Subject Summary - The Fermi Paradox and the Tape Recorder of Life

Habitable planets like this one are believed to be common throughout the universe, so where are all the aliens?
(Image: Me, via SpaceEngine)

Intelligent life exists in the universe.

This statement is self-evident, since we exist. You and I are confident of our own sapience and that of our fellow man, which begs the question of whether there are others out there who think like we do.

We are reasonably certain that some nonhuman animals are self-aware in the sense that they have a theory of mind, but even the most intelligent ape or crow fails to match up to the technological prowess of Homo sapiens. As for our search for equals in the cosmos beyond our planet, the results there have been even more dismal. While once it was taken almost for granted that we would find equals among the worlds of the Solar System, in all our centuries of searching we have not found so much as an alien microbe, let alone an alien man.

But still, the question of Are we alone? cries out for an answer. Even if all we can do is speculate in lieu of evidence, we are not entirely helpless in our search for cosmic brethren. We can still draw analogies from the life we know to trace out the rough image of the life we don’t, even as we continue to ply the dunes of Mars and oceans of Enceladus for signs of its existence.

Adventures in Astrobiological Spitballing

Central to the discussion of intelligent extraterrestrials is the (in)famous Fermi paradox, which refers to the puzzling failure to find alien civilizations despite the supposedly high probability of their existence. Considering that habitable-zone planets are apparently common^[1] and that the universe's age is more than enough for a motivated intelligent species to colonize the galaxy^[2] even with spacecraft far slower than light speed, one might expect our cosmic neighborhood to be teeming with advanced life. Since it evidently isn't, there must be some factors preventing intelligent life from colonizing the local cosmos.

To explore what these factors may be, we need a framework quantifying the various requirements for intelligent life to emerge, develop a civilization, and colonize the stars. For popularity's sake, we will be using the famous Drake equation^[3] as a starting place.

N = R_✴ • f_p • n_e • f_l • f_i • f_c • L

The terms of the Drake equation are as such:

N is the number of intelligent extraterrestrial civilizations we can detect.
R_✴ refers to the rate of star formation in the galaxy.
f_p refers to the proportion of stars that have planets.
n_e refers to the proportion of planets that are Earthlike.
f_l refers to the proportion of Earthlike planets that actually go on to develop life.
f_i refers to the proportion of life-bearing planets that go on to develop intelligent life.
f_c refers to the proportion of intelligent species which advance to a level where their existence is detectable.
L refers to the length of time that a civilization remains detectable.

The first problem with using applying the Drake equation in a useful way is that we don't know the values of any of the terms besides R_✴ and f_p. We lack the ability to characterize exoplanets to the degree necessary to determine whether any of them have habitable conditions, while our understanding of our own evolutionary and cultural history is insufficient to determine the likelihood of any given planet developing life, intelligence, or advanced technology, nor do we know how long civilizations are supposed to last for.

However, there is a simple (non-rigorous) solution: just wildly guess all of the factors that you can't determine! Frank Drake certainly did this, and he and his colleagues' best guess as of 1961 was as follows:

R_✴ = 1
0.2 < f_p < 0.5
1 < n_e < 5
f_l = 1
f_i = 1
0.1 < f_c < 0.2
1,000 < L < 100,000,000

Or, in English:

1 star forms in the galaxy per year (this is roughly correct)
Between 20% and 50% of stars have planets (exoplanet surveys suggest this fraction is closer to 100%)
Each star has between 1 and 5 Earth-like planets
All Earth-like planets develop life
All life becomes intelligent
Between 10% and 20% of civilizations become detectable
Civilizations are detectable for anywhere between 1000 years and 100 million years

Using Drake's numbers, the number of civilizations in the Milky Way is somewhere between 20 and 50 million, which isn't really all that helpful, though even his lowest estimate is higher than the number of definitively known civilizations (one). This either means that there are somewhere between 19 and 49,999,999 alien civilizations to find, or Drake's numbers are wrong.

To determine which of these it is, we need to look at Drake's three critical assumptions^[3]:

Every system with planets has at least one which is suitable for life. (hence n_e > 1)
All life-supporting planets inevitably develop intelligent life. (hence f_l & f_i are 1)
All intelligent life becomes technologically advanced and is detectable for thousands or millions of years.

But how true are these assumptions?

Habitable Worlds and Where to Find Them

Physical Solutions to the Fermi Paradox

Galaxy Messier 61 contains hundreds of millions of stars. But how many of them are circled by Earth’s distant cousins?
(Image: ESA/Hubble)

Let’s begin with Drake’s first assumption:

Every system with planets has at least one which is suitable for life.

Is this really true? Let’s look at the data.

Tidal locking plunges half of this super-Earth into freezing eternal night while the other side bakes under endless noonday sunshine. Is this a lifeless hell or an interesting alternative to more traditional Earth-like worlds?
(Image: Me, via SpaceEngine)

Before making any postulates about planetary habitability, let's start with the basics. All life on Earth from the smallest bacterium to the largest whale requires liquid water to survive, so it's a fair assumption that alien life ought to require it as well. In order for liquid water to exist stably on the surface of a celestial body, it must be in the circumstellar habitable zone, where it gets enough sunlight to melt water ice but not enough to boil away its oceans. Climate modelling suggests that planets that receive more than 110%^[4] the solar energy Earth does will experience runaway greenhouse effects that bake away their seas, while those that get less than 35%^[5] cool to frozen snowballs. Of the 5747^[6] confirmed exoplanets, some 176^[7] are within the aforementioned limits, which would naively suggest ~3% of planets are habitable.

However, this naive estimate cannot be trusted for a number of reasons, and indeed there are solid cases to be made for the proportion of habitable planets being much higher or lower than our 3% estimate.

