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I am setting a story on a tidally locked planet under a red star (original, I know).

So, this is the scenario: imagine a human being born on this planet in a mostly safe habitable zone mostly safe from harmful radiation for reasons. Humans are adapted to live under our yellow star, so in a twilight world under a red star giving off red light affect, how would it affect a human's skin and eyes growing up in this environment into adulthood?

I would assume pale skin and impaired eyesight. I jumped to that conclusion because that's how we imagine subterranean humanoids living in darkness develop in stories. But there is light in this place, it's just a different color and much weaker and probably not the right type of light. Would someone have to stock up on vitamin D supplements and artificial UVB lights before heading to this system? Would growing up in half-darkness actually improve your sight somehow?

And if you took this same human who grew up in this environment and put them here under our sun, do they just go blind and immediately get sunburnt?

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    $\begingroup$ Hello @GorillaGavy, welcome to Worldbuilding. When you get a moment, please read through the following four pages to better understand our expectations and limits: tour, help center, help center and How to Ask. Please edit your post to include the following details: How many generations of humans have been born to this world? Is the world primarily agricultural (most people are frequently outside) or industrial (most people are frequently inside)? On a scale of 1 (low)-to-10 (high), how much do people want to avoid mutation? $\endgroup$ Commented Mar 23 at 1:16
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    $\begingroup$ These people must have interstellar travel to get there. Do they not have artificial lights? What about the rest of their environment - what do they breathe, eat? The light seems like the easiest problem to solve. $\endgroup$ Commented Mar 23 at 19:34

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You should try to find a copy of the book Cycles of Fire https://x.com/retroscifiart/status/1792701395881394249s by William K Hartmann. It contained alot of paintings, some of them depicting worlds with red giants.

enter image description here

It is awesome! I distinctly remember there beeing a scenen, showing what our eyes would see- and what a native would see. The brain has a tendency to adapt and shift whatever is there into distinct spectres. Very dark red, becomes over time - the new blue.

The first thing is - it is dark and red. So a eternal gloom, like a late sunset. This would shift the pallet of what people percieve as colours outside very much. There also would be a inside pallet- used with artificial light.

People would develop very bright, almost translucent skin to get vitamin D. There pupils would be very open to catch the light - and if they have genetic engineering they would try to get something like cat eyes, reflecting light back a 2nd time through the sensor skin inside the eyes. If light is hard to come by eyes either vannish or get larger.

You could also genetically engineer something like the heatvision snakes have. https://en.wikipedia.org/wiki/Infrared_sensing_in_snakes Basically a cavern, dottet with little infrared pits, capable to take it all in.

People would build buildings different to catch more natural light - or even filter it up. So great mirrors, to focus the waning candle down into these buildings. Dark surfaces would be seen as a sort of light trap, which is bad luck or luxury to eat the little coming from the sky.

There would be a million names for the way red can shift, sparkle and have different textures. Things coloured red, would become the colour of the god(s).

Fire, burnin brighter then the sun, would be considered something spectacular and luxurious. Dim as the sun is, photosynthesis would be more ineffective. Such a world could feed fewer, would have fiercer competition and stronger defenses, as well as creatures capable to sleep away time.

enter image description here

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It's just a different light, humans live where the sun doesn't rise for months at a time. We just make our own light. We do the same thing at night time or in any area we can't see properly otherwise.

Humans adapt their surroundings to their needs. It's one of our defining traits. So if humans went somewhere where the light was sub-optimal for our eyes, they'd just create their own light. We'd adapt culturally, not physically. In the same way we're not physically adapted to go spacefaring in the first place.

If sunburn was an issue, they'd wear protection or shade their areas. Same with vitamins and anything else. We'd find out whats necessary, prepare properly, and carry on as usual. Not adapt to an alien environment. By the time we naturally adapted, we'd no longer be human I would think. Evolution is a slow process so we just skip it.

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    $\begingroup$ Agreed, it would take hundreds of thousands of years and exposing people to conditions likely to lead to death to people without a favourable mutation for humans to evolve. It's kind of like asking whether people living on space stations would eventually adapt to being able to survive unprotected in a vacuum. The possible exception is for genetic engineering to create the proposed mutations, but a) that's not evolution and b) any society that technologically advanced can more easily create robotics to remove the need for the genetic engineering. $\endgroup$ Commented Mar 23 at 2:02
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    $\begingroup$ To an extent we adapt, but we need sunlight and open space as a child for our eyesight to develop. Otherwise myopia sets in faster and becomes much more severe (see pmc.ncbi.nlm.nih.gov/articles/PMC7607527). I am leaving this as a comment rather than an answer because I have no idea what the light of a red dwarf is like or how human eyes (particularly the eyes of a baby/child) would respond. $\endgroup$ Commented Mar 23 at 22:27
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    $\begingroup$ @JoshRumbut, useful comment anyways. Things like this are awesome material for a more socially critical story, where such children do in fact appear (because they are too poor to afford the good lighting...). $\endgroup$ Commented Mar 24 at 12:54
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Cave-dwelling species lose sight because over thousands of generations, the ones without working eyes are slightly better at surviving and reproducing than those with working eyes. In other words, you need conditions where the organisms and their entire lineages preferentially die out. It's not the light-scarce conditions changing individuals, and that being passed on to their descendants. That's not how evolution works (there are exceptions, but those don't matter here).

