originally published May 25, 2013

The Great Filter

The Fermi Paradox.  The Silentum Universum.  The Search for Extraterrestrial Intelligence that comes up empty.  The appearance that we may be alone in the cosmos.  What’s up with that?

The “paradox” is named for Enrico Fermi, the inventor of the first nuclear reactor.  Here’s how it goes.  See,

there are hundreds of billions of stars in our galaxy,

and it’s probably commonplace for starts to have planets,

and it’s probably commonplace for a smallish but significant fraction of those planets to be rocky worlds with liquid water and organic chemicals, so they can sustain life,

and it’s probably not too terribly rare for worlds that can sustain life to develop life of their own,

and such life, once developed, probably tends over time to develop into complex and sophisticated forms — that is, into plants and animals,

and animal life probably has a good shot at developing into intelligent life,

and intelligent life should frequently develop the ability to use tools and technology,

and we can probably say that life in general tends to try to expand into new environments,

and probably, technology would eventually give intelligent lifeforms the ability to travel, or at least send robot emissaries, between the stars,

and such interstellar exploration, even if done slowly, could crisscross the galaxy in a few million years,

and the galaxy is at least ten billion years old...

...so it seems reasonable to conclude that not only should there be a great many intelligent species in the galaxy, but that they or their works should be spread everywhere by now.

And furthermore, since it looks likely that, as long as a technological civilization exists, it will want to keep finding ever more energy to use,

and the biggest available source of free energy is probably the galaxy’s many stars...

...then it seems likely that such civilizations, once spread through the stars, should not only be detectable, they should be obvious — they should be plain to the eye everywhere we look, because they would be harvesting starlight at the source instead of just letting it shine around everywhere.

But they’re not obvious, they’re not plain to the eye... they appear to be entirely undetectable.  Where are they?

Now it seems easy to imagine that there are other intelligent species, and maybe even other technological civilizations, here and there around the galaxy without us detecting them.  But if there are a few, why aren’t there a lot?  If they’re exploring a little volume of space, why aren’t they filling a large one?  Why aren’t they everywhere?  The idea that other civilizations like us exist, having technology but not starflight, appears unlikely.  Because in our own case, it represents a very narrow window of time in the world’s history.  From the first stone-tipped spear to the first probe launched out of the solar system took only about 0.01 percent of the lifespan of our planet.  It seems likely that in another thousand years, we’ll be heading out there ourselves, and in another million, some of us might well be on the far side of this galaxy.  What are the odds that there are many intelligent races, but out of the billions of years of galactic history, they’re all caught in that narrow in-between point?  It doesn’t make sense.

It really looks like we might be alone — like we might be the first species in the milky way to launch anything into space.  But how can that be, with billions of other planets out there, each capable of developing life?  Even if some of the steps up there have only a one percent chance, there are so many planets that intelligent life still ought to be abundant.

That is the Fermi Paradox: intelligent life should apparently be pretty common, as far as we can guess, but it appears that in practice it’s actually rare.


Now, the idea that it should be common rests on a pretty long chain of uncertain suppositions.  There should probably be lots of planets that can support life, there should probably be life arising on a significant fraction of the places that it can exist, it should probably develop complexity and intelligence reasonably often... we are clearly ignorant about how likely these things actually are, so the smart bet here is that one of these steps which, from our perspective, appears easy or inevitable in hindsight (or foresight, for the last part), is actually a very difficult hurdle that it’s almost miraculously rare to overcome.

This hypothetical difficult step, the one that stops most planets from being able to develop intelligent life and technological civilizations, is known as THE GREAT FILTER.  The term is applied to whichever step it is that is most difficult... and we don’t know which step that might be.  It could even be one we haven’t come to yet (which, if true, means we might be kind of doomed).

Can we figure out which of the steps are likely to be the difficult ones?  I think we can.

Once there is animal life, I have a hard time believing that there’s anything unlikely about the remaining steps.  In our planet’s evolution, we’ve seen nervous systems getting steadily more complex and sophisticated ever since nerves were first invented.  From simple worms to early semi-bony fish to us, it’s been a quite steady progression, constantly improving without any huge lucky leaps forward.  When I see how clever and adaptable a squirrel or raccoon or sea otter can be, it’s pretty obvious that they’ve already got most of what they need, and not much more has to be added, in any fundamental way, to get to a brain as good as ours.  Progressing from rudimentary worms and polyps to very smart animals such as dogs or dolphins is straightforward enough that on Earth, it’s happened two separate times in parallel: once with the vertebrates (culminating in higher mammals), and once in the molluscs (squid, octopi, and cuttlefish).  Even among the vertebrates, many branches have developed their higher levels of intelligence independently.

