I want to summarize and add some commentary to the article[1] posted by macu as a reply to another comment.

It's amazingly interesting and well argued. In short, the author says first that any sufficiently advanced civilization will at one point send self-replicating probes that will eventually colonize all of the galaxy and even universe. And it takes only one match to light a fire, and this is important--that is, for a sufficiently advanced planet, it would take only one rebel, one mad scientist, to set the colonization, by robots, of every habitable body in motion.

Assuming the above is true, then why do we not see any sign of these probes? It follows that there must be a Great Filter that prevented them from ever being created. Things that qualify as such a filter are not plenty, and include the original formation of life and the transition from procaryotic to eucaryotic (taking a couple billion years).

Thus if we find life so easily formed on Mars or anywhere else (especially eucaryotic cells), we can assume these past events are not really Great Filters. Then, the only reason we haven't found a single probe is that any sufficiently advanced civilization destroys itself before it ever gets to that point. If it's not nuclear weapons, then there must exist technologies that are sure on our path to discovery while at the same time guaranteeing our extinction--again, think in terms of things that only need one outlier to use it in a way that compromises the existence of all on Earth for good.

So he concludes that we must pray that we never find life anywhere else, because it would in turn give us hope (but not assure us) that there might exist no Great Filter ahead of us, and the Great Filter was indeed in our conception as life--thus we are the one single unfathomably lucky planet to ever have harbored life. If nothing potentially impedes our expansion, we will be the one sending probes and expanding to everywhere.

Adding my own expansion on this, I believe it's extremely unlikely that there is some irrevocably cataclysmical tech to be discovered before ever we are able to send self-replicating probes--ones who can mine the raw materials needed for unbounded expansion.

I don't think we're that far from that point, and taking from our own anecdotal existence since it's the only one there is, even if we do annihilate ourselves before sending them out by chance, other civilizations might not have done so, if they existed. Thus indeed the only tech that would assure a Great Filter is the self-replicating technology itself. And that is a strong point--the author seems to ignore the fact that these probes wouldn't really be the expansion of humanity, it would be the expansion of drones. And what might the precursor techs for such a replicating machine look like? If superior AI is needed at any rate, we could think that the tech leading to sure extinction is indeed robots who decimate us humans. And since they would do this as early as chance would afford it, the robots eliminating us would be as dumb as possible--thus we can assume they would be incapable of advancing technology on their own; or of coordinating an event like launching themselves into space, and thus would never leave Earth. In this scenario, we can assume every civilization that ever got a shot in going to space ended up extinct, their planet ridden with dumb self-replicating robots who can never launch themselves into space.

Since self-replicating robots are the hypothesis of the author's argument, then indeed it makes sense that every potential civilization either never reached this tech (the Great Filter is in life's conception, or otherwise somehow denies the existence of self-replicating robots), or it must absolutely reach it, in order to colonize space, but in doing so it declares it's extinction.

So, like the author, I also hope[2] that we never find life elsewhere, that we might put our tiny hopes in thriving as a race, the only one that ever existed.

[1]: http://www.nickbostrom.com/extraterrestrial.pdf

[2]: Just an hour ago, I would have jumped with joy if this NASA announcement was indeed confirming life.