Question

Why does photorespiration provide an indication that rubisco was present before the evolution of oxygenic photosynthesis?

Answer 1

Rubisco, the catalyst in the Calvin cycles, favors CO2 as a ligand more than O2. Photorespiration, however, occurs when the ratio of of CO2 to O2 is very low (meaning when there is more oxygen). The O2 levels were a great deal less in the beginning of this evolutionary change.