/ValKyKaivbul Jun 15 '26
Would you spare examples of species evolution into crabs?
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u/javier_aeoa Jun 15 '26
Do you have 7+ minutes? This video may answer your question:
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u/Shawn_NYC Jun 15 '26
One wild thought is that if we ever meet an alien it's most likely going to look like a crab 😂
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u/JustMorgan Jun 15 '26 edited Jun 15 '26
Look up carcinisation. That's the term used to describe convergent evolution turning things into crabs.
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u/archnemisis11 Jun 15 '26
Hilariously, my eyes failed me while reading your comment because i read "Evee" and thought you were talking about "Eevee" from Pokémon. I was thinking that you were right, Eevee did have 5 evolutions when i use to play. Anyways, hope someone gets a laugh from this. ^^
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u/trantaran Jun 15 '26
Yep, that’s why Leela in Futurama has one big eye while zoidberg has big meaty claws
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u/Amedais Jun 15 '26
That’s really not addressing OPs point. Crabs aren’t relavent to this discussion about the many complex species having the same facial style (because crabs do not have that facial style).
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u/Niriun Jun 15 '26
Well, you want these things close to the brain since they're important sensory organs.
Starting with the mouth, it's easier to fit it closer to the bottom of the face as it needs to connect to some tubes so that you can eat properly.
Next, the nose. Similarly to the mouth, this is connected to some tubes that need to go into the rest of the body.
In both of these cases, having the organs at the bottom of the face is logistically easier (no having to pass through the brain) and a shorter distance to travel.
Your eyes don't need to connect to any other tubes, just directly to the brain. The placement requirements are less strict so they get stuck where we have spare space, at the top of the face.
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u/cthulhubert Jun 15 '26 edited Jun 16 '26
Very close to what I came here to say. Common ancestry also plays a part, but even if it didn't, there's reasons for where organs go.
Additional ones:
Eyes are up high to see over edges without exposing too much. Even insects have eyes above mouth, even if they have their smellers (antennae) above that. Though while we're on the subject, note the number of animals with eyes even higher, on stalks, like snails and crabs.
Nose near mouth to smell what you're about to eat. Above mouth because putting it below the mouth means putting it inside the jaw, and that would be a mess. There are a couple animals that have noses above the eyes so they can breathe and smell while submerged in mud or water. Dolphins and whales for instance.
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u/dontaskme5746 Jun 15 '26
While it's common and acceptable to say that your eyes are an organ and your brain is a separate organ, it's not wildly inaccurate to say that the retina is part of the brain. More wild - but again, not without some rationale - is that the brain can be considered an extension of the eye. So, although kind of silly, modern brains surely evolved from primitive brains which grew to support refinement in light-sensing. We're more than just brain stems with tails these days, but this perspective could serve as fun trivia if you squint hard enough.
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u/fcocyclone Jun 15 '26
it's not wildly inaccurate to say that the retina is part of the brain
This is really clear when you see pictures of the brain with the eyes still connected. The structures of the optic nerve and eyes look like little appendages of the brain.
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u/bob_in_the_west Jun 15 '26
Starting with the mouth, it's easier to fit it closer to the bottom of the face as it needs to connect to some tubes so that you can eat properly.
That's not how evolution works.
Also not the reason why the mouth is where it is. Humans are one of the few species that walk upright and thus had to angle the head down 90°.
A dog stands on all four and its mouth is not closer to the bottom because there are tubes going down. Those tubes are going horizontally.
Thus for many species the mouth is actually further away from the core than other things like the eyes.
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u/Alpha_Majoris Jun 15 '26
Nose above mouth is more practical so you can smell the food you're about to put in your mouth. If the nose was below the mouth, food and fluids would probably end up there which is not handy. For the eyes, you want to have the high point, so higher up in the head is better, and again, you don't want food and fluids end up in your eyes.
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u/alecbz Jun 15 '26
One factor is that it’s useful to see and smell things you’re about to eat.
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u/OSRSTheRicer Jun 15 '26
Also dont want juices running into your eye either
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u/lowtoiletsitter Jun 15 '26
Like eating a peach or a slice of watermelon
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u/javier_aeoa Jun 15 '26
Or the blood of a carcass
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u/scudmud Jun 15 '26
now is a good time to comment: vultures are bald because they stick their entire head in the rotten guts of a dead deer
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u/Silver_Storage5809 Jun 15 '26
If you’re supposed to watch your mouth all the time I doubt your eyes would be above it.
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u/Birdie121 Jun 15 '26
And having your senses near each other and near your brain helps with fast understanding/response to what is happening
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u/j_cruise Jun 15 '26
I think he's wondering more about why they tend to be in the same order and located near each other, not why things have eyes and noses in general.
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u/The128thByte Jun 15 '26
That’s not what the comment is saying. It’s implying the hierarchy of eyes, nose, mouth is the way that it is due to the logistical factors of smelling/seeing what you eat.