Let's start with the pessimistic case. Many planetary scientists have argued that numerous factors have to be just right in order for a planet to be habitable, even if it is in the nominal 'habitable zone'. The number of critical parameters may be so great that few if any planets in the universe may be suitable, a proposition known as the Rare Earth hypothesis. The strictest formulation of Rare Earth parameters includes the following:

A habitable planet must be in a quiet and undisturbed spiral galaxy like the Milky Way^[8], since galactic collisions and interactions trigger life-extinguishing supernovae. Elliptical and irregular galaxies, which are formed in collisions, are therefore ineligible to host habitable planets.
A habitable planet must not be in a galaxy with an actively feeding supermassive black hole^[9], since those emit large amounts of deadly radiation.
A habitable planet must be in the galactic habitable zone^[10], a ring-shaped region encompassing the middle parts of a galactic disk. This region is far away from the crowded galactic core, where planets are exposed to numerous supernovae, but it is also close enough for there to be enough supernova-made metals to condense into rocky planets.
A habitable planet's sun must not be on an orbit which takes it through the spiral arms^[11] of its host galaxy, since the spiral arms are rich in massive stars that die as supernovae.
A habitable planet must have one sun^[12] in order for its orbit to be stable.
A habitable planet's sun must be stable^[12] and quiet. If it varies too much in brightness, the planet could be irregularly baked or frozen. If it is too active, then massive solar flares^[13] could strip the planet of its atmosphere and oceans.
Massive stars are too short-lived to keep their planets in the habitable zone long enough for life to evolve. A habitable planet may not orbit a star over ~1.5 times^[14] the Sun's mass.
Small, dim stars have close-in habitable zones where massive tidal forces cause planets to become tidally locked^[15], permanently pointing one face towards their sun. Water and air would freeze on the permanent night side, causing the oceans and atmosphere to collapse^[16]. A habitable planet may not orbit a star below ~0.7^[16] times the Sun's mass.
A habitable planet must have a circular orbit which stays in the habitable zone at all times, ensuring its temperature remains stable. This orbit must not be disrupted by the gravitational influence of other planets. Many real exoplanets have significantly elliptical orbits^[17].
A habitable planet must have a massive outer neighbor like Jupiter to defend it^[18] from errant asteroids.
If a planet is too small, its weak gravity will be unable to hold onto water vapor and its oceans will escape^[19] to space, as Mars's did.
If a planet is too large, it will accumulate a thick atmosphere of hydrogen and helium and become a gas giant^[20]. Gas giants lack surfaces on which life can develop.
A habitable planet must have a magnetic field to protect^[21] its atmosphere from solar radiation. Of the rocky planets in our Solar System, only Earth has a strong magnetic field.
A habitable planet must have plate tectonics, to drive the climate-stabilizing carbon cycle^[22]. Of all the planets in the Solar System, only Earth has large-scale tectonics.
A habitable planet must have a large moon to stabilize^[23] its axial tilt, preventing massive climate fluctuations. Of the Solar System's planets, only Earth has a moon large enough to do this.
A habitable planet must have water for life to develop in, but not so much water that its surface is drowned^[24] in globe-spanning oceans.
A habitable planet must not experience frequent mass extinctions, which would prevent life from becoming complex.

I could waste my time trying to find bounds on every one of these parameters and throwing them into a modified Drake equation, but I think it's safe to say that the sheer number of them ought to reduce the proportion of habitable planets by several orders of magnitude. Even just the gas giant criterion eliminates all but 30^[7] of our 176 original candidates, since planets are thought to transition from rocky to gaseous at about 6 Earth masses^[20].

The Rare Earth hypothesis may paint a hopeless picture of a barren, lifeless universe, but there is good reason to believe it may not be correct. New data and better computer modelling have in many cases overturned salient Rare Earth criteria:

The assertion that only quiet spiral galaxies can develop life may be overblown. New analyses of the Milky Way's stellar populations by ESA's Gaia telescope reveal that its last major collision with another galaxy occurred no more than 3 billion years ago^[25], well after microbial life on Earth had emerged^[26]. Since we are alive, it is evident that at least simple life forms can survive such events.
Even the most active supermassive black holes do not^[27] produce enough radiation to significantly impact the majority of planets lying more than a few thousand light-years away from them.
The inner edge of the galactic habitable zone may lie so deep in the galactic core as to be nonexistent - as stellar density increases, so too does planetary density, so some Earth-like planets will fortuitously survive^[28] the numerous cataclysms of the core no matter how many more are destroyed.
The outer edge of the galactic habitabe zone may also be meaninglessly broad, as it turns out that even stars on the outer rim of the galaxy with very few metals^[29] can still form rocky worlds.
Though it has been suggested^[30] that the Sun's crossings of the Milky Way's spiral arms cause mass extinctions by way of asteroid impacts or supernova radiation, only^[31] the Cretaceous-Paleogene mass extinction is definitively linked with an impact, while no extinction^[32] is definitively linked with supernovae.
It turns out that planets orbiting binary stars can and do have stable orbits^[33] and climactic patterns^[34].
It turns out that tidally-locked planets can have much milder climates^[35]^[36] than usually depicted. Even very thin atmospheres can transfer enough heat to the night side to prevent the oceans from freezing completely^[37].
It turns out that even planets with very elliptical orbits can sustain habitable conditions^[38].
Jupiter only protects Earth from asteroids that are problematic because of Jupiter^[18]. If it didn't exist, the impact rate might not be elevated significantly^[18].
Magnetic fields are not effective^[21] at protecting planetary atmospheres, and can actually make it easier for an atmosphere to escape in some circumstances^[39]. Consider Venus, which has an atmosphere 90 times thicker than Earth's but no magnetic field.
The Moon does stabilize the Earth's axial tilt somewhat, but even without it the variations would not be exceptionally extreme^[23].
The Moon also slows down the Earth's rotation. Were it not to exist, Earth would rotate fast enough^[23] to be stable on its own.
Life on Earth first appeared no more than 500 million years^[26] after the Earth formed and maintained a continuous lineage to the present day in spite of numerous apocalyptic events.

All of the Rare Earth criteria that were overturned suffered from the same weakness - they assumed that Earth's characteristics were vital for life. While this isn't unreasonable, since Earth is the only living world we know of, we shouldn't immediately assume that all of its unique and improbable characteristics are strictly necessary for all forms of life. We evolved to fit this planet, with its freakishly large moon, shifting crust and powerful magnetic field. What right do we have to assume that alien lifeforms could not find a way to do without?

