Photosystems I and II are named such because Photosystem I was actually discovered (and named) before Photosystem II, even though Photosystem II comes before Photosystem I during photosynthesis (i.e., Photosystem II precedes Photosystem I in the electron flow of photophosphorylation).

Photosystem I and II evolved independently, and I think that Photosystem II, which uses H₂O (water) as the electron donor, actually may have evolved before Photosystem I, namely in cyanobacteria (blue-green algae). If this is true, then Photosystem II is actually more primitive than Photosystem I. I think Photosystem II is also used in more organisms.