You remember those crystal trees I've spoken of from way back when in the Shadow Chasers campaign? (Which would be fantastic in GURPS). I've said before that those trees were the ancestors of the Eldumans of Kaleshmar and that the Guardian was their caretaker. Those trees were known as The Forests of the Umanle, but not until the Guardian came around, because trees can't speak and do things like come up with names for stuff. Because they're trees.
For the purposes of this post, we'll call the trees the Umanle. Even though they and their ancestors became the Elduman race, the Umanle persisted for quite some time. In the past I have described the Umanle as being pine trees with needles of blue crystal. This is basically true, but they're not so...conical I guess. Pine trees are conical right? Anyway, they are trees, so therr is a main trunk and a bunch of branches sprout off of it. The branches only grow out to a certain length though, and don't grow further than a few feet from the trunk. The branches on each tree grow out to the same exact length, so the Umanle have kind of a uniformly wide shape, rather than coming to a point at their top like pine tree. Their "needles" appear as clumps at the end of each branch and don't sprout from any other point on the branch. So basically, they are like pine trees in that they have needles, wee. Maybe they're more like weird cacti? Not sure, cacti are plants and I did conceive of the Umanle having some manner of bark-like skin. So, they're weird trees, until I retcon it. Heh.
So, the Umanle are trees that are psionically resonant with some blue crystal needles. What are the needles for? As with normal trees, the blue needs on the Umanle are how they absorb energy, except that Umanle don't use photosynthesis. These blue crystalline needles absorb psionic energy from the world around the trees. So when a silly little Kaleshmarian squirrel type creature wanders into The Forest of the Umanle and it gets close enough to one of those creepy blue trees, his mind basically drains out of his skull via the sucking psionic maw of the Umanle.
So the Umanle eat brains, there, I said it. Num num. But they're still just basically brainless, unthinking, trees. They don't think about melting the brains of Kaleshmarian squirrels, the same way oak trees don't think about nomming on some sunlight with their green leaves. These trees don't move or walk or reproduce in any manner of mundane way, so how do we get to the Guardian and the Eldumans?
The Forest of the Umanle is covered in the skeletal and decaying remains of all these Kaleshmarian critters. Now, to reproduce, these Umanle drop clusters of needles into the ground, and these in turn grow into their tree form, slowly developing a thick bark-like "skin" and eventually dropping their own clusters of needles to the floor of the forest.
The way I envision it happening is that one of these clusters of needles, or dozens of them, drop into something that no longer has a brain and was vaguely human looking. Now, the needles sprout crystalline root structures and kind of wind their way their way through this human looking creature and we have a bunch of slightly human looking stinking corpses full of crystals lying around on the forest floor doing not much of anything.
What happens is that these unintelligent tree-things begin using what is left of the corpses they inhabit to kind of move around and expand the borders of The Forest of the Umanle. The larger the forest grows, the more needles there are dropping, and the more corpses there are lying on the ground for them to grow into. At this point, the Forest has been eating brains enough that it kind of knows where they are in most creatures and that they kind of direct what these sacks of flesh do. This is why modern Eldumans have big lumps of crystal in their skulls that act as their brains.
The first proto-Eldumans were these shambling, half-decaying things that stumbled around The Forest of the Umanle not really knowing what they were doing. They weren't zombies in the conventional sense, they didn't storm into the caves of the kind-of-human things that the trees had possessed and eat out their brains with crumbling teeth. They just kind of stumbled around aimlessly, and if something intelligent came within range of the crystal structures within them, they stumbled towards it and ate its brain in the same way that the trees did. I'm imagining some kind of invisible microwave that cooks up the brain to a nice temperature and absorbs it like a vacuum while the physical remains leak out the nostrils and ears of the food source.
Long story short, these shambling crystal corpse things gain the intelligence and instinct of a predator and begin seeking out prey, rather than being opportunistic predators. They learn from the the minds they devour how to operate these flesh sacks and over time develop into what the Guardian was and eventually become full fledged members of Kaleshmar and move on to finally become the Eldumans of The Old Empire once Kaleshmar falls and The Old Empire rises.
So that is some history behind how the Eldumans evolved from trees into what they are in the current era of The Known World. A few questions remain though. The Forests of the Umanle still existed at the fall of Kaleshmar, and pieces of Kaleshmar impaled The Known World when it was destroyed. Does that mean that there are some crazy underground forests of blue crystal somewhere in Kusseth or Whurent or something like that? Or have they become some manner of deadly, underground, crystal fungi? Next question, what were these human-looking things that died and got used as meat puppets by the trees?