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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.

neocortex

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.

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