I'm finishing up the new book The Biological Universe, by emeritus evolutionary biologist Wallace Arthur. I made some critical comments on my sometime blog The Gyromantic Informicon [q.v.] about this, but what follows is my specific take on what he calls his Huge Hypothesis, summing up his view of the entire subject. If the general subject of life in the universe doesn't really capture your imagination, you might want to skip the rest of this.
He proposes that the evidence and reasonable inference supports the following "Huge Hypothesis." I paraphrase a good deal and add in some explanatory comments. These are, to use Thomas Huxley's phrase, "in the indicative," rather than the "potential," even though strictly speaking some of this has to be considered speculative. Most of it is pretty widely accepted in the scientific community today; the additional points I add at the end less so, but I believe they follow logically and are of the same order of certainty; namely, not certain but probable.
- Life first evolved somewhere in the universe not much later than 10 billion years ago. [Arthur restricts himself to the observable universe, a space about 93 billion light years across in all directions with us at the center and containing approximately 2 trillion galaxies; the entire universe is much, much larger and, applying the principles of isotropy and homogeneity on large scales, is presumably all much the same].
- The oldest instance of the origin of life was overwhelmingly likely to have been on a planet in a galaxy at great distance from the Milky Way, just because there are literally something like a trillion candidate galaxies, each containing hundreds of billions of planets, in the observable universe ("OU" for short).
- Since that time, there has been a steady increase in the number of locales where life has originated and thrived for a time, and at least a good proportion of them continue to have life at the present epoch, such that some form of life is now relatively common in the universe.
- QED, the number of planets in the OU with some form of life, mostly limited to microbial life, is many trillions. (Note: every spiral galaxy, and probably many other types of galaxies, as well, have billions to more than a trillion planets, and a typical spiral galaxy like the Milky Way has hundreds of billions of rocky planets situated in the "habitable zone" of their stars where liquid water is possible. The same should be true of most galaxies).
[Arthur includes an additional bullet point, that some systems have more than one inhabited planet; but I regard that as superfluous to the argument]. - Most or all of the life in the universe is chemically based on carbon compounds. (There are many reasons for including this inference, which I consider to be quite ironclad, but I won't go into it here. My own surmise is that you could correctly say "Essentially all". Fortunately, nucleosynthesis in stars results in the production of a good deal of carbon).
- Add-in, not included by Arthur: All, or nearly all, life in the OU has evolved a genetic information recording system that functions analogously to the nucleic acid system that evolved on Earth, although specific details vary considerably.
- Far and away most life in the OU is constructed of cells, although, again, the exact architecture varies considerably.
- Most life-bearing planets in the OU host only microbial life (single cell or small-aggregates of cells).
- A large number (but proportionally fewer) of the life-bearing planets in the OU also host multicellular life.
- At least some proportion of the biospheres that have evolved multicellular life have evolved "complex" multicellular organisms that conduct photosynthesis to utilize light energy directly (similar to Plants and other photosynthesizer macrobiota on Earth, such as "brown algae"); or that assume roles comparable to those of the Fungi and Animal kingdoms in the Earth biosphere (symbionts and parasites).
- On at least some of the biospheres that have evolved such "complex" multicellular "animals," some of them have evolved advanced motility, including analogs to skeletal (including exoskeletal) structure, musculature, nervous systems, and the beginnings of intelligence, in the sense of directed control by a "brain."
- With all intermediate levels occurring in numbers, some portion of the biospheres that have evolved such complex animal life have proceeded to the evolution of human-level intelligence, although exactly how that manifests varies considerably.
This is where Arthur's Huge Hypothesis ends, but therein lies my principal criticism of his thesis. I think he overestimates the numbers somewhat, especially of the last phase, but I don't disagree with any of the above. I suspect there may be some side roads that lead to unanticipated variants of the types of life we are familiar with, but the main ideas here I believe are sound. But I think they really miss the mark when it comes to a reasonable anticipation of our possible future, which, necessarily, means something analogous to what is already the state of being elsewhere, where human-level intelligence already evolved, in some cases no doubt, a very long time ago indeed. So I would add the following additional levels of development, further along in the sequence.
- Some portion of human-level intelligent life develops external symbolic manipulation analogous to language, and eventually culture, and then advanced science and technology. This gives organisms the ability to direct their own evolution from this point, at least to an extent.
- Some portion of the technological species develop artificial biohabitats and are no longer confined to the surfaces of their planets of origin. [I would adventure that we are on the cusp of this development, and that there is no guarantee we will proceed to it; presumably frequently in the past and future, beings at this level do not make this transition successfully or never even try, for whatever reason].
- Once at the level of "space-dwelling," most of the technological species proceed to colonize their star systems and later other stars, and to spread the form of life that originated on their planet to vast numbers of other locations in space, including but not limited to planets that did not and might never evolve life on their own, such that over time most of the life in the universe exists elsewhere than the planetary surfaces where it originated.
- There is virtually no natural limit to the expansion of life under the direction and impetus of intelligence; the future of the OU is for life to encompass a greater and greater proportion of the available locations where sustaining life is possible until some saturation level is reached in the distant future. [Comment: even if this development is relatively rare, it is a threshold; once it occurs, it tends to lead to a permanent change in the course of the development life over a very wide region of space, potentially including multiple galaxies before bumping into others similarly situated, because plausible rates of expansion of such extended biospheres entail small fractions of the age of the planets and galaxies in which they originate. So, ultimately, if this phase occurs at all, it will tend to fill all the available space everywhere].
