Free oxygen is a pretty good sign of life and if we were to see it in other planets' atmospheres that would be a good indication that life was there. But on th

Free oxygen is a pretty good sign of life and if we were to see it in other planets' atmospheres that would be a good indication that life was there. But on the other hand, there was life on Earth for a long period of time before photosynthesis was evolved, then there was a long period of time before large amounts accumulated in the atmosphere. So while this test is relatively easy to apply across a large number of planets it would tend to give a lot of false negatives. It seems plausible that on many planets you have life evolving but because it's slow to develop photosynthesis the solar wind strips away all the hydrogen before the production of oxygen creates a protective ozone layer. Maybe that even happened on Mars, though with the lack of magnetic field there and shallow gravity well it was going to lose its hydrogen eventually even with an ozone layer.