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.