The extraordinary low probability of intelligent life

Marc Defant gave a TEDx talk on the improbable events that had to happen in our planet’s history for us to eventually evolve, along with the implications for other intelligent life in the galaxy.

I find a lot to agree with in Defant’s remarks, although there are a couple points I’d quibble with.  The first, and I’m sure a lot of SETI (Search for Extraterrestrial Intelligence) enthusiasts will quickly point this out, is that we shouldn’t necessarily use the current lack of results from SETI as a data point.  It’s a big galaxy, and within the conceptual space where SETI could ever pay off, we shouldn’t necessarily expect it to have done so yet.

My other quibble is that Defant seems to present the formation of our solar system as a low probability event, or maybe he means a solar system with our current metallicity.  I can’t really see the case for either being unlikely.  There are hundreds of billions of stars in our galaxy, most with some sort of attending solar system.  So I’m not sure where he’s coming from on that one.

My own starting point for this isn’t SETI, but the fact that we have zero evidence for Earth having ever been colonized.  If the higher estimated numbers of civilizations in the galaxy are correct, the older ones should be billions of years older than we are.  They’ve had plenty of time to have colonized the entire galaxy many times over, even if 1% of lightspeed is the best propagation rate.

The usual response is that maybe they’re not interested in colonizing the galaxy, not even with their robotic progeny.  That might hold if there is one other civilization, but if there are thousands, hundreds, even a few dozen?  Across billions of years?  The idea that every other civilization wouldn’t be interested in sending its probes out throughout the galaxy seems remote, at least to me.

But to Defant’s broader point about the probability of intelligent life evolving, there are many events in our own evolutionary history that, if we were to rewind things, might never happen again.

Life seems to have gotten an early start on Earth.  Earth is roughly 4.54 billion years old, and the earliest fossils date to 3.7 billion years ago.  With the caveat that we’re unavoidably drawing conclusions from a sample of one planet’s history, the early start of life here seems promising for its likelihood under the right conditions.

But there are many other developments that seem far less certain.

One crucial step was the evolution of photosynthesis, at least 2.5 billion years ago.  The development of photosynthesis gave life a much more reliable energy source than what was available before, converting sunlight, water, and carbon dioxide into sugars.

And its waste product, oxygen, started the process of oxygenation, increasing the levels of oxygen in Earth’s atmosphere, which would be very important as time went on.  The early atmosphere didn’t have much oxygen.  Indeed, the rise of oxygen levels may have originally been a serious problem for the life that existed at the time.  But life adapted and eventually used oxygen as a catalyst for quicker access to free energy.

The good news with photosynthesis is that there are multiple chemical pathways for it, and it’s possible it evolved multiple times, making it an example of convergent evolution.  That means photosynthesis might be a reasonably probable development.  Still, oxygen producing photosynthesis doesn’t seem to have arisen until the Earth was more than halfway through its current history, which doesn’t make it seem very inevitable.

The rise of eukaryotes may be a more remote probability.  The earliest life were simple prokaryotes.  Eukaryotes, cells with organelles, complex specialization compartments, arose 1.6-2.1 billion years ago.  All animal and plant cells are eukaryotes, making this development a crucial building block for later complex life.

Eukaryotes are thought to have been the result of one organism attempting to consume another, but somehow instead of consuming it, the consuming organism entered into a symbiotic relationship with the consumed organism.  This low probability accident may have happened only once, although no one knows for sure.

Yet another crucial development was sexual reproduction, arising 1-1.2 billion years ago, or when Earth was 73% of its current age.  Sexual reproduction tremendously increased the amount of variation in offspring, which arguably accelerated evolution.  Who knows how long subsequent developments might have taken without it?

Oxygen had been introduced with the rise of certain types of photosynthesis, but due to geological factors, oxygen levels remained relatively low by current standards until 800 million or so years ago, when it began to rise substantially, just in time for the development of complex life.  The Cambrian explosion, the sudden appearance of a wide variety of animal life 540-500 million years ago, would not have been possible without these higher oxygen levels.

Complex life (animals and plants) arose in the last 600-700 million years, after the Earth had reached 84% of its current age.  When you consider how contingent complex life is on all the milestones above, it’s development looks far from certain.  Life may be pervasive in the universe, but complex life is probably relatively rare.

Okay, but once complex life developed, how likely is intelligent life?  There are many more low probability events even within the history of animal life.

