Not sure if this is addressed in the movie and I missed it, or if it's just a seemingly significant plot-point that's brought up and then ignored, but what was the origin of the "human vegetation" encountered on the alien planet?
I assume that
It could not have arrived with Ellie, since she reached the planet on an Engineer ship and wasn't really afforded the opportunity to load it up with Earth provisions before setting off (nor is there any guarantee or even plausible likelihood that "provisions" stocked on an exploration vessel's lifeboat would include seeds for sowing crops). And also that even if she had, David probably subdued her long before she got to the point of needing to cultivate food to survive (and since his first move upon arrival was apparently to kill all animal life on the planet, it's not like there'd be any insects around to pollinate the stuff anyways).
Was the wheat introduced to the planet, and if so, by whom?