Stepping from there to tool-using intelligence isn’t very impossible either.  I’ve watched a chimp create a tool out of a branch to hook things outside of his cage, and basic toolmaking is well documented for New Caledonian crows, and dolphins and octopi have both been filmed making purposeful use of found objects.  All of these evolved their abilities independently of the others.  The latter two are more impressive than they sound because dolphins have no hands, and octopi have short lifespans in which they must learn everything from scratch with no parental teaching.  Who can guess how capable they could become without these limitations?

Finally, once we get intelligent enough so that it’s possible to invent new uses and forms for stone or wooden tools on a regular basis — or for words — then there’s really no significant obstacle to progressing from there to rockets, and progress from chemical rockets to interstellar travel, though we have not achieved it yet, is a clear and straight path.  So I’m pretty confident that the Great Filter does not come after the development of complex plants and animals.

What about the other end of the chain of events?  There is a group who maintain that it’s the Earth itself which is miraculously unlikely.  They call this notion the Rare Earth Hypothesis.  Let’s take a look at that.  Here are some of the reasons they advance why Earth would be rare:

Solid planets cannot have formed in the earlier universe, because there weren’t enough heavy elements; everything was just hydrogen and helium.  Without an abundance of more complex elements, you can’t have any chemistry.

Even now, there are large portions of the universe that still lack heavy elements.  The outer fringes of our own galaxy, for instance, are too impoverished in such elements for life to be likely.  Most small galaxies are like this, apparently, so the Earth could only form in a large one.  And perhaps, some think, only in a large one that has grown out of messy collisions of smaller ones.

On the other hand, there are lots of places in the universe where circumstances are just, shall we say, a bit too exciting to be considered safe places for life.  For instance, the more central portions of our own galaxy.  Between the high density of stars, the high rate of supernovas, and the fun of having a giant black hole in the middle, any planet too close to the galactic center is likely to be repeatedly fried by blasts of powerful radiation.  (The same applies if any galactic collision is ongoing.)

In a lot of galaxies, there probably isn’t any safe zone between these two extremes.  But ours, being both quite large and (apparently) unusually calm, has got a narrow band between the hot interior and the barren edges, where planets like Earth can form, and life might persist for a good long time without getting wiped out.

But even here, things aren’t safe: the spiral arms are almost as dangerous as the center.  These are the regions where new stars form from clouds of gas and dust.  Many new stars, if they grow too large, die explosively after a short lifespan.  Their explosions help trigger further star formation in a chain reaction.  The spiral arms therefore need to be avoided.  But they don’t stay still: the chain reaction aspect causes them to propagate in waves around the galactic center.

Therefore (they hypothesize), perhaps life can only develop to its fullest on a planet whose sun is not only just the right distance out from the galactic center, but also has just the perfect orbital speed so that it always stays ahead of the spiral arms without falling into them.

Now, this last conclusion is dubious.  First of all, there are scientists who argue that there’s evidence that the Earth has, in fact, passed through spiral arms several times, with consequences that may have been tough but were certainly not fatal for life.  Now, recent observations show that our galaxy has four spiral arms, two large and two small... and most of the old stars, of the kind that life would need, have over time concentrated themselves into the two large ones.  If they last long enough, stars apparently tend to fall into the arms and become trapped there.  Our sun is currently in a small arm. Actually, it’s in a side branch, the “Orion spur”.  (Both of the lesser arms are full of gaps and forks, compared to the solid shapes of the major arms.)  Perhaps it’s stable there, and that’s been a lucky way for it to stay out of the thick part.  This could mean that though most stars end up in a bad spot, a significant minority are protected from that fate.  But on the other hand, galactic orbits are not simple or uniform or predictable; on average no such “trapping” is going to be reliable.  A given arm is going to continually gain and lose individual stars.  Our sun probably came originally from an entirely different region.  There are indications it might have started out much closer to the galactic center. That we now happen to be ensconced far away from the center and the major arms may be meaningless; for most of our planet’s history that may not have been the case at all.

So either the arms are pretty stable and therefore there’s a substantial minority of stars that are protected from the bad ones, or they’re not, and going through the arms seems to have been been relatively harmless after all.  Note that our current residence in a minor arm doesn’t imply that we’ve avoided star-forming regions; it happens that we’re passing through a small one right now, and it’s no problem.

Also, the notion that earlier stars couldn’t have planets with interesting chemistry was just dealt a blow by the Kepler observatory, which has found a solar system which appears to be twice the age of ours, with rocky planets.

Even if we don’t grant that the sun may have spent time in a major arm or other dangerous region, and assume that it must have had a very lucky orbital history that avoided all the bad zones, there’s still an insurmountable problem with this Rare Earth hypothesis.  Since we now know by direct observation that planets are abundant, it turns out that even if we stringently apply the limits called for in this theory, and then on top of that assume that only one star in a thousand has a rocky planet with organic chemicals at the right distance and temperature... that still leaves our galaxy with several million other Earths.

