It is said that when RUBISCO evolved, the atmosphere was high in carbon dioxide with very little oxygen. Because of this, photorespiration never occurred and there was no selection pressure against it. As a result, RUBISCO is so inefficient as it now catalyses photorespiration which is considered a waste of energy and carbon. Due to the fact it’s not a problem for plant nowadays, there’s no selection pressure for it to evolve in a better way. I don’t fully get what sort of change could have occurred if RUBISCO had catalyzed photorespiration at times it was evolving. How will there be a selection pressure against it ?

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