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My question is this: Is it plausible for most aliens to have eyes that are superficially similar to human eyes, given the context below?

The context is this:

  • I'm defining "convergent" as being any evolved light-sensing organ that's similar to a mammalian eye. It can have differences (e.g. designed to detect a different spectrum of light), but should be at least superficially similar in design.
  • An ancestral species deliberately seeded primitive microbes that evolved into the majority of sentient species, including humans, so there's no need to account for 'unconventional' forms of life. The aliens can be generalized to drink water, inhale oxygen, exhale carbon dioxide, etc.
  • Certain environmental conditions can make evolving organs to detect and differentiate light waves less probable, for instance intelligent life that evolved near the ocean floor of a world that receives most of its heat from tidal heating, and is far from its sun, but I'm taking it as a given that most of the seeded worlds were those in their star's normal zone of habitability, and thus the intelligent species evolved on them have evolved some kind of light-sensing organ.
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    $\begingroup$ It has already happened, the cephalopod eye is functionally and superficially similar to vertebrate eyes, but evolved independently. $\endgroup$ – Lex Dec 8 '17 at 16:38
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    $\begingroup$ Please note that you need one empty line between a normal paragraph and a markdown list. Otherwise the list items will only be displayed as one big paragraph that is very hard to read, even if the raw text looks like it should have a list. And you need two spaces at the end of a line before a linebreak to make a soft linebreak in the resulting layout. (Markdown can be weird at first, but the list at the top of the box when writing an answer or question on the desktop version gives many useful tips and has even advanced help texts - very helpful in the long run) $\endgroup$ – Secespitus Dec 8 '17 at 18:03
  • $\begingroup$ Anglerfish would have a hard time catching anything if all their prey didn't have photoreceptors. Switch to ultraviolet and it turns into a freaking circus down there. $\endgroup$ – Mazura Dec 9 '17 at 2:12
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Yes. Squids and humans make a good example here...actually there are a huge number of creatures that arrived at the same eye despite diverging long ago.

https://www.popsci.com/article/science/how-humans-and-squid-evolved-have-same-eyes

The most important of master control genes implicated in making eyes is called Pax6. The ancestral Pax6 gene probably orchestrated the formation of a very simple eye–merely a collection of light-sensing cells working together to inform a primitive organism of when it was out in the open versus in the dark, or in the shade. Today the legacy of that early Pax6 gene lives on in an incredible diversity of organisms, from birds and bees, to shellfish and whales, from squid to you and me. This means the Pax6 gene predates the evolutionary diversification of these lineages–during the Cambrian period, some 500 million years ago.

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Cephalopods have a camera eye with the same features as the vertebrate camera eye. Importantly, the cephalopod camera eye arose completely independently from ours. The last common ancestor of cephalopods and vertebrates existed more than 500 million years ago. Pax6 RNA splicing in cepahlopods is a wonderful demonstration of how evolution fashions equivalent solutions via entirely different routes. Using analogous structures, evolution can provide remarkable innovations.

We can see several instances on Earth where the eye evolved independently to around the same form...almost to the point where I would suggest the eye could be a semi-universal evolution.

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  • $\begingroup$ Though IIRC the cephalopod eye is actually better than the mammalian one in some ways: cephalove.blogspot.com/2010/05/octopus-visual-system.html $\endgroup$ – jamesqf Dec 8 '17 at 20:58
  • $\begingroup$ That's because our eye evolved in sea creatures and is not optimal for above-water use because light's refraction when, above water, travels from outside air to a liquid medium in the eye. That's not an issue in sea creatures where light goes from water->water. $\endgroup$ – durandal Dec 9 '17 at 1:03
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    $\begingroup$ @durandal: No, there are other reasons that don't involve air/water vs water/water. For instance, the cephalopod eye's camera-like focus - moving the lens instead of changing its shape - means we wouldn't lose the ability to focus closely as the lens hardens with age. The mammalian eye requiring light to pass through the retina to reach photosensors limits acuity and reduces potential night vision capability.,, $\endgroup$ – jamesqf Dec 9 '17 at 3:36
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No. While there are examples of convergent evolution of eyes on this planet, as Twelfth notes, there are more examples of (superficially) dissimilar eyes. Think of compound insect eyes that are very different from our eyes, to spiders that have lots of eyes and even the squid eye that is similar but superficially looks different. Even no-eye extraterrestrial intelligence is possible (though you exclude that in your question).

Even if you assume all earth-eyes to be the same there is also evolution to consider. Evolution works in the way that it favors the individual that any favorable genetic mutation, wich are random, but not necessarily the most optimised mutation possible. Any improvement is good and functions as the genetic base for further evolution. That results in a very wide tree of evolutionary directions, as witnessed by the enormous size of the tree of life.

The ramdomness of mutations results in there being a statistially good chance that eyes would be dissimilar to our own eyes even under the exact same evolutionary circumstances, due to the fact that the first mutation leading up to the eye would be different.

Then there are the conditions themselves that are vastly different throughout the universe. The planets being in the habitable zone does not mean they have similar conditions.

One could think of very bright stars maybe causing subdermal eyes or eyes with very dark filters, or dim stars like red dwarves causing aliens to see in infrared with their tongues like snakes more or less do.

Aliens could be seeing with echolocations much like bats and dolphins do, and any evolutionary eyes could have receeded like the tails in our ancestors.

Aliens could have been evolved into cyborgs like the Borg in Star Trek and be wearing camera implants, or have altered themselves genetically and are now sprouting whatever works best for them.

There are too many possibilities for only one solution to be the preferred outcome. Our eye, eventhough widely spread in our animal kingdom, is pretty unique in the universe.

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