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The story is as follows, a group of people (around 2000 from all around the world) leaves earth and lives on a different (earth like) planet for ca 6000 (earth) years. The planet is about 0.8 times the size of earth and has a similar gravity in the range of 0,8G to 1,2G. The planet has seasons, a year is about 4/5 of an earth year. The land masses on the planet are more fragmented than on earth, so there are 15 smaller continents and the distances between them are much smaller than those on earth. The people live in relative comfort, very few places have extreme temperature (like the poles and a couple of deserts) but the general climate is overall slightly colder than on earth. Loads of rain and fertile land but with obviously very different plants and animals. The planet is densely populated. ( <- I hope this improved my question, it is my first time posting)

How different would they be in appearance to earthlings? Would it be possible for them to have developed a hair/eye color that doesn't exist on earth, or is the time frame too short? and if it were possible could it have become a majority trait?

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  • $\begingroup$ Welcome to the Worldbuilding SE ! I am afraid that your question is too broad for an objective answer. Try editing it by adding more characteristics of your "Earth-like" planet. How similar it is from Earth ? In terms of gravity ? Atmosphere ? Stellar system ? Seasons ? ... $\endgroup$ – EngelOfChipolata Apr 10 '17 at 7:42
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    $\begingroup$ Welcome! The sentence "How different would they be [...]?" is indeed a bit broad. Other than that this looks like it could be answerable, if we take "earth-like" to mean the planet basically is earth. Are you interested in a specific eye or hair color? If so you should state your goal like "I would like to have my humans develop natural occuring pink hair" or something like that, though the question would then be more like "What differences does my planet need for this?". Please try to clarify in which ways your planet differs from ours. $\endgroup$ – Sec SE - clear Monica's name Apr 10 '17 at 7:59
  • $\begingroup$ @EngelOfChipolata I made a couple of changes, I hope that clears it up a bit $\endgroup$ – Anansi Apr 10 '17 at 8:06
  • $\begingroup$ @Secespitus I am not really fixated on a specific trait, I just need them to look very distinctly different from humans on earth. I would like them so be similar enough to recognize us as the same species, but at the same time have enough visual cues to make sure that they are easily (and from far away) distinguishable. My main concern is that 6000 years might not be enough for such strong traits to develop. $\endgroup$ – Anansi Apr 10 '17 at 8:12
  • $\begingroup$ "Could it have become a majority trait?" Most likely no, it wouldn't, unless the population was founded by a small number of people some of which already had that trait. See founder effect. Modern man has inhabited good old Earth for about 40 or 50 thousand years, and yet we do not look all alike. There are 1.5 billion Chinese, 1 billion Indians, 1 billion Africans, 1 billion European-looking people... and no clear majority. $\endgroup$ – AlexP Apr 10 '17 at 18:05
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They may appear to be a distinct people. This happens on a scale of a thousand years. Whether or not that’s seen as a “race” depends on culture. Norwegian and Scottish and German look different and they’re all Caucasian.

Hair or eye color: it could happen. When it does, it is sudden.

Look at the “founder effect” for a major source of the colony’s shared traits. As for whether a new trait takes over to become the norm, look at the time to shared common ancestor. This depends on the colony’s size and how isolated the subgroups may be. But I’ve seen figures of mere hundreds of years, so 6000 is no problem.

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  • $\begingroup$ I will look into that. $\endgroup$ – Anansi Apr 10 '17 at 8:15
  • $\begingroup$ @Anansi I added some links for you. $\endgroup$ – JDługosz Apr 10 '17 at 8:30
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With 6000 years without being on Earth, physical changes could be due to cultural or political changes as well as purely natural changes that JDługosz stated.

For example, Isaac Asimov, in the robot series, make a clear physical distinction between people stayed on Earth and those who live on other planets (Spacers). Even if terraforming made those other worlds similar to Earth, political and cultural differences are huge.

Spacers are tall, handsome, and they live 400 years. They have cleared all diseases and germs, and they are massively using birth engineering to select best criteria for babies.

Thus, I think if a "natural" cause does not satisfy the changes you want, you can easily base them on cultural differences (birth engineering, food, religion, ...).

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  • $\begingroup$ The genetical engineering (or birth engineering) is a great point, but then it would be easily attainable to the humans left on earth (once the two populations meet) and that would not work for the story. $\endgroup$ – Anansi Apr 10 '17 at 11:18
  • $\begingroup$ @Anansi Not necessarily, again, in Asimov stories, Spacer and people on Earth have some contact, but for cultural and political reasons, genetic engineering, robots and others technologies are used only on Spacer worlds. In fact, those who firstly left Earth, did not agree any more with the decisions took on Earth, and thus, left the Earth to build worlds according to their way of thinking. $\endgroup$ – EngelOfChipolata Apr 10 '17 at 13:18
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You state that the colonists were from all around earth. It is not unreasonable to assume that there will have been a thorough mixing of genes in the descendants of those people, leading to a distinct look unlike anything we have here on earth today.