Earth’s environment just so happens to be mostly aquatic, providing a place for life to begin, but with enough exposed land to allow the development of land animals.  In general, land animals are more intelligent than marine ones.  (Land animals can see much further than marine ones, increasingly the adaptive benefits of being able to plan ahead.)  A 100% water planet may have limited opportunities for intelligence to develop.  For example, mastering fire requires being in the atmosphere, not underwater.

Defant mentions the asteroid that took out the dinosaurs and gave mammals a chance to expand their ecological niche.  Without an asteroid strike of just the right size, mammals might not have ascended to their current role in the biosphere.  We might still be small scurrying animals hiding from the dinosaurs if that asteroid had never struck.

Of course, there have been a number of intelligent species that have evolved, not just among mammals but also among some bird species, the surviving descendants of dinosaurs.  Does this mean that, given the rise of complex life, human level intelligence is inevitable?  Not really.  While there are many intelligent species (dolphins, whales, elephants, crows, etc), the number of intelligent species that can manipulate the environment is much smaller, pretty much limited to the primates.

(Cephalopods, including octopusses, can manipulate their environment, but their short lives and marine environment appear to be obstacles for developing a civilization.)

Had our early primate ancestors not evolved to live in trees, developing a body plan to climb and swing among branches, we wouldn’t have the dexterity that we have, nor 3D vision, or the metacognitive ability to assess our confidence in making a particular jump or other move.  And had environmental changes not driven our later great ape ancestors to live in grasslands, forcing them to walk upright, and freeing their hands to carry things or manipulate the environment, a civilization building species may never have developed.

None of this is to say that another civilization producing species can’t develop using an utterly different chain of evolutionary events.  The point is that our own chain is a series of many low probability events.  In the 4.54 billion years of Earth’s history, only one species, among the billions that evolved, ever developed the capability of symbolic thought, the ability to have language, art, mathematics, and all the other tools necessary for civilization.

Considering all of this, it seems like we can reach the following conclusions.  Microscopic single celled life is likely fairly pervasive in the universe.  A substantial subset of this life probably uses some form of photosynthesis.  But complex life is probably rare.  How rare we can’t really say with our sample of one, but much rarer than photosynthesis.

And intelligent life capable of symbolic thought, of building civilizations?  I think the data is telling us that this type of life is probably profoundly rare.  So rare that there’s likely not another example in our galaxy, possibly not even in the local group, or conceivably not even in the local Laniakea supercluster.  The nearest other civilization may be hundreds of millions of light years away.

Alternatively, it’s possible that our sample size of one is utterly misleading us and there actually are hundreds or even thousands of civilizations in the galaxy.  If so, then given the fact that they’re not here, interstellar exploration, even using robots, may be impossible, or so monstrously difficult that hardly anyone bothers.  This is actually the scenario that SETI is banking on to a large extent.  If true, our best bet is to continue searching with SETI, since electromagnetic communication may be the only method we’ll ever have to interact with them.

What do you think?  Is there another scenario I’m missing here?

25 thoughts on “The extraordinary low probability of intelligent life

  1. I agree that the probability of intelligent life like ours in our galaxy seems to be low. But I don’t think the non-existence of intelligent alien life follows at all. But as you say, the strongest data point we have is that there are no aliens here on Earth right now, so it would appear to be the case that intelligent life in our galaxy is extremely rare, if not unique.

    Liked by 3 people

    1. Definitely there’s no reason to suspect intelligent aliens would be anything like us. Although there are limits to just how different they could be. They still have to exist within the same universe with the same laws of physics. Still, the probability that they’d look anything like Vulcans or Klingons is so low, we might as well regard it as zero.

      Liked by 2 people

      1. Very many wrong assumptions as i see it: first, the assumption that theres a relatively short way between existance of human level intelligent species and them passing the level of hunter gatherers and stone agers, not to mention ageicultural agers, bronze agers, industrial agers and atomic and digital agers, not talking about escaping their own planet gracitational pull or turning kardishev type 1 civilization, is ridiculously wrong. There are so many barriars to overcome, that has to do both with anatomical issues (the ability to physically create the needed tools for davelopment), ecological-zoological issues (chances that intelligence is ecologically and evolutionary so beneficial at all at that planet’s ecological system to a level that would let that species become dominant like in our situation. For example, if we lived alongside much more dangerous species like dino’s or giant mammals, we might struggle to survive so much that we would never have any time for development. Nowhere. Never. Another ecological issue is the ability to reach agricultural revolution in that planet’s ecological – botanical system/ working mechanism. It might not allow growing plants in an agricultural way because of a very specific type of) , social issues (chances that that intelligence species would have a high drive for being social. One assumption for why we and not the neandarthals “won” is that we were more social than them. Not more intelligent.) and planetary issues (abundance of metals near the surface, which without them no segnificant progress beyond agricultural revolution can be made, as well as more types of elements like germanium, option to create plastic like materials, and energy source like coal or oil, that are very necessary for digital and industrial revolutions. Oh and there is thr issue of the size of the planet. It is pretty much impossible to reach space at a super earth like planet, which is more than 2 times earth mass, without atleast a nuclear or antimatter engine, due to the too big gravitational pull that require exponentialy higher and higher amounts of fuel). So yea. Those are loads of very specific circumstances needed to occure to progress from “just” intelligent life forms to civilizational species that can develop.

