I'm confused about Nick Bostrom's comment [PDF] on Robin Hanson's Great Filter idea. Roughly, it says that in a universe like ours that lacks huge intergalactic civilizations, finding fish fossils on Mars would be very bad news, because it would imply that evolving to fish phase isn't the greatest hurdle that kills most young civilizations - which makes it more likely that the greatest hurdle is still ahead of us. I think that's wrong because finding fish fossils (and nothing more) on Mars would only indicate a big hurdle right after the fish stage, but shouldn't affect our beliefs about later stages, so we have nothing to fear after all. Am I making a mistake or misunderstanding Bostrom's reasoning?
Mars dried out a while ago. Finding fossils there would prove very little about the great filter - since they would probably be distant relatives of ours whose planet gave out on them (since the solar system is one big melting pot for life). Basically, it is a bad example.
We've had these for a year, I'm sure we all know what to do by now.
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