The Great Filter theory suggests that all advanced civilizations eventually destroy themselves before acquiring the capacity to colonize space — a notion that could explain why we've never been visited by aliens. But there may be another reason for the celestial silence. Yes, the Great Filter exists, but we've already passed it. Here's what this would mean.
Before we can get to the Great Filter hypothesis we have to appreciate what the Fermi Paradox is telling us.
The Fermi Paradox and the Great Silence
The so-called "Great Silence" is the contradictory and counter-intuitive observation that we have yet to see any evidence for the existence of aliens. The size and age of the Universe suggests that many technologically advanced extraterrestrial intelligences (ETIs) ought to exist — but this hypothesis seems inconsistent with the lack of observational evidence to support it.
Despite much of what popular culture and sci-fi would lead us to believe, the fact that we haven't been visited by ETIs is disturbing. Our galaxy is so ancient that it could have been colonized hundreds, if not thousands, of times over by now. Even the most conservative estimates show that we should have already made contact either directly or indirectly (such as from dormant Bracewell communication probes). [Image: Mark Gee.]
Unfortunately, most solutions to the FP don't hold for a number of reasons, including the realization that a colonization wave of superintelligent aliens would likely rework the fabric of all life in the cosmos (e.g. uplifting), or that these solutions are sociological in nature (i.e. they lack scientific rigor and don't necessarily apply to the actions of all advanced civilizations; all it would take is just one to think and behave differently — what astrobiologists refer to as the non-exclusivity problem).
There have been many attempts to resolve the Fermi Paradox, including the herculean attempt by Stephen Webb in his book Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life.
But one solution stands out from the others, mostly on account of its brute elegance: The Great Filter.
The Great Filter
Conceived in 1998 by Robin Hanson, the GF is the disturbing suggestion that there is some kind of absurdly difficult step in the evolution of life — one that precludes it from becoming interstellar.
And like the immutable laws of the universe, the GF is a stumbling block that holds true across the board; if it applies here on Earth, it applies everywhere.
Many look upon the GF as evidence that we'll destroy ourselves in the future. The basic idea is that every civilization destroys itself before developing space-faring technologies, hence the empty cosmos. Given our own trajectory and the ominous presence of apocalyptic weapons, this scenario certainly seems plausible. We're not even close to going interstellar, yet we're certainly capable of self-annihilation.
But that doesn't mean this interpretation of the GF is the correct one. Rather, it's quite possible that human civilization has already passed the Great Filter. Should this be the case, it would be exceptionally good news. Assuming there's no other filter awaiting us in the future, it means we might be the first and only intelligent civilization in the Milky Way.
It's a possibility, however, that demands explanation. If the filter is behind us, what was it? And how did we manage to get past it? Interestingly, there are some excellent candidates.
First and foremost there's the Rare Earth Hypothesis (REH), the suggestion that the emergence of life was extremely improbable for a confluence of reasons. The theory essentially suggests that we hit the jackpot here on Earth.
This argument, which was first articulated by geologist Peter Ward and astrobiologist Donald E. Brownlee, turns the whole Copernican Principle on its head. Instead of saying that we're nothing special or unique, the REH implies the exact opposite — that we are freakishly special and unique. What we see here on Earth in this solar system and in this part of the Galaxy may be a remarkable convergence of highly unlikely factors — factors that have resulted in a perfect storm of conditions suitable for the emergence of complex life.
It's important to note that Ward and Brownlee are not implying that it's one or two conditions that can explain habitability, but rather an entire array of happy accidents. For example, stars might have to be of the right kind (including adequate metallicity and safe distance from dangerous celestial objects), and planets must be in a stable orbit with a large moon. Other factors include the presence of gas giants, plate tectonics, and many others.
But even with all the right conditions, life was by no means guaranteed. It's quite possible that the Great Filter involved the next set of steps: the emergence of life and its ongoing evolution.
The Improbability of Life
Indeed, in addition to the various cosmological and chemical prerequisites for life, there were at least three critical stages that could all be considered candidates for the Great Filter: (1) the emergence of reproductive molecules (abiogenesis and the emergence of RNA), (2) simple single-celled life (prokaryotes), and (3) complex single-celled life (eukaryotes).
Chemists and biologists are still not entirely sure how the first self-replicating molecules came into existence. Unlike its big brother DNA, RNA is a single-stranded molecule that has a much shorter chain of nucleotides. Moreover, it usually needs DNA to reproduce itself, which would have been a problem given the absence of DNA in those early days.
That said, scientists know that RNA is capable of reproducing through autocatalysis. It does this by storing information similar to DNA, which allows it to become its own catalyst (a ribosome). This so-called RNA World Model suggests that RNA can function as both a gene and an enzyme — a pre-DNA configuration that eventually became the basis for all life.
Given that we've never detected life elsewhere, it's difficult to know how difficult this initial step was. But that said, this form of life emerged super-early in the Earth's history — about a billion years after its formation, and immediately after the cooling of rocks and the emergence of oceans.
But what we do know is that the next few steps — the leap from single-celled life to complex single-celled life — was exceedingly difficult, if not highly improbable. The process of copying a genetic molecule is extremely complex, involving the perfect configuration of proteins and other cellular components.
Here's how it likely happened: Once a self-replicating molecule emerged, the presence of RNA allowed for the formation of protobionts, a theoretic precursor to prokaryotic cells. These tightly bound bundles of organic molecules contained RNA within their membranes, which could have evolved into proper prokaryotic cells.
And here's where it gets interesting. After the formation of prokaryotes (about 3.5 billion years ago) nothing changed in the biological landscape for the next 1.8 billion years. Life in this primitive form was completely stuck. Imagine that — no evolution for almost two billion years. It was only after the endosymbiosis of multiple prokaryotes that complex single-cell life finally emerged — a change that was by no means guaranteed, and possibly unlikely.
And it's this highly improbable step, say some scientists, that's the Great Filter. Everything that happened afterward is a complete bonus.
Now that said, there may have been other filters as well. These could include the emergence of terrestrial organisms, hominids, and various civilizational stages, like the transition from stone age culture to agricultural to industrial. But unlike the first primordial stages already discussed, these are porous filters and not terribly unlikely.
More Filters Ahead?
So, if the GF is behind us, it would do much to explain the Fermi Paradox and the absence of extraterrestrial influence on the cosmos. Should that be the case, we may very well have a bright future ahead of us. The Milky Way Galaxy is literally ours for the taking, our future completely open-ended.
But before we jump to conclusions, it's only fair to point out that we're not out of the woods yet. There could very well be another GF in the future — one just as stingy as the filters of our past. The universe, while giving the appearance of bio-friendliness, may in reality be extremely hostile to intelligent life.
This io9 flashback is a slightly modified version of an article that was first published on io9 in 2012.