While we have, of course, persons with mixed ancestry, it is rather uncommon to have more than two or three racial characteristics combined in one person, in any case, those are single individuals and not a whole people.

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  • $\begingroup$ During a decade what you are stating is true, but over 6000 years ! With radiation casted during the trip, mutation will occur certainly. $\endgroup$ – EngelOfChipolata Apr 10 '17 at 9:33
  • $\begingroup$ @ths I don't think that just the 'mixing' would be enough to create a distinctive 'look'. $\endgroup$ – Anansi Apr 10 '17 at 11:12
  • $\begingroup$ @EngelOfChipolata I didn't even think about the radiation, but maybe the trip itself can actually be part of the changes. ;) $\endgroup$ – Anansi Apr 10 '17 at 11:13
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One thing you might think about is environmental influences on appearance. These might be much easier to make uniform (or majority) over a large population than genetic changes, given the restriction on how quickly genetic changes might spread without some outside circumstance like genetic engineering or founder effect. EngelOfChipolata touched on the possibility a bit when mentioning foods, but there are more possibilities.

So, how would these environmental changes work? Depends on what you want done, I can think of a lot of possibilities, plus the possibility of several stacking up to form a pretty striking difference fairly quickly.

  • One option would be simple bio-accumulation.

Something in the food, the water, or the air that causes changes. It could be as simple as the accumulation of some dye in the skin or hair or, heck, teeth or something - basically, the carnation-in-colored-water trick, only for humans. Or more seriously, it could be something like accumulation of minerals or other compounds - I recall an accumulation of silver can leave the skin a blue-grey color; and though it may be toxic in high dosages, it may be tolerable in lower chronic doses (and you people might more easily evolve higher internal tolerance or develop better treatment for the deleterious side effects without bothering about mere skin color). Of course, other minerals or compounds may cause different changes, some with greater side effects, others with few.

  • Another option could be physical conditions

Something like slightly lower gravity could make the body development tend towards noticeably tall and thin. Something like varying levels of sunlight or the radiation therein could encourage very pale, or very dark skin - since the basic mechanism for that selection is already evolved. Some quality of air composition or pressure might gently influence nose shapes (due to the benefits of altered nasal air passages), or lung development (possibly altering torso proportions slightly). These kinds of adaptions can be seen in populations adapted to high altitudes, for example. Nutrition or lifestyle-based physical demands might shape height, weight, or muscular development.

Of course, any of these adaptions are likely to be slight - genetics play a pretty big role in development, so environmental adaptions that are not genetic would, I think, average over a population (especially a widely dispersed one across many different climates) as slight modifications or tendencies rather than extreme ones - not to mention ones that have a decent chance of being already seen in one population or another on earth at some point in history. But, stacking a few of them together might make or help make a striking overall appearance shift.

  • Another possibility might be a disease.

Something like vitilego might do it - it causes patches of skin without melanin (the most common pigment) giving a patchwork appearance with dark and pale skin. Or something like, in one of Murray Leinster's Med Ship stories there was a mild disease, which had piggybacked on a worse one, that had the primary symptom of causing blue patches on the skin. If the disease is very mild, people might not think about curing it, they might not even notice beyond the color (especially if they have other, immediate concerns at the time it appeared). And some diseases are inheritable.

Otherwise, something like a virus (that meddles with DNA) might have a marker it leaves behind, that can as a side effect alter the DNA regulating skin or eye color in a way that makes it...produce unusual pigmentation. So anyone who caught the disease might have the color change, and any of their descendants might have it too, depending on the modification on question. That will spread a lot faster than inheritable mutation alone.

Or, even more simply, some otherwise harmless bacteria which tends to colonize human skin develops some property altering color - depending on the bacteria and/or its environmental conditions, it might result in coloration alterations in patches, in specific areas, or over the whole skin.

And, of course, some environmental effects can be a side effect of certain practices

Foods or other optional applications may have an effect on appearance, even long lasting ones, but they would likely not be consistent over a large population or in different climates without some extra encouragement - like ritual or religious use, medicinal use, or other practical uses like cleaning, pest control, or the like that makes that specific product ubiquitous.

