Benjamin Kay points to this:

But I [Nick Bostrom] hope that our Mars probes discover nothing. It would be good news if we find Mars to be sterile. Dead rocks and lifeless sands would lift my spirit. Conversely, if we discovered traces of some simple, extinct life-form–some bacteria, some algae–it would be bad news. If we found fossils of something more advanced, perhaps something that looked like the remnants of a trilobite or even the skeleton of a small mammal, it would be very bad news. The more complex the life-form we found, the more depressing the news would be. I would find it interesting, certainly–but a bad omen for the future of the human race.

What has all this got to do with finding life on Mars? Consider the implications of discovering that life had evolved independently on Mars (or some other planet in our solar system). That discovery would suggest that the emergence of life is not very improbable. If it happened independently twice here in our own backyard, it must surely have happened millions of times across the galaxy. This would mean that the Great Filter is less likely to be confronted during the early life of planets and therefore, for us, more likely still to come.

If we discovered some very simple life-forms on Mars, in its soil or under the ice at the polar caps, it would show that the Great Filter must come somewhere after that period in evolution. This would be disturbing, but we might still hope that the Great Filter was located in our past. If we discovered a more advanced life-form, such as some kind of multicellular organism, that would eliminate a much larger set of evolutionary transitions from consideration as the Great Filter. The effect would be to shift the probability more strongly against the hypothesis that the Great Filter is behind us. And if we discovered the fossils of some very complex life-form, such as a ­vertebrate-­like creature, we would have to conclude that this hypothesis is very improbable indeed. It would be by far the worst news ever printed.

Benjamin Kay writes,

It has a logical claim in it that seems similar to the doomsday argument you presented in Doomsday and Bayes but in a different context. I [Kay] found it an interesting example of inference from very limited data. Perhaps it is right for your blog.

It somehow reminds of the story about the person who brings a bomb on the plane with him to protect against terrorism because, hey, think how low the odds are of having two bombs on the plane. Also, the bit about vertebrates reminded me of the Great Chain of Being.