Which photosystem-1 or 2-most likely evolved first?

To determine which photosystem, either Photosystem I (PSI) or Photosystem II (PSII), most likely evolved first, we need to understand the process of photosynthesis and the evolutionary timeline.

Photosynthesis is the process by which plants, algae, and some bacteria convert sunlight into chemical energy. It involves two photosystems, PSI and PSII, working in series to harvest and use light energy. These photosystems consist of pigments that absorb light and reaction centers that facilitate the conversion of light energy into chemical energy.

To determine which photosystem evolved first, scientists have analyzed the evolutionary relationships between different organisms that perform photosynthesis.

Based on these studies, it is generally believed that PSII likely evolved first. PSII allows organisms to conduct oxygenic photosynthesis, which produces oxygen as a byproduct. Oxygenic photosynthesis is performed by cyanobacteria, which are considered the ancestors of modern-day plants and algae.

PSI, on the other hand, is involved in cyclic electron flow and non-cyclic electron flow, but it doesn't generate oxygen. It is found in both oxygenic and anoxygenic photosynthetic organisms.

To summarize, based on evolutionary evidence and the presence of oxygenic photosynthesis in cyanobacteria, it is suggested that PSII evolved before PSI. However, it's important to note that the exact timeline of photosystem evolution is still an area of active research, and further studies may provide more insights in the future.