I’m getting this out of the way before I forget it: here is a potential spoiler for more Avatar movies, if I’m right:
The glowing magical fiber-optic tree is the surviving neural core of a crashed spaceship, the corroded spars of which are still visible. The Na’vi are related to humans, from the future (of the future) or a parallel universe or a genetic engineering program. Either that or humans are the stunted pale survivors of a Na’vi colony on Earth. Or we have a common ancestry.
Whatever. Anyway, this explains why Na’vi have four limbs and boobs.