Is encephalization the great filter?

27 September 2015


Hominid encephalization reveals an exponential growth curve.

Hominid encephalization reveals an exponential growth curve.

The idea of the great filter was formulated by Robin Hanson. In the exposition below Hanson also names a number of steps (acknowledged to be non-exhaustive) in the development of explosively expanding life:

“Consider our best-guess evolutionary path to an explosion which leads to visible colonization of most of the visible universe… The Great Silence implies that one or more of these steps are very improbable; there is a ‘Great Filter’ along the path between simple dead stuff and explosive life. The vast vast majority of stuff that starts along this path never makes it. In fact, so far nothing among the billion trillion stars in our whole past universe has made it all the way along this path. (There may of course be such explosions outside our past light cone [Wesson 90].)”

Robin Hanson, The Great Filter — Are We Almost Past It? 15 Sept. 1998

Discussion of the Great Filter has focused on singling out one factor and identifying this one factor as the Great Filter, although Hanson is explicit that, “one or more of these steps are very improbable.” In the event that several steps in the development of explosively expanding life rather than some one single step is unlikely, the Great Filter may consist of several elements. I think that this is an important qualification to make, but at present I will adopt the conventional presumption that one step in the development of advanced civilization is improbable (or especially improbable) and constitutes the Great Filter.

Graph of the encephalization quotient of several mammals.

Graph of the encephalization quotient of several mammals.

What we know about the cosmos is consistent with it being rich in life, but poor in technologically advanced civilization. The more that we learn about exoplanetary systems (living, as we do, in the Golden Age of exoplanet discovery), the more our scientific understanding of the universe points toward a superfluity of habitable worlds (or, at least, potentially habitable worlds), even while no trace of intelligence has yet been seen or heard beyond Earth. Some of this may have to do with the amount of research funding that is channeled into astronomy and astrophysics in comparison to SETI research, which has received relatively little to date. This is about to change. A “Breakthrough Initiative” will be funneling a large amount of money into SETI — Breakthrough Listen — but there is no reason as yet to suppose that this effort will be any more successful than past efforts, though I would be quite pleased to be proved wrong.

Brain to body mass ratio is distinct from encephalization quotient (EQ).

Brain to body mass ratio is distinct from encephalization quotient (EQ).

The point that I made some time ago in SETI as a Process of Elimination still holds good: as our scientific instrumentation improves with each generation of technology, and our research methods become more sophisticated, we are able to exclude (and, correlatively, to include) an increasing number of possibilities and instances. In other words, progress in science comes about by falsifying certain hypotheses, as would be expected from a philosophy of science derived from the Popper-Lakatos axis. (It is often discussed in relation to SETI research that investigators are hesitant to publish negative results; perhaps if they better understood the crucial role of falsification in the methodology of the scientific research program that is SETI they would be more inspired to publish negative results.)

Comparative brain sizes of several mammals.

Comparative brain sizes of several mammals.

When, in the coming decades, we are able to obtain spectroscopic analyses of exoplanet atmospheres, our knowledge of what is going on on exoplanets — as opposed to merely knowing about their existence, location, size, orbital period, and so on, which is the kind of scientific knowledge we have only recently come into — will improve by an order of magnitude. At this point in time we will move from ne in the Drake equation (number of planets, per solar system, with an environment suitable for life) to fl (fraction of suitable planets on which life actually appears) and possibly also fc (fraction of civilizations that develop a technology that releases detectable signs of their existence into space, from which we can infer fi, fraction of life bearing planets on which intelligent life emerges) if exoplanet atmospheric signatures reveal signs of unambiguous industrial activity.

Frank Drake

We do not know the prevalence of life in our galaxy, much less in the universe at large — i.e., whether or not we live in a biota-rich GHZ, or even CHZ (cosmic habitable zone) — but we may soon be able to estimate the presence of life in the cosmos as we can now estimate the number of planets in the cosmos. It is entirely possible that the universe is teaming with life, even advanced life that is as sophisticated as the life of the terrestrial biosphere. I have written elsewhere that we may live in a “universe of stromatolites” (cf. A Needle in the Cosmic Haystack), but we may also be living in the universe rich in the ecological equivalents of sharks, koalas, and penguins. With one exception: the emergence of the cognitive capacity that makes abstract intelligence possible as well as the civilization that is predicated upon it.

Do we live in a universe of stromatolites?

Do we live in a universe of stromatolites?

In an earlier post, A Note on the Great Filter, I suggested that we are the Great Filter. I would now like to refine this: if I were to identify a “Great Filter” (i.e., a single element constituting the Great Filter) somewhere between plentiful life and absent advanced technological civilizations, I would put my finger on hominid encephalization. It was the rapid encephalization of our hominid ancestors that made what we recognize as intelligence and civilization possible. While there are many other large brains in the animal kingdom — the whale brain and the elephant brain are significantly larger than the human brain — and other mammals have brains as convoluted as the human brain — meaning more of the neocortex, which makes up the outer layer of gray matter — the encephalization quotient of the human brain is significantly greater than any other animal.