Icy moons, like Saturn’s Mimas, Enceladus, and Rhea here appear nothing like Earth. But might worlds as strange as these be suitable habitats for life?
(Image: NASA/Cassini)

Much of the astrophysical community is a lot more enthusiastic about the prevalence of life-bearing planets than the 'Rare Earthers', and indeed there are numerous reasons to believe that worlds capable of hosting at least simple lifeforms are much more common than our 3% estimate. Central to most of these arguments are extremophiles, organisms capable of surviving conditions wildly different to what we are used to. Many of them might find the extreme conditions of the planets and moons of our own Solar System quite homely^[40], let alone the comparatively clement ones of even the most alien habitable-zone planets. Indeed, they make a case against the usefulness of the habitable zone concept altogether, since they might thrive on worlds we would find unimaginably hostile. In fact, there are extraterrestrial locales we know of right now which could conceivably sustain Earth extremophiles. Let's look at a few.

Plumes of water and snow spew from Enceladus's south pole in this Cassini image. Chemicals detected in these emissions reveal the tantalizing possibility of life-sustaining hydrothermal vents beneath the icy crust.
(Image: NASA/Cassini)

First on our impromptu cosmic road trip is a little moon in our own cosmic backyard. Snow-white Enceladus is airless, irradiated and cold enough to freeze air. But while its surface may be an inhospitable wasteland, it conceals a cold, oxygen-rich^[41] ocean teeming with hydrothermal vents^[42] powered by intense volcanism. Similar vents on Earth are home to an enormous array of life from giant tubeworms to ghostly octopuses to iron-shelled snails that feed off of microbes which consume volcanic gas for life-giving energy, allowing them to live independently from sunlight. Were they transported to the lightless oceans of Enceladus, they would hardly notice a thing.

The azure clouds of K2-18b shine in the dim light of its red dwarf sun. Could life find a way on this hydrogen-swathed world?
(Image: Artist's concept for ESA/Webb)

Further afield, weirder worlds offer curious possibilities. 124 light years away lies the mini-Neptune K2-18b, a possible ocean world^[43] swathed in a thick hydrogen atmosphere^[44] thousands of times denser than ours. Though we would be crushed instantly if we tried to step out onto its surface, there are other lifeforms that might not mind - bacteria like Colwellia marinimaniae^[45] can thrive at pressures of over 1,200 Earth atmospheres. Nor would life be restricted to the high-pressure surface of such a world - on Earth, numerous species of microorganisms and small animals may drift in the air as aeroplankton. On a planet with a much thicker atmosphere, some lifeforms might even take up the sky-high lifestyle permanently.

A desert vision of the hot, massive super-Earth Kepler-452b. Could a world as different as this be a home for life?
(Image: Artist's concept for NASA/Ames/JPL-Caltech)

And yet even stranger places might be equally capable of supporting life. At some 1,800 light-years from Earth, Kepler-452b is by far our most distant hypothetical locale. This massive, dense super-Earth orbits slightly too close^[7] to its star; were Earth relocated to its orbit, we would end up as a Venusian hellhole in short order^[4]. But a massive planet should hold onto a thicker atmosphere, so the increased surface pressure could raise the boiling point of water^[46]. The replete volcanism of a large planet imight also add sulphuric acid to the oceans, further raising the boiling point^[47]. It is conceivable that a planet like this could sustain liquid oceans at surface temperatures well over 200°C! Such conditions may not be insurmountable for life. Hydrothermal vent archaea like Methanopyrus kandleri can survive and reproduce crushing pressures and temperatures as high as 122°C(!)^[48], while hot springs host microbes like Picrophilus torridus which shrug off concentrated acids^[49] and near-boiling water temperatures. Such lifeforms would find themselves quite at home on the steaming, acidic surface of Kepler-452b, no matter how impossible it might sound to us.

Though the range of extremes we have explored thus far may seem impressive, we have only used the limited biodiversity available to us on Earth to populate them. An intriguing proposal exists that may expand the habitable universe by orders of magnitude - that of alternate biochemistries. All known life on Earth constructs its genome from DNA, its enzymes from left-handed amino acids, and its food from right-handed sugars. On a deeper level, all Earth life is reliant on water as a solvent to facilitate its carbon-based chemistry. Some scientists have proposed that this is not the only way to construct a living organism, and that alternative chemicals could be substituted to create a plethora of bizarre forms.

At the most basic level, it is possible to construct one's genome out of an information-carrying molecule that is not DNA. On Earth, some viruses already do this, using RNA to carry their biological instructions instead. Though RNA and DNA behave very similarly, other nucleic acids have significantly different properties. Glycol nucleic acid (GNA) is exceptionally stable at high temperatures^[50] and could allow for life on very hot worlds like Kepler-452b, while genomes made from peptide nucleic acids (PNA) resist acidic environments^[51]. Though not as common in the cosmos as DNA and RNA, these genetic codes can form in nature^[52] and could conceivably become the basis of life in environments^[52] where their common counterparts fail.

At a higher level, the medium in which the reactions of life occur can be changed as well. Water is useful for life because it is a powerful solvent that can carry all sorts of biochemicals, but water-like chemicals such as ammonia^[53] and sulfuric acid^[53] might be able to do its job. These substances are liquid under vastly different temperature ranges than water is, so they could permit life on planets far too hot^[54] or cold^[55] for water oceans.

Even hydrophobic liquids like gasoline or liquid methane could conceivably support life of a very different variety. Though not as good at dissolving things, they are far less chemically harsh^[56] than water is, allowing life to make use of a whole world of delicate molecules to cope with the unconventional environment. The plastic resin acrylonitrile is even known to form cell-like structures^[57] in certain hydrophobic solvents. Life built from this exotic biochemistry would inhabit worlds devoid of liquid water, such as Saturn's frigid moon Titan, which is famous for its methane seas. It has been proposed that this type of exotic organism would convert acetylene and hydrogen gas into methane and indeed there is something^[58] on the surface of Titan that does just that!

Finally, some particularly speculative hypotheses propose that life may not need to be based on the chemistry of carbon at all. While carbon is unique among the elements for its tendency to bind to itself in large, complex molecular structures, some authors have proposed that other elements may be able to mimic its behavior adequately enough to sustain living systems. Silicon, which is chemically similar to carbon, is a common contender^[59]. Boron, carbon's neighbor on the periodic table, is even more versatile in certain ways^[60]. In extremely high-temperature conditions, even normally inert metals will become chemically active and link up with silicon and oxygen in complex structures speculatively permissive to life^[61]. Organisms based on boron or silicon might be exceptionally resistant to cold, inhabiting environments like Titan's freezing lakes, while metal-based lifeforms would be so heat-resistant as to be able to live in molten lava.