        Another wrong assumption is that intelligent life forms won’t look like us. It is possible that biologically, the only path to become intelligent is in a relatively human like physiological forms.

        Liked by 1 person

        1. Ethann, I don’t disagree with the overall gist of your first paragraph. Just because an intelligent species evolved doesn’t mean it would necessarily be a civilization producing one. There are barriers they would have to overcome and there’s no guarantee they would.

          On your second paragraph, the problem is that we just don’t know. I totally agree that physical dexterity is just as important as intelligence. Producing a civilization needs both. Dolphins, whales, elephants, crows, and other intelligent species won’t produce a civilization because they can’t manipulate the environment.

          On the other hand, ants do a pretty good job at manipulating the environment, indicating that the humanoid body plan isn’t the only one that might work. On the other other hand, ants don’t have hands, which seems to preclude them ever making finely crafted tools, even if they had the intelligence for it.

          Liked by 1 person

  2. Probability has been defined as “the measure of our ignorance.” We say, for example, the probability of a coin toss coming up heads or tails is 50%. But if we knew all the mechanisms in a single coin toss—the pressure and angle on the coin as it takes to the air, the atmosphere and its forces, and all the other facts of Newtonian physics—it wouldn’t be a question of probability at all; it would be a matter of certainty whether the coin comes up heads or tails (a “probability of 1”).

    So the question of the probability of various events in the history of intelligent life appearing on this planet gradually erodes into a question of discovery of mechanisms. The probability of an acorn becoming a majestic oak tree might seem close to zero (a “miracle”) to the uneducated mind; but when the complex mechanisms of its development are filled in, it is not.

    (That’s not to say that fully explained phenomena aren’t “miraculous” in some sense.)

    Liked by 4 people

    1. I agree with everything you write here, but it seems like we’re still stuck with probabilities for now, and unfortunately not even numerically precise ones. I don’t think there’s anything miraculous about our evolution, just low probability. Go far enough in the universe, and I do think you’ll eventually run into another intelligent species, or at least the remains of one.

      Liked by 1 person

      1. I have little doubt that there are millions–perhaps billions–of intelligent species in this universe. But because of the nature of time and space we can’t go far enough into the universe to run into them or or discover their remains.

        (Sorry I can’t click a “like” to your reply to Steve Morris. Seems I have to be registered and have a password with WordPress or something like that.)

        Liked by 2 people

  3. Most people concentrate on the vastness of the space in the universe, but also consider the vastness of time. How long do you think a species can survive a basically hostile universe? If you guess 10,000 years, then any civilization we might be able to connect with can be no older than that. That all of the advanced species from further back in time would already be extinct.

    And, yes, life is improbable, but also quite tenacious. Consider all of the extinction events that occurred on just this one planet, with “life” basically having to “start over.”

    And, just how many other solar systems have we explored? I’ll tell you. None. we haven’t even explored our own. We do not know whether the Jovian moons support living species. We do not know if Mars does or did support living species. How the hell can we make intelligent speculation about such a topic. Oh, I guess all one needs to speculate is a mouth and enough money to buy your way onto a Ted stage (It ain’t cheap.)

    I think speculators should be required to list percentages of knowledge available that have been acquired to date. Plus, for those inclined to conclude supernatural powers are involved, please consider that life has been on earth for 4,000,000 years, human species for 2-3,000,000 years, Homo sapiens, our species, for maybe 200,000 years, agricultural civilization for 10,000 years and organized science for about 400 years. Give us another 10,000 years and we should have answers for many of these hypothetical questions. Oops, we may be gone by then. Oh, well.

