Something that's bothering me, the ending could have been easily prevented

I don't understand why Ryland and Rocky decide to breed the Taumeaba until it can survive 8% nitrogen, surely they could have sent it on the beetle with a note saying "Hey I think you can breed this thing until it survives our solar system", there would have been no reason to do that stuff on the ship. And the whole reason Ryland went back for Rocky was because he bred the Taumeaba to escape xenonite, when there was no logical reason to even do that before reaching Earth/Erid.