Unfortunately for our speculations, each of these exotic biochemical systems has its weaknesses - silicon forms only weak bonds with itself and instead prefers to form inert crystals^[59] with oxygen, boron is rare in the universe^[62] and its compounds are dangerously explosive in the presence of oxygen, while high-temperature metallic chemistry is so poorly known it is unclear whether life would be possible at all. If they truly exist, non-carbon based lifeforms would be universal rarities, relegated to the toughest and most hostile of environments.

Regardless, it is evident that life is a lot more adaptable than we give it credit for. In the end, Drake may have had the right idea - it will always find a way.

Convergence vs. Contingency

Evolutionary Solutions to the Fermi Paradox

How often do complex lifeforms actually develop on habitable worlds?
(Image: Me, via SpaceEngine)

Our examination suggests that Drake’s first assumption is correct. Life, at least in its simplest form, is an adaptable and resilient thing that quite possibly pervades the universe. From that, Drake concludes:

All life-supporting planets inevitably develop a transmitting civilization.

But there is a vast gulf between simple microbe and thinking man. Is this gap surmountable? Let’s look at the data.

The view of evolution as a sequence leading up to humans is common in popular culture. But is it accurate?
(Image: March 11, 1876 Scientific American, p. 167)

Drake assumed that intelligent beings would inevitably^[63] emerge in any sufficiently advanced ecosystem. He, along with fellow astronomer Carl Sagan and paleontologist Simon Conway Morris, argued in the late 1990s to early 2000s that convergent evolution^[64] would ensure that something would eventually fill the 'intelligence niche' that humans occupy on Earth. They cited^[63] a trend of increasing complexity and intelligence among organisms from protozoa to invertebrates to reptiles to mammals to man, which they reasoned was a natural consequence of the complexficiation of ecosystems through Earth's history. Sagan in particular suggested^[63] that high intelligence is advantageous for all animals, so natural selection would eventually push all life to sapience. They also argued^[65] that statistically, Earth is probably a typical^[66] member of the habitable planet population, so we should expect that other planets are populated by lifeforms similar to humans.

It might seem intuitive that human intelligence would be advantageous to evolve, given how completely we have dominated our planet's ecosystems. But as with everything, context is necessary. To get that context, let's go back to the beginnning.

The primitive vertebrate *Haikouichthys ercaicunensis*. Was its descendants' rise to dominance inevitable, or simply a stroke of fickle fortune?
(Image: Nix Illustration, Creative Commons 4.0)

It is 518 million years ago, in the shallow seas of southern China.

You are wading through the warm waters of a shallow reef in what is today the Maotianshan Shale of Yunnan. Though the sun-soaked beaches and azure waves would not be out of place in the Bahamas of today, looking closer reveals an almost alien world. There are no coconut trees lining the shoreline, nor hermit crabs scuttling through the spray, nor colorful corals towering beneath the surface. The air is devoid of the cries of seabirds, the buzzing of insects, or the smell of sun-dried kelp, for none of those things had yet evolved^[67]. The sunshine is warm and mild, but you don a full-body wetsuit and air tank to keep out the unbreathable atmosphere^[68]. It will be another 50 million years^[69] before plants first crawl out of the ocean and pump the air full of life-giving oxygen. In the meantime, the Sun's rays bake the land into a lifeless Mars-scape.

But as you swim further from the lifeless beach, a whole world of strange life reveals itself. Forests of tubular Leptomitus sponges interweave with iridescent fireworks of proto-comb jellies Daihua and Stromatoveris and fields of tentacled Facivermis. Above and around them swarm the mobile inhabitants of the ancient reef - armored palaeoscolecid worms Maotianshania and Cricocosmia burrowing through the mud, multi-legged proto-arthropods Hallucigenia and Megadictyon clambering over the sponges, shrimp-like Synophalos and enigmatic Nectocaris leisurely drifting with the currents. Darting through the crashing waves are swarms of tadpole-tailed Beidazoon and proto-fish Haikouichthys, schooling tightly as fierce, predatory arthropods Lyrarapax and Amplectobelua use their powerful compound eyes to single out the weak and sick before ripping their targets to shreds in a flurry of powerful fins and lashing claws. Eoredlichia trilobites scuttle through the invertebrate forests secure in the knowledge that nothing has yet evolved which can crush their stalwart armor, while giant, filter-feeding Houcaris paddles unmolested through clouds of plankton, its two-meter body too large for even the most fearsome predator of this time to harm.

Plenty of alien-like organisms have inhabited Earth over life's 4 billion years of existence. We are all variations of a single template, warped on the surface but ultimately consistent throughout the ages. What would lifeforms truly different from us be like?
(Image: Creative Commons/Zhixin Sun, Fangchen Zhao, Han Zeng, Cui Luo, Heyo Van Iten, Maoyan Zhu)

Though these animals will never know, their fates will diverge soon. Five million years from now, an ice age^[70] will come for Earth, chilling these tropical reefs to extinction. Agile Lyrarapax, ferocious Amplectobelua and titanic Houcaris will meet their ends^[71], while strange Beidazoon will struggle through only to disappear some 5 million years later^[72]. Others, like Hallucigenia and Maotianshania, will march on slow and steady^[73] for another hundred million years. But for yet others, like the Eoredlichia and Haikouichthys, the ice age will be an opportunity. Freed from competition, their descendants will spread and diversify to dizzying heights^[74], founding vast biological dynasties that will change the world forever.

But on this day 518 million years ago, you do not know any of that. If you tried to predict whose descendants would dominate the world half a billion years in the future, would you even consider soft, tiny Haikouichthys over Lyrarapax or Amplectobelua? Drake, Sagan, and Morris would suggest so^[64]. According to them, Haikouichthys's vertebrate body plan had inherent advantages over its rivals which would ensure its future success. The oncoming ice ages would inevitably pare away the tyrannical arthropods, leaving the world for its silver-scaled descendants to inherit and conquer. Even if by chance its lineage did not survive, Drake, Sagan, and Morris would argue^[64] that fish-like animals would still inevitably dominate the oceans, as restrictive selective pressures forced other groups into the same roles. Perhaps Beidazoon would sprout eyes and drop its armor or Nectocaris would lose its tentacles and grow some scales, but in the end it would be the same. Give it a half billion years and basically-humans with big heads and long arms and upright bodies on two legs would still show up.

