We have no guarantees that some other compound will evolve that can convert sunlight, water and carbon dioxide into sugars. I know that copper-based substitutes for hemoglobin exist (in crustaceans), but I don't know enough biochem to guess if the chemical complexity of making carbohydrates from sunlight is unique in nature to only one compound.

It does appear that photosynthesis appeared very late, about half-way through life's history on this planet, so that suggests it is not an easy or inevitable chemical solution to the problem. It may be a binary gate, a bottleneck, in the evolutionary process leading to high-energy life forms.

We know anaerobic life evolved very early in Earth's history (almost as soon as it cooled off!). This, and the presence of simple organic compounds in the interstellar medium, and the presence of extremophiles on our own world, argues some forms of primitive life are probably widespread throughout the universe. But the transitions to an oxygen metabolism and the emergence of multicellular organisms (both essential to complex life-forms capable of achieving sentience and rapid evolution) may be a fluke unique to our world.

I certainly hope not, but that is what the meager evidence suggests so far. There may be other possible chlorophylls, just as there may be other possible DNAs; the fact they aren't around today may simply be because more efficient modern versions just crowded them out by out-competing them. But until we find alternatives here, or in space, we must be prepared to accept that these two compounds are both necessary preconditions for complex life forms.

As much as I hate to admit it, we may very well be alone.