WARNING: THIS POST WAXES INTENSELY PHILOSOPHICAL. Except when it is waxing biological. Read at your own peril.

The Fermi Paradox ponders the question of why we've yet to observe any sign of intelligent alien civilization.  Speculative theories to explain the Fermi Paradox abound, including the Zoo hypothesis, UFO Conspiracy, Transcendence, Simulated Reality, and many other speculations closer to religion than to practical science. The Great Filter is so far the most useful hypothesis addressing the Fermi Paradox. It takes a process approach to analyzing the problem, positing that an advanced civilization must go through an evolutionary sequence of achievements to culminate at Kardashev level 2 or higher galactic colonization which would be observable even to an intelligent planet bound species such as our own. To explain the observed deficit of galactic society, at least one of these steps must pose a difficult if not impossible barrier to pass. The architect of this logic, Robin Hanson, defined the steps thus (my graphics and captions):

Now if the impossible barrier is truly impossible, it must be the last step (else we wouldn't exist).  This is the Copernican perspective which says not only are we not the center of the universe, we are just like every other doomed wannabe civilization on a bazillion planets across the universe.

Alternatively, the Rare Earth hypothesis much more optimistically says no, the hard part is the astronomically (literal) improbable odds of life originating, but because the universe is so big it had to happen somewhere, and we won the cosmic lottery so the universe is now ours to colonize. 

There is also the admittedly circular Anthropic Priniciple, which says we are here because we are here and we observe what we observe because we are here. 

There are a couple of exceptions I take to Prof Hanson's list. Foremost is the sexual reproduction step. From my understanding of Matt Ridley's "Red Queen*", sexual reproduction (i.e., gene recombination) is a natural consequence of cellular competition.  Prof Hanson is to be forgiven however, as this line of thinking evolved after his original thesis. Hanson has also acknowledged that his list is likely incomplete. I'd agree- galactic conquest at #9 should really be number n, and be preceded by ellipsis. Beyond the obvious first step (develop CELSS and expand beyond Earth), we really don't know what steps to take. I also think there is a critical past step related to the formation of our moon, which is responsible for the magnetosphere, plate tectonics, and the carbon cycle governing much of the evolutionary path that took place on Earth. 

This week we got hints of a breakthrough in microbiology where a possible relative of the missing link to eukaryotes was discovered. This is quite exciting because understanding how the seemingly unlikely symbiosis between two distinct domains of life occurred gives us hints about whether that was the Great Filter. Achaea and bacteria are genetically so distinct that some have speculated that one or the other may originate from elsewhere, with Mars being a likely candidate. Our own eukaryotic ancestors were the bastard hybrids of these original life forms. The more unlikely their chance meeting, the more likely that occurrence was the Great Filter. If we discover either archeans or bacteria on Mars, it would credit this theory and be great news for the future of our species. Jury's out if we discover both. If we discover eukaryotes, we're probably doomed. If we discover multicellular life that isn't eukaryotic the list will need to be rewritten (and it won't be good). If we discover nothing, things are looking up.

Looks like we gotta go to Mars. It's the next step in our continued survival anyway.

*Does it say something about me, that I gave a copy of "The Red Queen" to my wife on our very first date?