Some photosynthetic bacteria (e.g. ,purple sulfur bacteria)have only photosystem I,whereas others (e.g. ,cyanobacteria)have both photosystem I and photosystem II.No photosynthetic bacteria have only photosystem II.Which of the following might be a possible explanation for this observation?

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This observation suggests that photosynthesis with only photosystem I is more ancestral and was present in the common ancestor of photosynthetic bacteria. Photosystem II likely arose later in evolution and became widespread in photosynthetic organisms that use both photosystems.

Some Photosynthetic Bacteria (E