    And for those who think it is a telling fact that the universe we live in seems to support life, I say, well if it didn’t we couldn’t draw that conclusion. In order for the idea of life to be wondered at, the fricking universe has to support it. Sheesh,

    Liked by 3 people

    1. Good point about the lifetime issue. It might be that thousands of civilizations have existed in the galaxy, but we’re the only living one right now. Still, assuming that any of those civilizations had any interstellar capability, you’d think we’d find ruins. Perhaps not on Earth with its tectonic geology, but on Mars or the moon where the geology has been stable for billions of years.

      Granted that we might still find something somewhere else in the solar system, but if there have been thousands of civilizations throughout the eons, it seems like at least some would have left relics where we would have seen them on bodies that we’ve extensively mapped to a fairly high resolution from orbit.

      I actually don’t think that life, in and of itself, is that improbable, only intelligent life capable of symbolic thought. Definitely there’s a lot we don’t know (and I think I was honest about that in the post), but I disagree that speculating based on what we do know is a waste of time. We do it all the time. We have to be careful in not inveighing against speculation only when we don’t like the logical conclusions.

      Liked by 2 people

  4. And then there is a singularitarian view which gives us the Highlander theory: there can be only one. This theory suggests that intelligent life is to the universe as fire is to a pile of oily rags. If you leave a pile of oily rags in a hot dry place, eventually you will get a spark (spontaneous combustion) which then produces flame that quickly consumes the pile. While a spark to start the flame is almost inevitable, the chance of a second spark happening after the first, but before the pile is in flames is negligible.

    Just one more of the “necessary coincidences” I’ve heard about is a large moon. The large moon in conjunction with a combination of oceans and lands gives you tides. Cycles of wetting and drying are good for producing the chemicals for starting life.


    Liked by 2 people

    1. I think there’s something to be said for the idea that the first civilization to arise in a region of the universe may end up dominating that region. Which means, we may be the elder race. But I think the speed of light limit will prevent that from applying to the whole universe. There may be other civilizations that just haven’t had time to reach us yet. Given the ongoing expansion of space, we may never get a chance to encounter any but a few of them, if any at all.

      I’ve heard that about the moon too. Who knows how common or uncommon a large moon is around Earth sized worlds? The list of similar attributes that might be crucial seems endless.

      Liked by 1 person

  5. I tend to agree with most of what you’ve said here. Life like ours and intelligent life like ours seems an unlikely event and shouldn’t be too widespread. A different chain of events may be possible but it is something we can’t comprehend or talk about.

    The unknown variables seem too many to make any realistic estimates, it could be that intelligent civilizations do evolve but never think about other extraterrestrial life to be bothered by it or perhaps they usually destroy themselves or they do exist and are out of reach or perhaps they do try to communicate but we don’t have right apparatus to pick them up (and vice versa).

    The biggest issue is as you say that we only have a single example which makes it difficult to induce any sensible ideas. It would be a monumental discovery if we were to ever find life within our solar system, that would mean it’s likely very prevalent. The specific chain of events that led to us could either be the key causal mechanisms or perhaps intelligent life is more inevitable and there are numerous other causal mechanisms for its development. Eventually, we are restricted by our own condition and ability to conceptualize, so perhaps we can (and do) only look for ‘intelligence’ as it is a human trait. If that is true then it is unlikely that there is any other ‘intelligence’ in our galaxy or even cluster, other forms of life should still be very possible.

    Liked by 2 people

    1. I’ve often wondered if an alien intelligence might be so radically different that we’d have trouble recognizing it as a fellow intelligence. What usually brings me back from that is that they’d have to live in the same universe we do, subject to the same laws of physics. That seems to put a constraint on just how different they could be, although given the variety of life on Earth, we should still expect them to be very different.

      Totally agreed that if we find life anywhere else in the solar system, and confirm that we or it didn’t seed the other, then it means that life is inevitable, and it pervades every corner of the universe. If we don’t find life anywhere else in the solar system, even in any of the underground oceans, then it means it’s not inevitable and probably only occurs in some fraction of the environments where it’s possible.


  6. After reading a book on exoplanets last week I was struck once again by Fermi’s paradox, and came to many of your conclusions. I also wondered if life itself may be rare, but as you said, we may know more as we explore our own galaxy. Or we may not, it could still be rather numerous but not quite have happened elsewhere in our solar system.

    I will say, if we are the first and only intelligent life forms in this vast, vast universe, it is a bit disturbing.

    One last thing, if humans have spread to multiple solar systems in sufficient numbers, and given that by then we would have conquered most of biology, I find it difficult to believe that we would somehow die out. Why would that happen?