*Velociraptor* was a small, nocturnal ambush predator functionally similar to medium-sized cats. Notably, it looks nothing like a cat.
(Image: Prehistoric Planet S1:4)

Unfortunately for Drake, Sagan, and Morris, both fossil and living organisms argue against the inevitability of intelligence. Morris in particular argued strongly that evolutionary pressures are predictable enough that organisms filling the same ecological functions should look and act roughly the same^[64]. This is true to some extent. Eyes, Morris's favorite example of convergent evolution, have evolved in lineages as distant as vertebrates, mollusks, jellyfish, and dinoflagellates. However, most of the truly incredible examples of body-plan convergence Morris points to only occurred because the ancestors of the organisms involved were already physically similar. Of the "many attempts of Nature to evolve a crab", for instance, all save one occurred in a single group of crustaceans, the Reptantia, whose common ancestor was a lobster-like thing easy for selective pressures to reshape into a crab^[75] The crab shape, along with numerous other claimed convergences, is therefore mostly a product of lineage and not environment. Though convergence is undoubtedly important, I personally think the evidence is insufficient to justify Morris's extensive application of the concept.

Evolution has no goal^[76], so every adaptation a lineage goes through must provide some advantage^[77] in the environment at the time it appears. As such, organisms adapting to a certain ecological role are only pressured to change the parts of their anatomy which are important for that role while ancestral traits that don't get in the way remain. For some animals, such as aquatic swimmers, there are legitimately few solutions for moving efficiently around the environment and so we get very similar fish-shapes across groups as distant as lizards and sea slugs. For others, the restrictions are much less severe; Dryosaurus, ostriches, kangaroos, and horses look basically nothing alike despite being functionally identical medium-sized, quick-footed herbivores. Morris argues that the niche of 'intelligent being' is relatively restrictive^[64] such that its occupants should evolve to be human-like regardless of ancestry. However, dolphins, parrots, crows, octopuses and elephants don't resemble us much at all, suggesting that relatively high intelligence at least may be useful for a variety of ecological roles and not just the one that humans occupy.

Drake and Sagan's proposal that intelligence has steadily increased throughout evolutionary history doesn't really hold water either. While there are certainly some very intelligent animals today, people forget that animals are only a very small part of global biodiversity. There are somewhere on the order of 5-20 million animal species compared to upwards of 100 million protists and 1 trillion bacteria^[78](!!!), none of whom are particularly intelligent. Sagan argued that since bacteria and protists are our ancestors, they did evolve intelligence^[63], but this argument makes the mistake of assuming living protists and bacteria are primitive forms unchanged for billions of years and not cousin lineages who have been playing the game of life for just as long as we have. Considering that bacteria can evolve to overcome the numerous poisons^[79] we throw at them in days while we vertebrates struggle to resist a few degrees of global warming despite all our smarts, perhaps being unintelligent is the better strategy!

Earth's history also argues against the common view of the evolutionary march of progress towards bigger, better, and smarter. Supposedly 'superior' lineages have consistently languished in the shadows while their 'inferior' counterparts ruled the world, while numerous powerful and sophisticated creatures have been extinguished by chance misfortune. Lyrarapax and Amplectobelua, rather than being primitive failed experiments that would inevitably be outcompeted by the fishes, were active, sophisticated animals who could hold their own against vertebrate rivals. Their swimming style was far more stable and efficient^[80] than that of finless Haikouichthys, while the power of their eyes would be unrivaled^[81] until dragonflies evolved 200 million years later. Dinosaurs, the grandest sovereigns of natural history, were held down^[82] by bulky crocodile-relatives until the splitting of Pangaea climate-changed the crocs away, while modern mammals were overshadowed by those same dinosaurs^[83] and their own sibling lineages^[84] until a random space rock killed every warm-blooded animal bigger than a cat. Even today, mammals dominate the terrestrial megafauna even though birds are smarter on average^[85].

Finally, Drake, Morris, and Sagan's claim that an 'intelligence niche' exists is not as rock-solid as it might seem. We share millions of years of shared history with even the most removed Earth bacterium, while we are related to alien life only by shared chemical overtures, if that. If convergence towards intelligence is powerful enough for it to arise independently in numerous completely unrelated lifeforms on different planets, as Drake and Sagan claim, would we not expect it to be even more common among Earth animals closely related to known intelligent lineages (us), who already possess anatomy we know can be modified into an intelligent form? Terrestrial ecosystems have been essentially modern^[86] since the Carboniferous period, yet in the 300 million year-long period that gives us we find only a single example of civilization - ourselves. Corvids (crows, jays, etc.) and dolphins are of comparable intelligence to ourselves and have existed for over 10 million years^[87]^[88], but neither of them have managed to construct a technological civilization.

If the evolutionary conditions that produce civilization-building species are rare enough that they only appeared once on Earth, what reason do we have to expect them to be common on alien planets? At least on the point of intelligence, I think it is safe to say that Drake was wrong.

Ozymandias, King of Kings

Our modern technological society is built on the fossilized remains of ancient rainforests and tropical reefs. On a world without them, could our equivalents achieve nearly as much as we?
(Image: Me, via SpaceEngine)

For the sake of argument, let’s assume that intelligent life manages to evolve. Once it does, will it follow our path and expand to the stars? Drake says:

All intelligent life becomes technologically advanced and is detectable for thousands or millions of years.

Is that true? Let’s consider some possibilities.

Are they advanced?

The controlled use of fire is typically regarded as the first of the great human inventions. While numerous animals and plants use natural fires to eliminate competition or flush out prey, only humans and some Australian raptors^[89] intentionally spread them and only humans can start them. Beyond its role in cooking food and fending off predators, the mastery of fire allowed humans to construct complex tools from materials such as ceramic and metal^[90]. Without fire, the great revolutions of agriculture, industry, and information could have not possibly occurred^[91], and we would be stuck clubbing rabbits on the African savanna no matter how smart we became.

Now imagine intelligence emerged in the icy seas of Enceladus which we explored back in Section 1. Perhaps octopus- or dolphin-like beings call these lightless waters home, sandwiched between volcanic rock and kilometres of ice. Like their less-intelligent kin, they would huddle around the life-giving warmth of hydrothermal vents and the ecosystems they support. The vents are more than hot enough to cook food, but are much too cold to purify or forge most metals^[92]. Our space octopuses or space dolphins would be restricted to stone, shells, and a few native metals such as gold or platinum. They would still be capable of constructing complex societies but without workable metals they would never develop the industrial base^[93] needed to leave Enceladus or broadcast their existence to aliens (us). They might not even realize that there was a world outside of their sealed ocean!

