Oxygen would slowly deplete into carbon dioxide as there is no photosynthesis to create more. Earth would not be able to support human life later on.
The answer to your question is Mutualism.
Answer: The Central Nervous System and The Peripheral Nervous System
Basically, the P-P chain turns 4 protons into: 1 neutrino, 1 gamma ray, 1 positron, 1 helium nucleus.
A gamma ray = energy.
The chain converts hydrogen into helium and makes energy.
This is all occurring through fusion reactions where 2 protons basically smash into each other. But, it could also be e.g. a proton smashing with a deuterium nucleus. Or, other nuclei smashing together to make something else.
Hope this helps!