    Liked by 3 people

    1. I tend to doubt that we’re the only intelligent species in the whole universe. I totally agree that it would be disturbing if we were. But I think it’s possible that our nearest neighbors are so far away that we may never have a chance to encounter them.

      I do think if we’ve spread to multiple solar systems, our chance for long term survival (measured in millions or billions of years) goes up substantially. I can think of things that might go wrong enough that an interstellar society could destroy itself, but the speed of light limit seems like it would prevent extinction from some short term conflagration.

      Liked by 1 person

  7. I’m inclined to agree with you: microbial life is, I think, fairly common across the universe. Complex life is rare, and intelligent life is rarer still. We may turn out to be alone after all.

    However, there’s a part of this that I keep coming back to, and it relates to that bit about the Solar System being rare. It wasn’t that long ago that people really did think star systems like ours might be rare or even that our Solar System might be unique. After all, until the 1990’s we had no clear evidence of planets orbiting other stars.

    Then all of a sudden, we started finding exoplanets everywhere. It goes back to that old line that absence of evidence is not evidence of absence. At some point, we may yet start picking up SETI signals, and once we have a better idea what we should be looking for, we might find alien civilizations are more common that we thought.

    Liked by 2 people

    1. Definitely we don’t know what we don’t know. I sometimes worry whether we’re the animals scurrying along outside of power plants or other technological installations, completely oblivious to the significance of what they’re seeing.

      Personally, I always thought the idea of the solar system being rare was unwarranted. In the absence of information, it made more sense to assume it was ordinary and that planets were common. I might have felt different if Earth were the only planet in the solar system. I don’t recall too many astronomers really crediting that notion too much. It was usually mentioned as a caution about assuming there were exoplanets in the absence of evidence. I know everyone was shocked by how prevalent “hot jupiters” turned out to be.

      Liked by 2 people

      1. I was young, so maybe I misunderstood, but people seemed to take the “our Solar System is unique” argument pretty seriously. I remember a science teacher telling us in class that if there were other planets out there, we should have found them by now. This would have been in the early 1990’s, so it was just before the first exoplanets were discovered.

        Liked by 2 people

  8. It’s not possible to calculate the probability of life at all given that science doesn’t know how the universe was created or why existence exists. That is, calculating a probability assumes that it is probabilistic. But science doesn’t even know what it is in the first place.

    Liked by 2 people

    1. Definitely coming up with rigorous precise probabilities isn’t possible. But we do understand a good deal of the chemistry that life is based on, and the prevalence of those chemicals in the universe, along with evolutionary history of life on Earth. I think we can come up with rough back of the napkin type probabilities by looking at what we do know.


  9. Congratulations, for the nice article.
    Have you ever considered cognitive diversity in our planet?
    It goes from unicellular creatures to humans.
    Apparently only humans possess a high level of intelligence and for us it seems that we have developed a great technological civilization.
    In fact, we underestimate the inferior organisms and we seek signals from extraterrestrial civilizations.
    We are infinitely primitive, we are just starting to discover the mysteries of the universe, from dark matter to the Higgs boson.
    We totally ignore other creatures on the planet.
    Consider this:
    here we are for E.T., tiny, insignificant organisms still uninteresting to be visited.

    Liked by 2 people

  10. Yeah, all good points that virtually every astronomer disagrees with.
    The logic they use always amused me.
    It’s like… There’s a blue rock on this planet & because there are so many planets there must be many blue rocks.
    The problem with that is the object they are looking for is the DNA molecule or something near its complexity. Not a blue rock

    So essentially, they are looking for the most complex object in existence but they completely disregard it and instead just look for a planet like ours instead. It’s preposterous.

    Just take something almost infinitely less complex like a Lincoln penny. For copper to somehow get shaped by time into the right shape and something resembling Lincoln’ face is so less complex than DNA that the probabilities wouldn’t even fit on the same page. A penny doesn’t reproduce, won’t eventually talk… Its nothing…. Yet I would bet there is not another in the entire universe. In fact most people would agree it would be absurd. Or a small rock that through time and collision looked exactly like you & your wife….which would be something like 10 to the 20000th more probable than DNA. Some things won’t form twice… Most, not even once.

    It shows the bias being used. It’s wish making, yet it’s proclaimed on every SCIENCE Channel show on TV as a fact. Boy, I love Star Trek. I’m a sci-fi guy through & through but this current cast of characters on supposed Science documentaries are embarrassing themselves to the point that they’re in a cult. They are supposed to about Truth but instead they look like unstable weirdos.


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