Numerous planetary circumstances can make advanced civilization difficult or impossible. Fire is impossible in an atmosphere less than 16% oxygen^[94], trapping even terrestrial sapients on planets with such atmospheres in a perpetual stone age. Intelligent species on ocean or desert worlds would be scattered across habitable oases or islands separated by long stretches of inhospitable terrain, which might discourage the long-distance commerce and information exchange critical to innovation on Earth^[95]. Planets younger than Earth or those with unfavorable geologic histories might not have the fossil fuels that burgeoing industry relies on; Earth's vast coal and oil deposits were formed by fortuitous arrangements of Carboniferous swamp forests^[96] and shallow seas^[97], respectively, and did not begin forming until 'just' some 300 million years ago. Even if everything is fine with the planet, the anatomy of the planet's inhabitants may not be permissive; dinosaurs ruled the Earth for over 100 million years, but happened to have extremely inflexible hands^[98] that would likely make fine toolmaking difficult^[99]. Carnivorous sapients would also struggle, since preindustrial animal agriculture might not be enough^[100] to support a sedentary population large enough to necessitate technological advancement.

Even on Earth, we find examples of civilizations trapped in pre-industrial states due to circumstances beyond their control. The megafauna of the Americas was devastated by the end-Pleistocene extinctions^[101]. While Eurasians could yoke oxen as labor for high-intensity agriculture and ride horses to exchange goods and ideas, Americans were stuck with dogs and llamas. With no means to project power or trade across continents, there was never any incentive^[102] to invent things like iron tools, natural science, or market capitalism that would promote the development of industries critical for spaceflight.

Considering the preponderance of factors that can slow or totally stop the technological advancement of an intelligent civilization, it seems rather absurd to assume that everyone must become advanced enough to build radio and communicate with us.

Can they survive?

Once a civilization collapses, its traces are rapidly removed from its former habitat by natural processes. Were we to disappear today, for example, all signs of our existence would be gone by ~5 million years with the exception of microplastics in Holocene sediments, radioactive isotope anomalies caused by 1950s atmospheric nuclear testing and the various defunct space probes floating around the Solar System. As a result of this, our ability to detect alien civilizations is strongly related to how long such civilizations last on average - if they are short-lived, then the likelihood of one nearby being around for us to detect is low and vice versa.

We know very little about how long advanced civilizations should last. Our species has been in existence for some 200,000 years while our current global industrial society has existed for some 200. Despite the efforts of internet doomsayers, we have discovered no sign of an impending human extinction, though there are many things that could conceivably wipe us out or at least terminate our global, industrial (hence detectable) civilization.

We could spitball about killer asteroids, nuclear wars and anthropogenic climate change all we want, but our responses to those things are determined by a lot of public policy decisions which I don't want to even attempt to predict with any sort of rigor. However, we may not need to mire ourselves in politics and economics in order to get an answer to this question.

I

…But can we find them?

Even on a planet with all the conditions necessary for industrial civilization to develop, it may remain elusive if the anatomy or society of the native intelligent species are not permissive. A species that sees the world through echolocation might develop an industrial society but never discover the wider cosmos, for they would see the sky as an empty void. A flying species might lack the concepts of tribe and nation, spared from the vast conflicts that drive the need for technological supremacy. A hive species like ants or bees might advance rapidly in the face of brutal inter-hive aggression, but wipe itself out with the irresponsible use of apocalyptic superweapons. A solitary species, conversely, would struggle to get much of anything done.

Sociological conditions might prevent an advanced society from leaving its homeworld. Many people and institutions believe that space travel is not worth our time when social issues remain unaddressed. Or perhaps in the future as virtual reality advances we will lose our motivation to physically leave the Earth. Or perhaps we will die, laid low by natural disaster, internal strife, or technological irresponsibility. Or perhaps interstellar travel is simply too hard and we will never invent spacecraft capable of weathering the vast gulfs of void between the stars. In any case, a civilization that stays on its home world is limited in life.

…Or can they find us?

If a motivated society manages to establish an offworld colony, its survival across the millenia becomes much more assured. But that does not necessarily ensure that such a society will be detectable! It makes sense from an economic perspective to minimize the leakage of power and communications from your networks; aliens light-years away will probably not be interested in buying the products whose advertisements fund your TV coverage. Our current instrumentation is very limited in capability and if alien civilizations are not doing things like converting an entire star cluster's light output into waste heat, we will probably not notice them. However, we would expect such a society to eventually colonize every star or advance to a level where whatever they do is noticeable, but if intelligent and space-faring life appear rarely enough, it may be too early for

Or perhaps the solution is more sinister. If we make the (speculative) assumption that it has been long enough for intelligent civilization to become technologically advanced and extensively established in our galaxy, we must then conclude that intelligent life conceals its existence for some reason. One of the more compelling explanations is the Dark Forest hypothesis, which proposes that communicative civilizations are destroyed by others. Warfare between interstellar civilizations is likely to be highly asymmetric, as near-lightspeed projectiles are used to utterly obliterate enemy systems with no chance for counterplay, so an antagonistic civilization can easily and definitively destroy its enemies as soon as it learns of their precise locations. Since an anatagonistic civilization is unlikely to be forthright about its intentions, even ostensibly friendly communications cannot be trusted - at interstellar distances, it is impossible to spy on your neighbor to ascertain their true intentions. A civilization wary of this risk will never respond to hails, lest it reveal its own location and invite destruction. One that is paranoid about it will go about decisively eliminating the risk by preemptively destroying anyone who reveals themselves. Unfortunately, this means that a paranoid civilization is entirely indistinguishable from a hostile one! The existence of even a single paranoid civilization will encourage others near them to become cautious or even paranoid themselves, becoming more extreme as the paranoid civilization(s) wipe out those who don't get the memo. Eventually, paranoid civilizations become widespread across the galaxy, and so everyone either shuts up or gets killed. As we have not learned to shut up and are in fact actively shouting into the void in search of others, we may be in quite a bit of trouble if the Dark Forest is correct!