Brain size in absolute terms may have to exceed a certain threshold before intelligence of the sort we seek to measure can be said to be present. Neurons are of a nearly constant size, so the minimal neuronal structure necessary to control bodily functions take up about the same space in a mouse and an elephant. Factors other than sheer brain size are relevant to brain function, as, for example, the portion of the brain made up by the cerebral cortex and the amount of convolutions (therefore outer surface area, and the cerebral cortex is outer layer). Hence the introduction of encephalization quotient: encephalization quotient is not simply a ratio of brain mass to body mass, but is also based on the expected brain size for a given body plan — this introduces an admitted interpretive element into EQ, but that does not vitiate the measure. When, in the distant future, we can compare EQs over many different species from many different biospheres, we can firm up these numbers. Someday this will be the work of astroneurology.

The 'WOW!' signal -- fugitive signature of intelligence in an otherwise lonely universe? Perhaps astroneurology will someday study neural architecture across biospheres and arrive at a non-anthropocentric measure of intelligence that could account for something like the 'WOW!' signal.

The ‘WOW!’ signal — fugitive signature of intelligence in an otherwise lonely universe? Perhaps astroneurology will someday study neural architecture across biospheres and arrive at a non-anthropocentric measure of intelligence that could account for something like the ‘WOW!’ signal.

The human brain (with its distinctive and even disproportionate EQ) has not changed since anatomical modernity — at least a hundred thousand years, and maybe as much as three hundred thousand years — and human thought has probably not greatly changed since the advent of cognitive modernity, perhaps seventy thousand years ago. We must continually remind ourselves that even the earliest anatomically modern human beings had a brain structurally indistinguishable from the human brain today. With the blindingly rapid gains of technological civilization over the past hundred years it is increasingly difficult to maintain a sense of connection to the past, not to mention the distant past. But when the human brain appeared in its modern form, it was unprecedented in its cognitive capacity — it was and still is an extreme outlier. There was nothing else like it on the planet, and from this brain followed control of fire, language, technology, art, and eventually civilization.

. . . . .


. . . . .

Grand Strategy Annex

. . . . .

project astrolabe logo smaller

. . . . .

. . . . .

9 Responses to “Is encephalization the great filter?”

  1. xcalibur said

    Long ago, when I first started reading about astronomy, I theorized that exoplanets would be fairly common, and I’ve recently been proven right. This gives me confidence in my hypothesis on exobiology: life is common throughout the universe, but it’s like a pyramid. Simple lifeforms (microbes etc.) are very common or even ubiquitous, and as life gets more complex, it becomes less common. The capstone of sentience is probably rare. Then of course, the challenge of building civilization, and then advanced civilization, and not self-destructing, are further filters on ETI. Given this, I think the fermi paradox doesn’t really contradict alien life.

    I’m not surprised to see a high encephalization quotient for dolphins. I’m aware that they’re very smart animals – maybe not to the extent of hitchhiker’s guide to the galaxy, but they’re up there.

    • geopolicraticus said

      Agreed. The Fermi paradox isn’t really about extraterrestrial life per se as it is about extraterrestrial civilization, and moreover civilization of a particular kindthe kind of civilizations that build starships or engage in SETI.

      The average dolphin (and related marine mammals) brain is larger than the average human brain in absolute terms, but human beings come in at almost double of the EQ. As I noted, the EQ measure involves certain assumptions about the size of a brain relative to the body weight and size. Whales and elephants have even larger brains, and size does matter, because neuron size is nearly constant, but size is far from being the only consideration that matters. In any case, comparative study of neural architecture is fascinating and, I think, an insufficiently exploited window into encephalization and, by extension, intelligence.

      Best wishes,


  2. Erik Landahl said

    Hi Nick,

    Does the stagnation of the human EQ for the past 300,000 years imply a reasonable possibility of any of the following: a) humans have reached the ultimate EQ we will reach; b) the human EQ is sufficiently large to allow humans to survive passage through an encephalization-based great filter; c) because of the human EQ stagnation, we are at risk of not possessing a sufficiently high EQ to survive passage through an encephalization-based great filter; or d) Strong AI may be a necessity to create a level of intelligence sufficiently high to survive passage through an encephalization-based great filter.

    Thanks much.