So if Earthlike planets that can support life must be abundant, and the progression from animal life to interstellar civilization is straightforward, that leaves only one gap in the middle where things might be highly difficult: the actual creation of animal life.


It does seem very reasonable to assume that going from the potential for life to the actuality must be what’s difficult.  This is the step that’s most impossible to visualize in our minds in a plausible way.  This is the part that seems most obviously like what must call for a divine miracle.  Surely it’s that initial spark, the very first cell, that must be the really impossible stroke of luck that made our existence achievable.

But hold on there.  Why is it that so many of the scientists who’ve studied this question keep concluding that it must actually have been kind of easy?  What’s up with that?

The current consensus from the fossil record is that Earth apparently had widespread life of some sort only a tenth of a billion years after it cooled off — that is, when the planet’s surface was only two or three percent of its current age.  There are some who argue that life quite likely existed around the same time on Mars, despite the fact that Mars remained habitable for only a brief time in the history of the solar system — perhaps even that it started there first, and a Martian meteor might have kickstarted things here.  If we ever find traces of Martian life — and there are a few who argue that we already have — then those who maintain that the creation of life from nonlife must be surpassingly difficult will have some mighty tough explaining to do.  There are even some who speculate that life may have started multiple times — for instance, that it may have formed before the Late Heavy Bombardment and then had to start over after that ended.

On the other hand, complex multicellular life of the sort that forms recognizable plants and animals is a much later development.  Life of the sort that we would instinctively recognize without having to use a microscope — things like scorpions and snails and seaweed — didn’t really get going until the Cambrian Explosion, which took more than three and a half billion years to happen.  The Earth’s solid surface was 85% of its current age; it took dozens of times longer than the initial creation of basic life did.

The unlikely step might be the initial formation of basic protobacterial life.  But I think it’s more likely that the really unlikely part was the progression from bacteria to complex plants and animals.  After all, in our own history, this is the only step which took billions of years — more time than all the other steps combined.  Since then, the entire remaining span of evolution from worms to people took only about one eighth as long.

Let’s take a look at what it took to get to that step.  Life had to develop sexual reproduction, in which different organisms exchange genetic material to produce offspring different from the parents.  This greatly sped up the process of evolution. We needed eukaryotic cells, with mitochondria and nuclei.  Finally, that allowed us to progress to multicellular organisms, in which a group of cells with a single common genetic code could differentiate into distinct tissues.  With tissues you can have organs and muscles, and most importantly, nerves.

All of these developmental steps are under careful study by those trying to figure out The Great Filter.  Each of them is considered a key hurdle.  But I’m not convinced that all of them are absolutely necessary as the sole path to complex life.  And I think I’ve noticed another key hurdle which, as yet, I have not heard anyone else mention.


What is it that makes mobile animal life possible?  The fact that we have an oxygen atmosphere.  Without it, there would be no way for a critter of larger than microscopic size to harvest and expend energy in a mobile lifestyle — you’d have to painstakingly sit around and photosynthesize for hours before building up the energy to move yourself a few feet.  And without mobility, you can’t take advantage of the opportunities that can be afforded by intelligence.  The Earth’s biosphere is full of organic materials that animals can use as fuel, but without oxygen, they’re nearly worthless.  Without the oxygen atmosphere, I think complex life would essentially be limited to plants.  And though a walking talking plant is not utterly inconceivable, they’d have to live their mobile lives at a very slow pace compared to animals.  So it would take such beings much longer to evolve any sort of intelligence than it takes for animals.  And that’s if we grant the assumption that plants would find any benefit at all in developing rudimentary sorts of intelligence.  To me it seems that in a plant world, intelligence would be rather useless as a survival strategy.  It’s the animal lifestyle, dependent on finding resources, which gets value from intelligence.

I think it’s safe to say that the oxygen atmosphere is a key component of what made the evolution of human intelligence possible.  It enabled animals to live roaming lives where energy sources (food) are available in the environment all around them, but can still be difficult to exploit once found.  It lets them live in a way that can benefit from cleverness.  So how did the oxygen get there?  Photosynthesis, of course.  It’s a waste product of plants.  And where does the food come from?  It also comes from plants.

Wha?  Our plants produce both oxygen-requiring fuels, such as carbohydrates, and the oxygen to make them useful?  Why doesn’t the plant just use both halves itself??  Like, why wouldn’t they store the oxygen and photosynthesis products in the daytime and then expend them at night?  Our existence as animals comes down to the happenstance fact that plants use photosynthesis to produce biologically useful sources of chemical energy...  and then throw half of that energy away.