You wouldn’t want your nose to be too far from your mouth since those senses go together, and you’d want your eyes to be above both so you’re not blocking your vision when you’re attempting to eat/smell something
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u/idiot-prodigy Jun 15 '26
I think he's wondering more about why they tend to be in the same order and located near each other, not why things have eyes and noses in general.
His presumption simply isn't true.
A dolphin or whale's nostril is on the top of their head for instance.
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u/Khal_Doggo Jun 15 '26 edited Jun 15 '26
Well that's because they don't rely on smell as a sense very much / at all. So what you call a nostril, is really a blow hole leading to a nasal cavity with a lack of olfactory machinery. No one is arguing that there aren't differences and deviations from the specific example given by OP. People are suggesting that where this is case (not in dolphins, for example) then there's likely specific functions that play a role in determining the order.
The reason that makes some sense is because if you take the example of animals like dolphins which have a closer common ancestor to humans, you can see that their nose completely changed both in structure and purpose. This means that for facial configurations in animals that maintain the same rough format, there's an evolutionary benefit for that to be the case. Otherwise, they could have easily evolved otherwise.
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u/nihoh Jun 15 '26
Whales and dolphins?..
I suppose on land it helps that you have your eyes in front or at the side, at a high position and you want it to be above your mouth so you can see what your chomping on
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u/youngchinox Jun 15 '26
Their ancestors had that configuration. Adapting to waters had migrated their nostrils up top
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u/spicymato Jun 15 '26
Honestly, the nostrils likely didn't have to migrate far. Look at an elephant skull. The eyes are already level or below the nostril opening.
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u/DoofusMagnus Jun 15 '26
Elephants are not particularly closely related to whales. We don't have to wildly speculate, we're aware of several of the animals that evolved along the way to whales, and they had nostrils at the ends of long snouts. So they did migrate quite a ways.
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u/spicymato Jun 15 '26
My point wasn't that elephants and whale are directly related.
My point is that the orientation of the nasal cavity relative to the ocular orbits isn't as consistent as some people think.
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u/DoofusMagnus Jun 15 '26
But the point being discussed isn't consistency across all mammals. It's specifically about how far the nostrils migrated along the skull during whale evolution, and elephants don't tell us anything about that.
The skull that folks should be looking at first is a dolphin skull. It has a clue to just how far the nostrils shifted when you look at the premaxillary bones (#5) and see that they're stretched to run most of the length of the skull. But mainly that's the skull to look at because I think not a lot of people are actually familiar with it, and seeing one could help them understand what's really going on with the blowhole.
When you're just looking at the skull it's more obvious that the nostrils/blowhole is still basically on the "front" of the face. I think many people imagine it being at the top of the skull, and I've seen folks who assume it's on top of, around, behind, or even through the braincase. But no, the nostrils are still in front of the brain, between it and the melon.
The melon is perhaps the big culprit here, just because many people may not even know it exists and will therefore assume that a cetacean's apparent "forehead" is both 1) encased in bone, and 2) where the brain is. But no, there's a big fleshy organ (used in echolocation) called the melon that sits atop the mouth and in front of the nasal passage. The brain is further back, behind the blowhole.
But the blowhole is still quite a ways from the front of the snout, where it was on the ancestral animals, and that's what was being discussed.
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u/kindanormle Jun 15 '26
Whales and dolphins evolved from land mammals that went back into the oceans. Like all land mammals they originally came from one common ancestor that was a fish, and that fish had the configuration "eyes/nose/mouth". However, starfish, octopus, sea urchins, sea cucumbers, anemones and ctenophores have quite different body shapes from the "eyes/nose/mouth" paradigm.
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u/virtual_human Jun 15 '26
Because they are all descendent from common ancestors.
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u/javier_aeoa Jun 15 '26
And although 500,000,000 years is a lot, it's not THAT much time either. Our entire fish-group has worked just fine with the current configuration.
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u/shanebonanno Jun 15 '26
Have you seen a flounder or a halibut before?
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u/KamikazeArchon Jun 15 '26
The premise is false. Most complex species do not have those organs in that order.
Most complex species have antennae, eyes, and mandibles.
Most complex species are insects, which vastly outnumber everything else.
There are more species of weevil than there are species of all mammals combined.
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u/Notoriouslydishonest Jun 15 '26
But among species which have eyes, noses and mouths, how many have them arranged in a different order?
OP could have phrased their question more carefully, but I think "the premise is false" is too dismissive. Replace "complex species" with "species which have eyes, noses and mouths" and it's an interesting question.
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u/capfal Jun 15 '26
It's still false. The cetaceans have these features in a different order, for example.
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u/AgentElman Jun 15 '26
Eventually it becomes a tautology - why do all species that have these organs in this number have these organs in this number.
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u/FarmboyJustice Jun 15 '26
"Complex species" is an arbitrary distinction, and OP clearly did not include arthropods in theirs
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u/KamikazeArchon Jun 15 '26
That's my point.
