It's just speculation at the moment. As far as we know, the Engineers made more than just humans. Maybe all vegetation we are familiar with comes from them as well. Maybe wheat was engineered on Paradise and brought here? We don't know just yet.
We do know that Scott isn't thinking small. I get that. I just hope he isn't thinking clumsy, like he did with 'PROMETHEUS'. A movie of Grand Ideas and Ambition wrapped in coating of Stupid. 'PROMETHEUS' wasn't far from being an amazing movie all round, it just fell short in a few areas. Key areas, at that. Sadly, areas that just a little script doctoring could have greatly repaired.
I'm hoping 'ALIEN: Covenant' doesn't follow too closely in those footsteps. There's so much about this entry that looks like it's set to redefine the series in a positive way. I just hope it doesn't disappoint.
-Windebieste.