The fly overflyover scene at the beginning showed green areas as well as lots of rock and water meaning there was at least plant life on the planet before they seeded. Maybe they were adding roses to the garden. If it was a few million years ago, all they had to do was "engineer" a way to nudge us in the humanoid direction and see what happens...the soup was at the right point (potatoes boiled, meat cooked) to add some spice.