Retrospective

In the end, we have no way of really knowing what strange creatures may or may not inhabit the worlds we glimpse in our telescopes, but even our own planet’s storied history paints a less than inevitable picture of our own existence. Through Gouldian contingency or sheer misfortune, an alien civilization might well be directed upon a path towards undetectability or simply fail to exist at all, and even if it survives and thrives we need not expect it to become obvious to our probings.

It is not inconceivable that the universe is full of life. That strange parodies of plants and animals might be scattered through the vast cosmos, an uncountable number of evolutionary stories playing out in parallel across the stars. There might even be beings like us, intelligences able to ask themselves, “Are we alone?“

But we cannot expect them to come to us. Fortunate as we are to be able to see the wider universe for what it is, we must seek our cosmic brothers and sisters ourselves. And if we do not find them, then it is all the more important that we cultivate the spark of sapience we carry into a blaze that rivals the stars themselves.

All we have to do is look.

Footnotes

[1] For reference, see this.

[2] For example, consider Jones (1976).

[3] You can find Drake's original postulates here.

[4] As per Gómez-Leal et al. (2019).

[5] As per Kopparapu (2013).

[6] As of August 21, 2024. A dynamic list of confirmed and candidate exoplanets is kept at the NASA Exoplanet Archive.

[7] I searched the newest version of NASA's Habitable Worlds Catalog CSV for all planets satisfying the solar flux requirements. At the time of writing, it is accurate up to March 21, 2024.

[8] As per Hammer et al. (2007).

[9] As per Scharf (2012).

[10] Concept explored by Gowanlock et al. (2010).

[11] An overview of this concept is found in Sundin (2006).

[12] It's hard to find uncritical references to these ideas outside of Ward and Brownlee's original Rare Earth book, but a critical analysis of them (along with other Rare Earth hypothesis postulates) can be found in Kasting (2001).

[13] As per Vida et al. (2017).

[14] For a general discussion of the habitability of massive stars, see this article.

[15] Using Gladman et al. (1996)'s tidal locking formula, the habitable zone distance for stars of various masses, and Earth's historical tidal dissipation factor of ~100.

[16] As per Cohen (1981).

[17] As per Kane et al. (2012).

[18] For an overall analysis of this topic, see Horner and Jones (2010)

[19] This infographic provides an extensive overview of the various methods by which atmospheric gases may escape a planet.

[20] As per D'Angelo and Bodenheimer (2013).

[21] For an investigation into this topic, see Ramstad and Barabash (2021).

[22] As per Brownlee (2010).

[23] For an investigation into this topic, see Lissauer et al. (2012).

[24] As per modelling by Cowan and Abbot (2014).

[25] As per Donlon et al. (2024).

[26] As per Noffke et al. (2013).

[27] As per Lingam et al. (2019).

[28] As per Prantzos (2007).

[29] Exemplified by the 5-planet system of Kepler-444, which has less than a third of the Sun's metal content per Campante et al. (2015).

[30] As per Filipović et al. (2013).

[31] Koeberl et al. (2004) addresses the impact hypothesis for the Permian-Triassic extinction event, but similar impact hypotheses have been proposed for numerous major and minor extinctions. Few are as rigorously dated and confirmed as the Cretaceous-Paleogene.

[32] As per Barash (2014).

[33] Numerous planets in binary star systems have been discovered, the first of which was Kepler-16 b.

[34] As per Popp and Eggl (2017).

[35] Joshi et al. (1997) refutes the claim that atmospheric collapse is likely for tidally-locked Earth-like planets.

[36] As per Merlis and Schneider (2010).

[37] As per Hu and Yang (2013).

[38] As per Dressing et al. (2010).

[39] As per Sakai et al. (2018).

[40] For the survival of a complex terrestrial organism under simulated Martian conditions, refer to Lorenz et al. (2023).

[41] As per Ray et al. (2021).

[42] As per Postberg et al. (2018).

[43] As per Changeat et al. (2022).

[44] As per Madhusudhan et al. (2020).

[45] Described by Kusube et al. (2020).

[46] As per Ramirez (2020).

[47] According to the European Chemicals Agency report.

[48] As per Takai (2008).

[49] As per Schleper (1996).

[50] As per Egli et al. (2023).

[51] As per Zhao et al. (2015).

[52] As per Nelson et al. (2000).

[53] As per Bains et al. (2021).

[54] Investigated by Seager et al. (2023).

[55] Investigated by Martin et al. (2020).

[56] Consult the National Research Council's The Limits of Organic Life in Planetary Systems.

[57] As per Stevenson et al. (2015).

[58] As per McKay and Smith (2005).

[59] As per Petkowski et al. (2020).

[60] As per Scorei (2012).

[61] This was explored once in David W. Koerner and Simon LeVay's 2001 book Here Be Dragons: The Scientific Quest for Extraterrestrial Life, but the concept has received little attention since then. A class of metal compounds called heteropoly acids have been investigated as the basis of an alternative biochemistry, but they exist at moderate temperatures and are rare outside the lab.

[62] Since stellar nucleosynthesis goes straight from helium to carbon, boron is not produced by stars. It is instead generated by stray cosmic rays disintegrating heavier elements, so it is always a minority element wherever it is found.

[63] It's pretty hard to find Drake and Sagan's original arguments since the lecture documentation has long since dropped off the internet. However, this rebuttal does a decent job at repeating them (albeit in a critical manner).

[64] Morris makes his arguments in several books, none of which are available online. You can find a short review of one of them, Life's Solution, on JSTOR that I would say adequately summarizes Morris's views on the subject of convergence.

[65] As per Snyder-Beattie et al. (2021).

[66] This argument is known as the Copernician principle. Basically, if you pick a random data point from a large set, it will probably be close to the average of all the data. Since Earth is one data point picked from a (presumably) large set of life-bearing planets, we expect its properties to be reasonably typical of all life-bearing planets. Opposing the Copernician principle is the anthropic principle, which says that scientists who can do statistics will (obviously) inhabit planets where intelligent life can emerge. They cannot naively assume their planet is typical of all planets that have life because it must have conditions that allow intelligent life to exist no matter how rare those conditions are among the set of all planets that have any form of life.

[67] If you are curious, coconuts evolved ~66 million years ago, hermit crabs ~200 million years ago, modern corals ~240 million years ago, seabirds ~100 million years ago, flying insects ~326 million years ago, and kelp ~32 million years ago.

