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 a very improbable event. If it happened independently twice here in our own back yard, it must surely have happened millions times across the galaxy. This would mean that the Great Filter is less likely to occur in the early life of planets and is therefore 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 exist 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
multi‐cellular organism, that would eliminate a much larger stretch of potential locations where the Great Filter could be. The effect would be to shift the probability more strongly to the hypothesis that the Great Filter is ahead of us, not behind us. And if we discovered the fossils of some very complex life form, such as of some vertebrate‐like
creature, we would have to conclude that the probability is very great that the bulk of the Great Filter is ahead of us. Such a discovery would be a crushing blow. It would be by far the worst news ever printed on a newspaper cover.
— from Where are they?: Why I hope the search for extraterrestrial life finds nothing by Nick Bostrom
Essentially: there ought to be a lot of extraterrestrial civilizations, but we haven’t seen any. Either we’re the only advanced civilization in the observed part of our universe (unlikely), or all the other advanced civilizations have disappeared through a ‘great filter’ that destroys them under some condition — population, resource use, warfare, etc.
If we find evidence of alien life, it reduces the probability that life is very rare, thus increasing the probability that there is a great filter.
And, if there is a great filter, we have to ask whether it’s behind us (which would be a relief) or whether it’s still ahead of us. If we find fossils of a dead lifeform that was very simple, we could argue that the great filter is something we passed through millions or billions of years ago. If we find fossils of complex lifeforms, it means we may be living through the great filter right now.