The answers that fit in the blanks are: mitochondria and oxygen
The first thing about this question you need to understand is how cell respiration works, and why does it exist in the first time. Well, it's a common theory between the scientists that the mitochondria is a bacteria due to its own DNA, but why would a cell have a bacteria inside of it? Well, in science you need to see things from the point that if that's worth it for me (as a cell) why not? It'll provide me ATP during the process, I'll have energy enough to survive the environment, I'll start using oxygen too, it helps developing more and more ATP, and it shows its presence on the atmosphere, so, why not right?
But remember, in science, time flows really fast, these things took cells millions and millions of years to really happen, and that's how science works.