[68] As per Jiang et al. (2022).

[69] As per Rubinstein et al. (2010).

[70] As per Zhang et al. (2023).

[71] Amplectobelua and Lyrarapax are not known to have survived past 518 million years ago, but their lineage last appears in the fossil record with Guanshancaris at around the right time. The taxonomy of Houcaris is pretty messed up and we're not sure how long the genus itself survived for, but its family Tamisiocarididae last appears with Echidnacaris, also just before the ice age.

[72] As with Amplectobelua and Lyrarapax, Beidazoon itself isn't known past 518 million years ago. A relative, Banffia, is known from the 508 million year-old Burgess Shale, extending their range 5 million years post-ice age. A similar animal, Skeemella, is known from as late as 501 million years ago but it isn't clear whether it is actually related to Beidazoon and kin. All records are per McMenamin (2019).

[73] A close relative of Hallucigenia, Carbotubulus, is known from the 300 million year-old Mazon Creek, while Maotianshania's lineage, the palaeoscolecids, are found up to about 426 million years ago.

[74] Some 22,000 species of trilobite existed over some 270 million years. Vertebrates, as should be obvious, are alive and diverse today.

[75] Carcinisation is a parallelism, a phenomenon where similar environmental pressures force organisms with the same ancestral body plan to evolve a certain way. All 'crabs' evolved from squat lobster-like animals such as Eocarcinus, whose body plan doesn't take too many changes to turn into a crab. The one exception to this are the extinct Cyclida, whose closest living relatives are the entirely un-crab-like copepods.

[76] For a pretty good explanation of how evolution actually works moment-to-moment, see this article by Nature.

[77] Some neutral or harmful traits like sickle cell anemia in humans are linked to positive ones like malaria resistance and can be brought along for the ride, while neutral mutations can get 'fixed' by random drift in small populations. But the overall phenotype of those traits is still selected because of its immediate utility. For example, dinosaurs evolved feathers to keep warm - the fact that feathers can be used to fly didn't matter when the first fuzzy dinosaur was busy not freezing to death on a cold Triassic night. All 'intermediate' features like protofeathers and proto-eyes were very useful for their bearers at the time they evolved, even if their descendants use theirs for different purposes.

[78] As per Wiens (2023).

[79] As per Bonomo et al. (2024).

[80] As per Sheppard et al. (2018).

[81] As per Paterson et al. (2011).

[82] As per Brusatte et al. (2010).

[83] There no consensus on whether dinosaurs were in decline before the asteroid or not. There were some climate instabilities and sea level fluctuations during the latest Cretaceous that caused localized extinctions in North America, giant herbivores worldwide. However, dinosaurs overall had overcome similar crises in the past and we have very little fossil data on the latest Cretaceous outside North America, so it's probably wrong to say that all dinosaurs would have gone extinct even without the asteroid.

[84] As per Brocklehurst et al. (2021).

[85] As per Isler and Van Schaik (2009).

[86] As per Benton (2010).

[87] As per Mourer-Chauviré (2004).

[88] As per Murakami et al. (2014).

[89] As per Bonta et al. (2017).

[90] Some elements, such as copper, gold, silver and iron (as meteorites), occur in metallic form in nature, but fire is needed to shape them into usable forms. Furthermore, native metals are very rare so we couldn't use metal extensively until copper smelting was invented some 10,000 years ago (Murr 2015). We're not exactly sure how this happened, but no one can deny its utility to civilization.

[91] Numerous animals cultivate and harvest foodstuffs without the need for fire, so it is not strictly necessary. However, these animals are all small - for large animals like ourselves, fire is probably necessary to make the equipment etc. necessary to feed a decent-sized population.

[92] Hydrothermal vents are hot enough to melt tin and lead, but much higher temperatures are needed to extract them from their ores in the first place.

[93] Since the concept of a fully underwater advanced civilization is just really cool, there's quite a bit of discussion in various forums about how to make this work. Ideas like using the thermite reaction to make underwater fire or breeding electric animals to electroplate things get thrown around a lot, but most of them aren't particularly feasible. Thermite-type reactions can work underwater, but they need pure metals (e.g. aluminium) which wouldn't be found in a corrosive environment. Electric eels make enough energy to electroplate, but they can't sustain that output long enough to actually do it. Nor is it likely that space dolphins and co. could make solutions with the required chemical purity for electroplating to make anything useful.

[94] As per Belcher et al. (2010).

[95] Intuitively, an isolated community has less innovation power simply because it has fewer people to invent things in it. On Earth, the people of Easter Island developed a unique written script, but due to their isolation they did not spread it to any other Polynesian island (that we know of).

[96] Most (but not all) coal is from the Carboniferous and Early Permian periods some ~300 million years ago. It is often stated that those deposits formed because plants evolved wood that could not be broken down by decomposers and accumulated into coal, but newer studies find that this is not the case. Instead, coal production peaked during this time because forests were dominated by fast-growing, short-lived lycophyte trees that lived in a continuously wet environment on the east side of the Central Pangean Mountains.

[97] The famous giant petroleum deposits of the Middle East were formed at the bottom of the shallow Tethys Ocean, which occupied the area from 150 to about 14 million years ago, remaining as the Mediterranean, Black and Caspian Seas. Were continental movements different, the Tethys likely would not exist.

[98] Dinosaur hands are permanently supinated, so they cannot face their palms downward with their arms at their sides. This is even true for quadrupedal dinosaurs, whose hands just extended to touch the ground (VanBuren and Bonnan 2013).

[99] Some birds do manage to produce surprisingly sophisticated tools despite having the same inflexibility as other dinosaurs, so clearly inflexibility is not a deal-breaker for tool use. However, they would have issues working with heavy objects, causing problems for industry.

[100] Since livestock expend energy while they are alive, they use more calories of feed than they produce in meat - it takes 6 kg of feed to make 1 kg of beef, for example. A carnivorous civilization can support far fewer individuals per unit of farmland than an omnivorous or herbivorous one, which could discourage the formation of cities and the specialization of labor that drove technological innovation on Earth.

[101] As per Barnosky et al. (2004).

[102] Per Eichler et al. (2017), extensive metalworking in South America started some 2700 years ago. Despite this, no New World civilization ever phased out stone tools for metal ones.