When life first started, it didn’t have photosynthesis.  It must have simply worked by scavenging up useful chemicals that were left in the environment by inorganic processes.  When the Earth’s oceans first formed, they were probably loaded with useful chemicals — the so-called “primordial soup”.  Early life probably found pools of these chemicals, exhausted them in bursts of growth, and then had population crashes.  This is why some speculate, as I mentioned above, that life may have had to start up many separate times.

At some point, life got settled in at places where there was a steady supply of new chemical fuel, such as at the famous hydrothermal vents on the ocean floor.  These places may, at certain times, have been the final refuge for life at times when the Earth’s surface became uninhabitable, for instance when the seas froze.  (We still have no idea how they ever got thawed out again.)

The first crude photosynthesis appeared on the scene some 3.5 billion years ago.  At first, it did not produce oxygen.  But then at 3 billion years ago — that is, when the Earth was about one third its current age — a better kind of photosynthesis appeared, which used sunlight more efficiently.  But this new kind of photosynthesis was still inefficient.  It could easily have been replaced by a third kind without much energy lost in waste products...  and if it had been, we wouldn’t exist.

At first, the result of oxygen-producing photosynthesis was the same as any other situation where wasteful life forms pollute the environment with their own waste products: a lot of destruction and dieback.  This was called the Oxygen Catastrophe.  But once oxygen was an established fact of life, some organisms evolved to be able to make use of it.  There doesn’t appear to be any definite timing of when that happened.  But it laid the groundwork for the grand bargain between plants and animals that we see today, in which plants turn water and carbon dioxide into oxygen and carbon-hydrogen compounds, and animals turn carbon-hydrogen compounds and oxygen into carbon dioxide and water.

Nowadays, all the elements of life — carbon, nitrogen, phosphorus, and so on — move through the ecosystem in great cycles.  And the oxygen cycle is the most important, and possibly the most surprising.

Early life didn’t have those closed cycles.  It worked by one-way processes (much as our industrial processes generally do).  As a result, early life was, in some sense, not sustainable.  It depended on exhaustible outside resources.  I would speculate that its story is probably less one of growth and thriving and progress than of occasional blooms followed by hard crashes.  Maybe it was only thanks to certain chemical resources that continually refreshed the necessary resources in certain spots, such as the hydrothermal vents, that it didn’t crash completely and go extinct.  Or perhaps it sometimes did, but the residue was close enough to alive that it didn’t take too much miraculosity for it to start again.

Early photosynthesis helped a lot — it greatly reduced the need to find energetic chemicals in the environment.  Instead, it only needed materials that could do useful things once energy was added via sunlight.  But it still didn’t really produce a sustainable cycle.  It was only the accident that led some bacteria and algae to start producing tons of waste oxygen, enabling other critters to use the oxygen to extract energy from carbon-hydrogen compounds (the easiest source of these being to gobble up the very cells that had made the oxygen), that allowed life to finally achieve a closed cycle.  With that cycle established, it was now finally possible for the whole system to keep going at full speed continuously, without fear of any resource ever running out.  Once that was done, the planet could fill with life from top to bottom, with both plants and animals having abundant resources everywhere.  Only then did the long term continuity of life around the world finally become something that one could count on and take for granted, rather than being a desperate venture that might fall into ruin at any time.

So that’s my theory of what constitutes the Great Filter — the reason why intelligent life is rare.  That it isn’t life itself which is miraculous, but sustainable life with a closed environmental cycle, and further, a cycle of such a nature that some organisms participating in it get to enjoy an Edenic abundance of resources that normally ought not to be available, particularly oxygen.  Think how unlikely it is that we would end up with a world of plants which waste half of their sunlight energy and half of their own growth, just to fuel animals which happen to replenish the atmospheric carbon dioxide a tiny bit.  (The ratio of O2 to CO2 in the air was about 750 to 1 in preindustrial air, though now it’s almost down to 500 to 1.)  That strikes me as an enormously generous deal for animals — the plants really got the short end of the bargain.  We’re damn lucky that our plants happen to be just lame enough to be so successful and yet so thoroughly taken advantage of.  And on any planet where plant life evolves, and then manages to come up with better terms than that for using its own sunlight energy... or conversely, where it settles for a poorer version of photosynthesis and never gets as far as our plants did... there won’t be any intelligent life.  The resources just won’t be there for critters to be able to move about and explore a challenging but provident environment — the kind of environment that rewards intelligence.

This hypothesis of mine has the advantage of being highly disprovable: if our astronomical observations detect even one instance of another planet with an oxygenated atmosphere, I’ll abandon this idea at once.  (Note: thin traces of frigid oxygen found around icy bodies, as is detectable on some moons in the outer solar system, don’t count.)

I don’t know how you feel about this issue, but for me, considering this topic gives me a greater appreciation than ever for the irreplaceable preciousness of our home planet and its web of life.  We need to be responsible stewards, or we lose something whose like may not be found in a thousand galaxies.


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