CO2 breathing animals?
Obviously from what we see today, evolution branched off into two sides, the co2 producers, and the co2 users. Now all animals that walk or swim the earth today breath in oxygen and out co2. It may sound crazy, but have we not even attempted to produce oxygen making creatures. Evolution just didn't really branch off to this kind of creature and it would be very difficult to create. Todays Autotrophs don't produce enough energy for a large animal. Trees and small single cell organisms use tiny amounts of energy, but could a hybrid allotroph, which gets the majority of its energy from eating like a normal cow, but then it also uses co2 and sunlight maybe, and then you can eat it. Just a crazy thought that has always bugged me.