The only way that'd be possible would be genetic engineering, and of the scale you're describing, I'd say would be beyond the colonists. As for it being by evolution, part of the Principle of Parsimony states that the simplest explanation is more likely, kinda like Occam's Razor. It is often applied to determine the order at which structures in organisms evolved. Basically, more complex systems take considerably longer to evolve then small adaptations. Example: Skeletal system and, say.. the pinna (outer portion of ear) can be said to have evolved AFTER the skeletal system, as its a less complex structure. It took several million years for the skeletal system to come into existence. Chances of humans converting from primarily oxygen based organisms (Which is also directly related to how we metabolize stuff), yet another several million years, is especially unlikely. Chances are, they'd just terriform the planet, or go somewhat "biodome," OP2 style. It'd be far easier then trying to genetically engineer an entire population of which there is no known efficent way to do so with an already grown organism. Pre-natally, when the child is still just a few cells, not as difficult. But even then, thats an 18 year wait. Much faster to build a whole new colony.