So, maybe there's an oil that makes one's skin turn a bit more orange with every application, but people use it anyway because of health benefits (like sunscreen), or pest control (like citronella). Maybe there's a beverage that makes the eyes, whites included, turn a bit greener the more it is consumed - but people keep drinking because of religious ritual or just culturally accepted practice (ie, equivalent of alcohol - it does show up just about everywhere). Maybe there's a plant used for cleaning (like we use lemon in so many cleaning products, or like we use chlorine in cleaning the water) that has a pigment like henna does, making everything dye redder and redder as long is its used, and lasts long enough for the effects to be overlapping.

The original colonists might not have cared about side effects next to the benefits - and later generations might not have known, especially if it is ubiquitously used. Or it might have turned into culturally accepted practice, interwoven in beauty standards or something.

These effects will all be limited

That is, some will occur only as long as the environmental influence is present, and may fade if the person leaves the planet, stops using the product, or technology finds a non-altering equivalent. Others may last a lifetime, since the change once made cannot be undone, but kids born without the influence will lack the trait. And still others may take generations to fade (if selected away from or genetically diluted), or may never fade at all without interference since the relevant factor remains present - the diseases which alter DNA or which are carried along and contagious or inherited by each generation.

So, you might have a very striking appearance change that slowly fades as someone leaves the planet and/or its customs, and develops as someone moves there and picks the customs (and conditions) up. You might have kids physically different from their parents depending on the environment in which they're born. You may have an overall, very dramatic appearance change consisting of several elements, some of which are very immediate and some of which are more long lasting, some of which are present more in some cultures or areas than others. You may have evidence of historical influences on the changes - including manufacturing, medicinal knowledge, and fashion - as what is known about these factors changes with technology and common knowledge.

And all of them will be varied by individuals, by climate or populations, and by the presence or absence of other environmental or cultural factors. Planet-wide, with a diverse population and many different climates, ecosystems, and lifestyles, you would likely find a lot of variation in whatever traits you choose - which can still be quite noticeable, it is just more likely to be slight tendencies, or frequency or populations with any given trait, or more visible in some people and less in others (possibly with social cues tied to the less and the more).

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"developed a hair/eye color that doesn't exist on earth"
Most likely no. Even if "orange" eyes would be an advantage - this won't happen so simply. It has to be a major advantage, otherwise orange eyes will disappear immediately.
If tall people have an advantage and short people won't have that many children the population will become taller and taller.
On "earth like planet" there is no reason to differ so much.

Only "very different plants and animals" - those who have problem with food, have lower chance to survive - some different stomach could appear, tongue, teeth, but you want to increase the time frame to 10000 years to have minor changes on whole population, and to 50000 (or more) to have major changes like the number of teeth. (Even if I think the human system is pretty perfect and does not change on different continents on earth that much)

There is also a small chance that humans will create two species (but most likely they will not) - more info

Also the available options increase with the size of population.

similar question/answer
Some sources: Evolution, Natural selection

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    $\begingroup$ Why didn’t blue eyes disappear immediatly? Ditto with red hair. There’s no advantage, some disadvantage, happened in that time scale, and stuck around to dominate some populations. $\endgroup$ – JDługosz Apr 10 '17 at 8:32
  • $\begingroup$ Well, just because something is not advantageous doesn't mean it will disappear due to evolution. That's not how evolution works. Evolution lets things disappear that are disadvantageous in an impactful way. If for example the orange eyes are genetically dominant, it will spread. $\endgroup$ – Doomed Mind Apr 10 '17 at 8:33
  • $\begingroup$ @JDługosz Well if you downvote me because it is not advantage - well: en.wikipedia.org/wiki/Sexual_selection also: "The non-tanning skin associated with red hair may have been advantageous in far-northern climates where sunlight is scarce" And yes, orange eyes will disappear if it is not advantage - even if it is "stronger gen", there will be only 50% chance per child to give those genes. If you have stagnant population, it will disappear on 99.99% in 5000 years. (read: "The planet is densely populated" ! , unlike earth where blue eyes gens has been spread and population grew) $\endgroup$ – Jan Ivan Apr 10 '17 at 8:56
  • $\begingroup$ ... maybe still someone will has orange eyes, but not more than few ppl. $\endgroup$ – Jan Ivan Apr 10 '17 at 9:04
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    $\begingroup$ “may be advantageous”—in some areas— is a far cry from “must be a major advantage” and the trait has not disappeared immediatly in other areas. Blue eyes are a disadvantage in higher altitude or sunny areas — any advantage someone might find will be slight and contraversial, and doesn’t apply to all areas. $\endgroup$ – JDługosz Apr 10 '17 at 9:05

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