Comment Re:Chlorrophyll makes a big assumption (Score 4, Informative) 46
It's not so much a big assumption as it is a starting point. There is probably a biosphere somewhere in the Universe that uses a red or yellow pigment for photosynthesis. The problem is that detecting it at a distance is much harder, because while we might see the spectral signature we couldn't be sure that it's life.
Looking for a biosphere that is very similar to that of Earth makes it much more likely that we'll be able to detect that it is in fact "life" and not something else. While we may miss 99% of the life in the Universe with this approach, if/when we do detect it, our confidence will be much higher.