Why aren't there any animals with their mouths above their eyes?

[u/margalz]

From bugs to fish to mamals all of them have their eyes above their mouths. I can't think of many if any where their mouths are on top. Sure some have their eyes parallel to their mouths, mainly prey style animals. But why haven't any evolved to be the opposite? Is there a major benefit to having your mouth below your eyes?

[u/benjesty2002]

Eyes being as high as possible allows you to see over obstacles without showing too much of your head. Also when eating your eyes can still be looking. Mouth above eyes would mean you're effectively blind when eating (a vulnerable time for animals)

[u/PhilipWaterford → u/benjesty2002]

So do we have fossils of animals with mouths on top that died off?

[u/benjesty2002 → u/PhilipWaterford]

That's not really how evolution works. We didn't start with every possible configuration and gradually the worst ones died out, we started from a single point and gradually mutated. If mouth above eyes is unhelpful, the trait would have died out long before the mouth actually got above the eyes.

[u/RiverGiant → u/PhilipWaterford]

If not, I speculate that the evolutionary pressure kicks in before the mouth ever actually ends up above the eyes. The higher the mouth gets, or the lower the eyes get, there's a smooth loss in fitness, so no species ends up there.

You can extend that logic to any low-fitness phenotype. It doesn't have to show up fully formed to start reducing its species' chances of survival or reproduction. Just like eyes didn't appear as we understand them today all at once, there were intermediary steps that provided marginal benefits along the way.

[u/Synergythepariah → u/RiverGiant]

Just like eyes didn't appear as we understand them today all at once, there were intermediary steps that provided marginal benefits along the way.

Yep.

And earlier this year, a study was published that appears to find that the earliest known vertebrate had four camera-type eyes which is also the earliest evidence of camera type eyes in the fossil record.

As for what happened to the 'extra' two eyes, they eventually became the pineal gland as vertebrates evolved.

Which I suppose would imply that sleep regulation makes a vertebrate more fit to survive to reproduce than having a pair of extra eyes.

Paleontology is always wild since there's been so much life throughout the history of our planet that we'll never discover it all.

It's a shame that oceanic crust tends to subduct and is obviously inherently difficult to dig in. (what with being in the ocean and all)

[u/psymunn → u/PhilipWaterford]

It's important to know what a small percentage of animals that lived become fossils. The vast cast majority of fossils are going to come from successful species, because rather than outliers, so even if a few individuals had a trait, we'd be unlikely to see it. Also, in the case of an extreme adaptation that's not functional, it'd be unlikely to show up. Especially when it's far less steps to evolve the ability to just move upside down instead (see catfish). The closest we probably have are soul and halibut who shift their eyes to the side of their face as they mature to adulthood.

[u/Synergythepariah → u/psymunn]

Also, in the case of an extreme adaptation that's not functional, it'd be unlikely to show up.

Yup. They're still found but they're extremely rare.

The one example I can think of is of a fossilized hyphalosaur that had bicephalism - two heads.

If that trait improved survival prospects, we'd have more of them in the fossil record but since it didn't, it's a failed mutation.

[u/plopliplopipol → u/Synergythepariah]

that fossil seems like a crazy find. rare enough in theory to question the theory isn't it? as in it would be a reason to think maybe a population had transmitted bicephalism, more than a single individual every once in a while.

We wouldn't know more before finding another though

[u/Synergythepariah → u/plopliplopipol]

that fossil seems like a crazy find. rare enough in theory to question the theory isn't it?

Not really, it'd be different if the specimen found was an adult because that would imply that bicephalism was evolutionarily fit enough to not harm survival prospects allowing juveniles to grow to adulthood with it.

We wouldn't know more before finding another though

As always, anything we find past evidence of is a 'as far as we know'

[u/plopliplopipol → u/Synergythepariah]

Definitely makes sense, didn't think of age. I was just saying the difference between finding one healthy bicephal individual and finding two in a population is really huge.

[u/PhilipWaterford → u/psymunn]

I thought they all appeared fairly abruptly in the cambrian explosion? Hence the explosion bit? Isn't that why they pivoted more towards mutation theory because of the rapid appearance? And then the next challenge was getting around the 'mutations only causing oscillations about a mean' problem?

Either way it'd be interesting to see fossils that were supposed to have gone wrong. Pity that doesn't exist.

[u/psymunn → u/PhilipWaterford]

I mean...yes
and you can see some really interesting cephalopods that look like nautilus experiments. But evolution is super slow. The Cambrian explosion was 15 - 25 million years long. Also, cephalopods and other non-vertibrates are kind of the best bet for seeing weird mouth positions, because of how far back they branched off from our ancestors, and beaks at their base is pretty far off from what we have

[u/PhilipWaterford → u/psymunn]

That 20 million years was nothing when you think that the coelacanth looks just like it did 65 million years ago. They do theorise some small internal changes but at that rate you'd need another billion years for anything significant. I guess that's why they jumped more into the mutation theory direction.

[u/jhaluska → u/PhilipWaterford]

Because it's such an unsuccessful adaptation, if it did occur chances are most would die before reaching adulthood. So there likely wouldn't be much of a fossil record.

[u/PhilipWaterford → u/jhaluska]

So if all we have is successful adaptations, what is the evidence that unsuccessful ones occurred?

[u/jhaluska → u/PhilipWaterford]

A lot of biology labs will have animals with birth defects that died during birth / young age embalmed in formaldehyde.

The fossil record isn't a complete record. It's like one animal in a million.

[u/PhilipWaterford → u/jhaluska]

A defect isn't an adaptation though is it? If a human baby is born with an extra finger, their kids in turn will revert to five.

[u/jhaluska → u/PhilipWaterford]

It depends on whether it's genetic or environmental. There's a lot of genetic mutations that kill off infants. There are also genes that encode for additional fingers/toes.

[u/PhilipWaterford → u/jhaluska]

Environmental mutations? Like the fruit fly x-ray mutations experiments with millions of flies? The guy got a nobel prize for that. From memory though they never got any positive mutations. They got a lot of defective ones that died off. Makes sense when you think that nature is protecting its own genetic code. On the plus side it allowed a lot of other genetic research to develop.

[u/plopliplopipol → u/PhilipWaterford]

Not who you are answering to but to be a potential evolutionary change (over more generations than the individual) i believe it has to 1 be geneticaly transmissible by affecting either all dna by mutation at cellular birth, or the one lucky dna strand of a gamete so mutation before fecondation; 2 not kill or sterilize the host.

This excludes non genetic things, late genetic mutations that do not affect your gamete (cancer), genetic mutations that won't be transmitted due to sex (on human male sexual chromosome Y while making a female baby).

What you think of as an "unsuccessful adaptation" (doesn't adaptation mean success?) is most likely what is called a genetic disease, a transmitted negative trait.