Humans don't really do that anymore. We're still affected by evolution, of course, but not like this. If your eyesight is bad, you don't die - you buy glasses. If your skin type is too easily damaged by the Sun, you use sunscreen - if it's too bad at producing vitamin D, you consume supplements. If your hair isn't glossy enough, you use a conditioner.

So overall, you'd only get the same changes you get here on Earth when you stay out of the Sun - less chance of eye damage from UV, and depending on your skin type, somewhat paler skin (but not really different from Temperate winter). And needless to say, none of these changes are inherited - your kids are not going to have skin any paler if you move back to Earth :)

So, how about those stories? Well, people in the past didn't understand evolution - not scientists, not sci-fi writers, certainly not the authors of even older mythologies. Even today, the vast majority of people don't understand evolution at all, even in countries not steeped in millennia old superstitions about how life develops. You shouldn't expect people to get complicated things right just because "we" have known the right answers for a century.

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    $\begingroup$ The textbook example of genes that would kill off all their bearers if not for civilization is Hemophilia. Most individuals in the wild would die before they reached reproductive age. $\endgroup$ Commented Mar 23 at 14:29
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The problem with your description is this.

If humans reached the world your describing? Then they've long surpassed the point where they need to adapt to local environmental conditions. Instead they'd take 'home' with them. Colonist's arriving at the system in question would be bringing the technology they need to survive and prosper. And having done so? Then here are two basic paths they can choose follow either individually or in tandem. (Probably the latter.)

  1. They can use technology to alter local environmental conditions to suit their needs by building artificial biomes above or below the surface including greenhouses & farms, living quarters and factories etc.

  2. They can modify the genes of their children (or other species like food crops) to be better adapted to local environmental conditions in the 'twilight' zone.

  3. They do both i.e. 'tweaking' the genome of their descendants to be able to better tolerate surface conditions on the planet to at least a limited degree while also maintaining near Earth level environmental conditions inside the cities and towns they live and work in when they're not outside for limited but not short periods of time.

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    $\begingroup$ Unless the colony was too small or they crash-landed or some other catastrophe pushed the people back in terms of knowledge and tech. $\endgroup$ Commented Mar 23 at 12:36
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    $\begingroup$ @Ivana The size of the initial colony doesn't really matter. This isn't a group of explorers sailing to a new world, clearing land and building houses by hand. They'd have access to prefabricated parts, robotic construction techniques and nuclear power of one form or another. People could start arriving en mass once the initial construction phase was completed. And if there was some disaster and they lost their tech base? Well humans probably wouldn't survive on a tidally locked red stat probably wouldn't survive in the long term anyway. $\endgroup$ Commented Mar 24 at 2:43
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They changed through genetic engineering, not evolution.

We assume a technology level on par with interstellar space travel. That may not mean FTL travel, but certainly means genetic engineering. IRL, as of 2020's already can make minor modifications to an animal's genome.

Your Red Dwarf settlers are adapted to the atmosphere, light, temperature, and other aspects. They were before they even set foot on the planet.

Imagine a generation ship, going to colonize the Red Dwarf Planet. The people can be slowly modified and selected through generations to better fit living in the Red Dwarf Planet. Their children's eyes and skin change, the body becomes stockier to better live under the high gravity (if any), etc.

A handful or a dozen of generations later, the Red Dwarf Planet people are ready to disembark.

The advantages of adapting the people to live on the exo-planet are many. For one, not needing to use protection suits, and also the reduced risk of disease or disability due to unfavorable living conditions.

Hells, it could even be seen from a humanitarian standpoint, that it would be cruel to send normal Earth people to suffer under the conditions of an exo-planet. If the genetic engineering technology is advanced enough (eugenics), cosmetic modifications, designer babies, can be commonplace. Making your kid with red eyes and green skin to live on the harsh sulphuric atmostphere of CoRoT-24 c could be seen with the same gravitas as getting a tattoo nowadays.

It's a blend of futuristic societal and technological changes.

This is what humanity did in the Warhammer 40k and Dune universes, for example. In WH40k, you have the Ogryns, Squats, Rat-Kins, who where designed for their worlds.

In both universes you have the respective navigator subspecies too, people who changed to better travel in space.

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Would growing up in half-darkness actually improve your sight somehow?

To some degree, yes. Of course, you'd still be far below eyesight evolved/designed for that environment but a "native" will see far better than a fresh arrival.

And if you took this same human who grew up in this environment and put them here under our sun, do they just go blind and immediately get sunburnt?

They'd buy super-dark shades, long-sleeved clothes and SPF 200 sunscreen in the in-orbit duty free shop. Otherwise yes to the sunscreen and no to the going blind (permanently I assume). They'd adapt in a couple of days to that (of course with longer-lasting unease about the brightness).

That requires them not being biologically/genetically modified for your planet, which is what a humanity capable of reaching it definitely would be capable of.

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