    • geopolicraticus said

      Hi Erik,

      All of these are wonderful questions, all of which suggest further questions and avenues of research. I will give an extemporaneous response, but this is really a research program more than questions that can be given definitive answers at the present time.

      a) human beings in their present form (i.e., anatomical modernity) have certainly reached a stable plateau, and the selection we have seen in subsequent human evolution has been stabilizing selection, not a directional selection that would either increase or reduce EQ, but I don’t think we can rule out some selective pressure coming to bear that would further increase or reduce EQ. However, I think an unprecedented selection pressure is unlikely, as it is by definition unprecedented. Unprecedented events do occur, but not very often.

      b) My assumption when I wrote this post was that human beings and our civilization have already made it through an encephalization filter; however, now that you have asked the question and made my assumption explicit, the more interesting inquiry is to challenge the validity of this assumption. Might a species require even greater encephalization in order to achieve an interstellar civilization that would nullify the Fermi paradox, i.e., fill the universe with life and civilization so that both are no longer invisible on a cosmological scale? These questions make question a, above, newly relevant. If human beings can still experience further encephalization, despite limitations such as the size of the birth canal and the weight of our head on our shoulders, etc., we could yet pass through a more severely selective encephalization filter.

      c) I think my responses to b, above, partially address this question, but, yes, certainly, we could construct scenarios in which human beings, or indeed any intelligent species possessing disproportionate encephalization in relation to every other species in its biosphere, is at risk of ultimate extinction due to an inability to continue some kind of directional selection that would result in ongoing gains of encephalization.

      d) Strong AI would be a technological fix to the intrinsic biological-based limitations of human intelligence. Then, however, the feasibility and practicability of strong AI, and its particular possibilities and limitations, would then simply substitute for the above questions instead of questions about encephalization. I know that there any many people (and not only singulatarians) who are of the view that only AI-based intelligence survives and expands on a cosmological scale. On this cf. my post The Wilderness Hypothesis, in which I quote a Centauri Dreams comment to this effect on one of my posts.

      Thank you so much for your questions. You have given me a lot of think about.



  3. Shaun Farrell said

    EQ is not the development of technologically advanced civilisation. Visible colonisation of the visible universe requires the development of a particular type of civilisation with the physical capability to achieve it and the cultural imperative to seek to achieve it. Humans have had the mental capacity to create that for many millennia, they have had developed social structures of sufficient complexity to progress in that direction for several millennia. It is only in the last few centuries that one particular strand of civilisation developed in particular and unusual circumstances the mindset combined with the social structures conducive to such development. On the admittedly limited evidence of one planet and its batch of civilisations, technology and a culture of questioning inherent in the scientific method necessary for the development of a civilisation capable of such expansion is not a forgone conclusion.

    A highly EQ creature could well evolve on another planet and if a social creature, develop a civilisation, that civilisation may not be nor ever develop into one with the a technological bent nor one with the social imperative (whether by reason of their social structure or the inherent nature of the creature) to expand out of its native environment.

    The development of intelligence therefor may well not be the great or sole great filter.

  4. Gregor Hartmann said

    Hi Nick.

    People looking for an alternative to primate intelligence seem keen on dolphins, but cephalopods (such as octopi) are worth thinking about in connection with encephalization.

    Some of the research makes them sound creepily smart. They’re curious, they solve problems, they get bored, they play, they practice deceit. Behaviors we associate with humans.

    Found this on a Scientific American blog: “There’s a famous story from the Brighton Aquarium in England 100 years ago that an octopus there got out of its tank at night when no one was watching, went to the tank next door and ate one of the lumpfish and went back to his own tank and was sitting there the next morning. The aquarium lost several lumpfish before they figured out who was responsible.”

    Since these jokers live in the sea, they’re not going to be using radio waves to communicate, hence will not be detectable by SETI. But if there are lots of water worlds in the galaxy…

  5. Mark Suggitt said

    “…human brain…has not changed since anatomical modernity… maybe as much as [300kyrs] …human thought has probably not greatly changed since the advent of cognitive modernity [70kyrs] ago.”

    Cranial volume has dropped rapidly in H.Sapiens for the past 20kyrs, losing ~150cc of volume in that time…the size of a tennis ball. So there’s that. Is this a trimming of excess? Loss in effective intellect? Physically smaller bodies? Better energy efficiency? And why…: Gene expression, childhood brain development conditions, population density, diet, domestication … a calculus of those and more. The current brain may appear to have the same structure, size notwithstanding, but we can only infer that the thought-product of a 70kyr specimen is comparable to 21st c human. It might be. The essential cognitive characteristics that led to the first civilizations may be more recent acquisition, may not have been present in pre-20kyr humans.

    So I pause at your statement, while enjoying the larger idea of encephalization as the great filter, only because it implies steady state and I believe there is reason to think it is steadily adapting and that it doesn’t lessen your argument.

  6. if the Great Filter were really still ahead of us wouldn’t the EM spectrum be alive with the death throes of other civilizations as they approached that boundary?

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: