One can speculate: the fact that our evolutionary ancestors lived in trees, that trees produce most of the earth's oxygen, that they make habitats and ecosystems, that they provide us with food, fuel and 
(One; can speculate; the fact that our evolutionary ancestors lived in trees) (our evolutionary ancestors; lived; L:in trees) (trees; produce; most of the earth's oxygen) (they; make; habitats and ecosystems) (they; provide; us; with food, fuel)
One our evolutionary ancestors trees they they
can speculate lived produce make provide
the fact that our evolutionary ancestors lived in trees
L:in trees
most of the earth's oxygen
habitats and ecosystems
us with food, fuel