Thousands of years ago (approximately 12 000 years), a group of humans (let's call them First Group) reached out and settled in forests with huge trees (Tall trees : atmospheric pressure) which made them "evolve" (I know that the time lapse is too short for talking about evolution) a little bit differently from other human groups (Second Group). Indeed, those forests have a very rich biodiversity and a lot of food everywere, so the main challenge is not really finding food but live with the other species and survive to the very big ones. It's safer, for small species like humans, to live in hights (the branches are very large and huge, so it's easy to walk on them). And they are powerfull enough to preserve there habitat in those forests.

The thing is, it's essential that no interbreeding happens between the two groups until present day (if it's even possible), and I have some trouble figuring this out :

What could be the reason(s) for the First Group to be able to adapt to and live in those forests but not the Second Group ?

Note : if the time lapse is not plausible, changing it is not a problem.

Thanks !

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    $\begingroup$ Any reproductive pressure is still an evolutionary pressure, no matter how short or long that pressure is being applied. People from the first group who didn't take to the trees simply didn't breed for some reason, so the ones who did climb (and their children who were better able to climb) were the ones who survived. -- Perhaps there's a predator that can't climb, or a toxin that is unique to the forest floor. The second group would simply live outside of the forest, with different pressures. $\endgroup$ – Ghedipunk Jan 6 '20 at 17:55
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    $\begingroup$ To have two human populations not interbreed at all they must be geographically separated; if there is no geographical separation, the two populations will interbreed. Humans are very good at interbreeding... Therefore you must have the Arboreal Group on an isolated island somewhere (think, Sentinelese people), or separated by some other kind of geographical barrier. $\endgroup$ – AlexP Jan 6 '20 at 18:10
  • $\begingroup$ @Ghedipunk Oh, I though it would be more complicated than that. So it's possible that the Second Group just never tries to explore/live in the forest in that time lapse ? $\endgroup$ – Emie Jan 6 '20 at 18:57
  • $\begingroup$ As AlexP points out, they need to be separated or they will breed together. (And 12,000 years is enough for some significant changes, but not enough to prevent interbreeding afterwards.) Maybe the second group tries and gets killed, and never learned the trick of climbing to the canopy layer (though this won't last for thousands of years; humans are a determined bunch). Unless there is a physical barrier and certain death for the few who cross that barrier, people will try. Perhaps a good barrier could be a fast moving stream. $\endgroup$ – Ghedipunk Jan 6 '20 at 19:05
  • $\begingroup$ Streams do change their course, too, which could explain why group 2 is suddenly able to visit the forest (which is just as much of a mystery as why they didn't visit the forest in the first place... Why not before and why now?) $\endgroup$ – Ghedipunk Jan 6 '20 at 19:05

Perhaps the first group found a difficult path to an inaccessible area where the trees grew. Sometime later circumstances changed so that the path was no longer accessible so there would no longer be anyway for the second group to reach the trees.

The later change in circumstances could be a number of things for example A very difficult mountain pass might be permanently closed due to a major landslide, earthquake or change in climatic conditions such as much greater snow fall.

A desert area that was previously just barely crossable might become impassable because the area of desert expands or some vital oasis dries up.

A very cold sub artic coastline that was barely navigable might become totally frozen due to climate change and become impassable.

Volcanic or glacial activity might block a route.

A vast depression in the land used as a route between two areas might flood as happened in the distant past with the Black sea and also the Mediterranean sea.


They are geographically separated. Vertically.

Your tree people never come down to the forest floor except by accident or when they die. It is dark down there, and wet. There is quicksand. And there are monsters. The tree people know what is down there because they hunt for the things further down with things like fishing rods. The tree people do not go too high for fear of eagles and lightning, and also the sun hurts their huge eyes.

The non-tree people do not enter the deep forest for the same reason: on ground level it is dark, wet, dangerous, and there are monsters. Climbing to a reasonable height and entering the forest at altitude is not something that occurs to these folks, who are farmers and ranchers. Wood for building is much more easily obtained by felling small, 100 year old trees. The big trees that compose this forest are so big they are impractical to work with.

These two groups do not come into contact with each other, ever.

  • $\begingroup$ "Huge eyes" ? ^^ $\endgroup$ – Emie Jan 7 '20 at 8:48
  • $\begingroup$ @Ernie -adapted for low light. Crepuscular, if you will. $\endgroup$ – Willk Jan 7 '20 at 13:33

I doubt 12,000 years would mean significant enough evolution for both groups to be different species (unable to breed with each other), but it is posible, specially if the way of life in that place is very different to the one we know.

The element I think would make the second group not be able to adapt, like the first one did, is competition. If the second group of humans started relatively equal to the first one, it would be likely that the best strategy (ecological niche) is the one the first group took (not necessarily, but likely). Therefore, if they evolve enough (both phisiologically and phsycologically; heck, even culturally, since the first group, I imagine, already started from a civilized society) to not recognize the second group as their equals, but as a different species, they would probably not co-operate with the second group, but compete with it for the resources and territory.

If the other species living along the first group also evolved to be better at hunting/evading humans, the second group would be even in a higher disadvantage, because all other species around would have evolved so that it would be harder for the second group to adapt than it was to the first one, both by the competition with the first group, and by the adaptations other species had against humans.

  • $\begingroup$ Yes, the 2 groups, when they split, were as developped - physically and phsycologically - as the Homo-sapiens 15 000 years ago. So, for the other species of the forest being able to grow better at hunting/evading humans, I would need a strong competition between them, no ? $\endgroup$ – Emie Jan 6 '20 at 20:18
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    $\begingroup$ Between the other species? Well, they probably already had their niches and roles and were more or less adapted to each other, and the arrival of the first group forced all other species to change and adapt. I imagine the same happening in almost any environment, not just a tall tree forest, but in general. If a bonobo group was placed in chimpanzee territory, the small evolutionary advantages of the chimpanzees for their territory, I think, would make it very hard for the bonobos to survive, even though they are very similar physiologically, as an example. $\endgroup$ – Iván Flores Vázquez Jan 6 '20 at 20:24

I like the geographic solutions mentioned above. The first and second groups separated by a body of water or a collapsed cavern/mountain passage sounds entirely plausable and for me would require no further explanation. But for the sake of spitballing ideas... A totally different option may be based in unseen reasons. One or both may live in fear of mingling for superstitious or religious reasons. Perhaps one group has developed a tolerance to a local mushroom spore fatal to the other. Maybe both options are in play are true and one is tied into the other. This should require additional background to support the plausability. But maybe you've got an idea for a tidbit of lore that would fit here but haven't yet found a place to slide it in.


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