We are directed by current modes of scientific thought to shun all teleology, but I think it's fair to assess that the "function" of advanced human-level intelligence (and beyond) is to make something like the final three stages of my "even huger hypothesis" possible. I envision an "Age of Life" that is just getting underway in a universe that will eventually be quite literally filled with life.
I hasten to emphasize the obvious: most of the last two phases mentioned above lie in the future, in most locales. If some regions of the OU have advanced to the level of galaxy-spanning civilizations already, this would likely be apparent in some way were it already common, at least in the relatively nearby regions, say out to 500 million light years. Because, of course, if such developments were to have occurred at that distance, say, 450 million years ago, we would not see any evidence, because the light from that time would not have reached us yet. There could well be the first instances of extremely advanced civilization in parts of space that we just can't see yet. (This assumes, as I think is reasonable, that engineering on a literally galactic scale would change the quality of enough of the light coming from natural luminous sources that the presence of artificial technology would be inferrable).
I agree with much of what you write, but I have a small quibble and then a really big one. First the small: our own galaxy is over 13 billion years old. If a galactic civilization had already arisen, presumably we would know about it by now. This is just standard great filter stuff. If it hasn’t happened here, yet, then why not… and why would we expect it to happen inside the next 13 billion years, or in even the tiniest fraction of other galaxies. We don’t know. This effects your optimistic prediction for an Age of Life.
ReplyDeleteThe much bigger issue for me is in our notion of life itself. It seems to me that we are inherently short sighted… we only see what we know how to see. Even more, can only imagine what we can understand. As you’ve eloquently described, the universe is a Big Place. Looking at published science fiction, I’m always drawn to the examples of “Dragon’s Egg” by Robert L. Forward, and especially “The Black Cloud” by Fred Hoyle. “Dragon’s Egg” envisions life evolved on the surface of a neutron star, while “The Black Cloud” envisions a sentient interstellar gas cloud (and indeed, a whole society of these entities). Both of these novels were written by scientists.
This just dips our toe in the water. “Life”, as broadly as we can construe it, seem to require some sort of self-contained system maintaining homeostasis. That’s a very broad umbrella. Fred Hoyle imagines something extremely remote from our carbon and cell based life biases. Why not go elsewhere? Could there not be self-contained, homeostatic systems based in the interior of planets? The interior of stars? I would posit that every argument against this runs into the criticism that we are ignorant of more than 99% of what we *know* must be out there. We simply have no idea about what can arise in these places over the scales we know exist.
And I would go further with this. We have this word and concept “life” that is formed from our experience. We can only imagine what we can understand. But we’re not so helpless… we can at least understand that we are limited, and know that much more exists. We can “imagine” being able to understand more. So in my mind, there are no doubt unimaginably many instances of things in the universe which are beyond our ability to understand. Things which we might try to call “life”, but which are not self-contained. Or which fail to maintain homeostasis in some sense, yet continue in a “lifelike manner”. Sentience could arise in any of these cases, although of course sentience is another extremely short-sighted term. Stanislaw Lem’s “Solaris” envisions an entire planet which seems to the humans visiting it to be alive in a sense, and perhaps sentient in a sense, but which ultimately can’t be understood in those frameworks at all.
So this is my optimism… the universe is bigger and more fantastic than we can imagine, and there is no end to wonder.
Arthur actually mentions Hoyle's Black Cloud. Are you familiar with the OTHER Arthur, Isaac Arthur (who was actually named by his parents after Isaac Asimov)? He has done a whole series of YouTube videos on futurism, with a lot of emphasis on the great filters, Fermi conundrum (it's not really a paradox), interstellar migration and the potential of life to colonize large swaths of the universe, etc. You're right of course that our imagination is limited, but it's also true that while science proceeds in fits and starts, and sometimes great leaps, things that are actually correctly ascertained may be refined, or shown to be special cases of larger truths, but they are rarely actually abandoned as false, and when they are they're usually pretty wobbly to begin with (such as Phlogiston theory and the concept of aether as the medium for the transmission of electromagnetic waves). So the broad outlines of physics and chemistry are real and we know about them and they'll never be different; our understanding will be better, but F will always equal ma in ordinary situations. So the idea that a disorganized cloud of mostly hydrogen with high entropy and some impurities as complex as amino acids could spontaneously organize itself into a high order intelligence with the equivalent of language, with no perveivable natural selective pressures driving its evolution, seems a bit far fetched to me. Plus, the idea of really exotic forms of life achieving advanced intelligence and science still begs the Fermi question... if this is happening at scale all over the universe, why don't we see the effects of it?
ReplyDeleteI am attracted... call it a hunch... to the ideas that the universe from the long range point of view of the evolution and prevalence of life is YET YOUNG; and that, teleology or no teleology, the function of advanced intelligence, at least from time to time, is to facilitate the expansion of life into a much wider array of habitats and vast areas where life has not yet arisen, and might never do so without the intervention of intelligent beings.