Did Eyelids Evolve?

fixed.

unless you're a starfish. or any other radially symmetric animal (like corals), in which even/odd limb numbering is somewhat randomly evo-devo. i think you even find the occasional odd-numbered cephalopod. now, what you're getting basically confused about is that all land animals evolved from tetrapods, animals with four feet. this is by far not majority of life on the planet. and if we start counting tails as limbs (especially prehensile ones) you'll find that a lot of land animals actually have an odd number of limbs. ...because, you know, that fifth limb actually aids a lot in locomotion and balance. generally more that the front two -- just ask t. rex.i think you'll also find that faily advanced animals (such as yourself) lack duplications for various rather important internal organs, such as the heart and brain.

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i think you'll find that the earliest primitive photo-sensitive organs were all sub-dermal. ie: they started out covered by skin. but we're talking flatworms here, and long, long before the complex eyeball.of course, the path from something like that to the human occular system comprises several hundred million years of evolutionary history, and it's rather convoluted. the simplest answer to your initial quandry is "we got our eyelids from our ancestors, who also had eyelids."indeed, you will have to go pretty far down the evolutionary scale to find an animal anywhere near our path that does not have eyelids. to the fish, i believe. and even then, some of them (such as sharks) have similar structures, as has already been pointed out. and sharks are very primitive fish.

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...well, here i am caring about accuracy again lol.

my brain is organic, and fully intact, thank you. maybe your brain is missing something, or cannot process something, but mine's just fine. as you'll notice, i said "brain" in singular. i have one brain -- this was one example of an organ i brought up above that is not duplicated. it's also example of how an organ can gradually duplicate. primitve amniote brains have only a single hemisphere. my brain has two. i would call that "transitional." again, this is simply your failure to see something. evidence is there. if you want to understand the human reproductive system, first look at the primitive amniotic ones, followed by the monotremes (like the platypus) and marsupials and then placental mammals. you'll see a rather clear progression. like so: define ID. because now you're pushing its definition so far as to be vacuous, and easily eliminated by ockham's razor as an extraneous variable. if the observed mutatations are enough, what do we need ID for, exactly? take a biology class or five. yes. afterall, it's more or less identical to the reproductive system of every other primate. which is almost identical to every other placental mammal. which is only slightly modified from marsupials. which is only slightly modified from monotremes. which is only slightly modified from other amniotes (like reptiles)... etc. you get the picture.

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