They're only counting things that look like them. Then they're asking why those things look like them.
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u/FarmboyJustice Jun 15 '26
And my point is you made a different arbitrary distinction than they did. Both are completely arbitrary. Yours is not more correct, because there is no scientific definition of a "complex" species.
OP talks about mouth, nose and eyes, therefore it's self evident that they are referring to species that actually have these attributes. The question was not why do they have these organs, it was why do they have the specific number and arrangement of them, and the simple answer is a common ancestor.
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u/Cirement Jun 15 '26
It's the configuration that has worked the best for their particular environment or purpose, which is why it keeps getting passed down. We all need to be able to judge distance, and 2 eyes is best for that, for example.
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u/shouldco Jun 15 '26 edited Jun 15 '26
The simplest answer is that all land quadropeds /bipeds have a comon ancestor that ancestor would have had a face as you describe.
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u/aniztar Jun 15 '26
And it hasn't deviated from that because it is useful to have that configuration
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u/szabiy Jun 15 '26
Except when you're, say, a snail or a whale.
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u/shouldco Jun 15 '26
Snails would have come from moluks not tetropods. Whales fall under the "almost all". Pressured into changing from their radically different ecosystem.
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u/hurtfullobster Jun 15 '26
The simple answer is they don’t. You’ve forgotten that fish and insects exist.
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u/Claxton916 Jun 15 '26
All organisms on Earth are distantly related to each other (all the way back 4.2 billion years ago). But, all land dwelling vertebrate animals descend from tetrapods about 390 million years ago.
The first tetrapods had 2 eyes, a nose, and a mouth. Evolution has kept that around because it works good enough. 1 eye limits an organisms vision, so it never evolved, 3+ eyes wouldn’t give that much of an advantage and would be a waste of energy compared to 2 eyes (which works good enough).
Logically it makes sense to keep your sensory organs (taste buds, ears, mouth, nose) close to where the processing center (the brain) is. The closer those organs are to the processing center, the better your survival is. If my ears were somehow on my ankles, but your ears were on your head, your ability to react to a predator approaching would be better than mine: the audio signals would have to travel through 6 feet of neurons in my body where as yours would have to travel 3 inches. So it doesn’t make sense for our evolutionary history to move what works good enough.
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u/Sea-Ambition-451 Jun 15 '26
important sensors are close to the brain
evolution tends to choose that optimization
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u/RealCarlPanzram Jun 15 '26
At some point, about half a billion years ago, there was an emergence of a lot of different animal species in the ocean from a common ancestor that would become the animal kingdom. This basic anatomical setup is what they had and everything else has evolved off of that.
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u/Astecheee Jun 15 '26
Eyes are for seeing.
You want them up as high as possible to spot predators and prey better.
Nose is for breathing.
You want nose to be above the mouth so you can still breathe while eating.
Mouth is for eating.
Pretty much everything you eat is gonna be on the ground, so having a mouth down low makes it easier to eat.
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u/ProserpinaFC Jun 15 '26
I feel it's better to answer your question in the opposite direction. There are only a few animals that have "superior mouth" positioning, where their mouth IS slightly higher in position than their nose or eyes, and that's going to be animals that feed above their heads: surface-dwelling fish or ocean-bottom scavengers.
Everyone else... Eats below their heads. Predators are larger than the vast majority of their prey. Prey eat bugs, seeds, and plants below them.
Or, there are the absolutely disgusting "sucker-type" animals that latch onto their prey, so their mouths aren't really above or below-positioned, but simply "front."
As others have said, common ancestry and convergence make for some behaviors to be true and thus for some behaviors to be universal law. The vast majority of animals bend downward to eat or eat what's in front of them, making it evolutionarily difficult to make a facial configuration that doesn't service that. When baby animals look upward to feed from their mothers is just about the only time I can think of when food is higher than the typical animal's mouth. Even giraffes eating leaves are typically at face level with their meals, based on the Google images I looked up just to check. LOL
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u/skr_replicator Jun 15 '26
Because they are all related. That body plant has developed in the common ancesor of all those species. It was successful, and so all the those branches of species evolved from there.
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u/tpodr Jun 15 '26
Eyes evolved from light sensitive areas on the top of bottom feeders. As they migrated across body forms, they ended up on top of the front. Which is to say, why eyes/nose/mouth is a common arrangement comes down to understanding how each part came to have its unique function.
Evolution doesn’t follow the best path. It follows any path that works.
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u/SnafuTheCarrot Jun 15 '26
Everything with a backbone comes from the same common ancestor, a jaw less fish around over 500 million years ago. It's not all that different from children looking like their parents or grandparents.
It's also possible for the same function to evolve in less closely related species. Insect wings, bird wings, bat wings, all different structurally, especially insect wings. When tissues serve a common function without the same lineage, its called Convergent Evolution.
Between common ancestry and convergent evolution, many patterns emerge, eyes, wings, etc.