I’ve written several posts in which I have mooted thought experiments in astrobiology, namely, Astrobiology Thought Experiment, Another Astrobiological Thought Experiment, a comment on the latter, A Sentience-Rich Biosphere, and Addendum on Anthropic Panspermia, and Two Thought Experiments on the Same, as well as other thought experiments occurring occasionally in other posts. The terrestrial biosphere is so complex that when we begin to think of it in scientific terms, the possibility for variations on the theme of the terrestrial biosphere is almost endless.
One interesting variation on the theme of the terrestrial biosphere is the idea of a “shadow biosphere” (also called “shadow life”), that is to say, another kind of life on Earth, right here, right now, but so different from life as we know it that we cannot recognize the shadow biosphere on Earth even though we could be coexisting with it right now.
I learned about the shadow biosphere idea from Paul Davies’ book The Eerie Silence: Renewing Our Search for Alien Intelligence, but the idea has been credited to biochemists Steven A. Benner, Alonso Ricardo, and Matthew A. Carrigan, for their 2004 paper “Is there a common chemical model for life in the universe?” in which the authors asked, “Does an alternative biosphere exist on Earth?” and called a possible shadow biosphere, “non-terran life on Earth.” Davies collaborated on another paper in 2009 with several authors, including Benner, “Signatures of a Shadow Biosphere,” that further developed the idea. (The above image is drawn from this second paper.)
While on Earth a second biosphere would have to be a “shadow” biosphere, and so difficult to find that we have not previously noticed it, and cannot even find it now when knowingly searching for another genesis on Earth, this would not necessarily be true on some other planet with a biosphere. And this leads us to yet another astrobiological thought experiment: imagine a world with two or more biochemically distinct biospheres existing side-by-side. These distinct forms of life from distinct origin of life episodes might divide up a planet in geographical regions, in which different biomes were the result of different forms of life (i.e., ecologically separate, as in the top illustration above). Or different forms of life on the same planet might be layered like geological strata, with multiple concurrent, concentric biospheres (i.e., ecologically integrated).
In what is known as the “three domain system” worked out by the molecular phylogeny of Carl Woese, according to which life consists of the archaea, bacteria, and eukaryote domains, we cannot know at the present time that these three domains originated from one and the same origin of life event. It has been postulated that bacteria originated separately from archaea and eukarya, in which case their biochemical integration in the contemporaneous biosphere is already an example of multiple origins of life events of Earth. (I previously wrote about this in Last Universal Common Ancestor.) In this case, the “shadow biosphere” is no shadow at all, because it has become a full partner in the primary manifestation of terrestrial life; that is to say, the “shadow life” is biochemically integrated.
How might biochemical integration of initially distinct forms of life derived from distinct origin of life events come about? What is known as the endosymbiotic theory of the origin of the eukaryotic cell holds that a proto-eukaryote cell engulfed, without digesting, an autotrophic bacterium, which latter then continued to function inside the now more complex eukaryotic cell, and the whole composed of two formerly distinct cells coevolved into the eukaryote of today. (This is not the only theory of the evolution of the eukaryotic cell; there is also, e.g., the “inside-out” theory of eukaryotic cell evolution, as well as other theories.) Further endosymbiotic events continue today. It might be possible that two biochemically distinct organisms resulting from unique origin events might attain endosymbiosis, resulting in more complex forms of life.
Another mechanism for the emergence of more complex forms of life from less complex precursors could involve something like the known ability of phospholipids to spontaneously organize themselves in the presence of water, because one end is hydrophilic (”water-loving”) while the other end is hydrophobic (”water-fearing”). This self-organization results in structures such as micelles, liposomes, and bilayers, which latter can grow into significant membranes.
This kind of self-organizing structure suggests the possibility of the emergence of structures from the interaction of biochemically distinct forms of life, which, when they meet, may prove to have xenophilic and xenophobic constituents, which then spontaneously organize in the presence of biochemical xenomorphs. If both forms of life have these xenophilic and xenophobic reactions, the resulting self-organizing structures will be that much more complex, and so may contribute to new emergent complexities that we do not see in life emergent from a single origin event.
Given that nature is parsimonious even when it gives rise to emergent complexity, and that the rudimentary constituents of life are likely to be consistent across the observable universe, and robustly consistent within a single planetary system derived from a single protoplanetary disk, one would expect life to exhibit variations on a theme, and that these variations would not be radically distinct from each other. Needless to say, if multiple distinct origin of life events occurred on the surface of a single planet, the biochemistry of these distinct events would be even more robustly consistent, with their organic and inorganic precursors derived from the same planet, the same lithosphere, the same oceans, and the same atmosphere. Following from the same chemical precursors, and the same molecular precursors, the macromolecules of life could well interact with other macromolecules of distinct origin in remarkable ways.
This interaction, whether on a single world or across multiple worlds, would be selective, and natural selection would result in descent with modification. Exclusively xenophobic reactions would mean the end of either or both forms of life. Successful interactions resulting in novel emergent complexity would have an opportunity to reproduce itself and leave its biochemical legacy. The evolution of multiple worlds, or of multiple origin of life events on a single world, would be the same evolution that we know on Earth, only more complex as its juggles more parameters. And from that greater complexity, new forms may evolve.
Interaction across multiple worlds may be significant under special circumstances. In multi-planetary ecosystems, with planets spaced as closely as we find them in the TRAPPIST-1 system (on which cf. NASA Telescope Reveals Largest Batch of Earth-Size, Habitable-Zone Planets Around Single Star), if life arose on one such planet the likelihood of that life being carried to the other planets by lithopanspermia is relatively high. If life arose separately on several closely spaced planets, with slight biochemical differences between the distinct origin of life events on the several planets, and circumstances within that planetary system were conducive to lithopanspermia, this would mean that each of the planets would eventually have tinctures of life from the other planets, and if these varieties of life could live together without destroying each other, the mixed biospheres of multi-planetary habitable zones where there has been independent origins of life on multiple worlds would suggest a diversity of life not realized on Earth.
There are two papers that discuss the increased likelihood of lithopanspermia in the TRAPPIST-1 system, Enhanced interplanetary panspermia in the TRAPPIST-1 system by Manasvi Lingam and Abraham Loeb, and Fast litho-panspermia in the habitable zone of the TRAPPIST-1 system, by Sebastiaan Krijt, Timothy J. Bowling, Richard J. Lyons, and Fred J. Ciesla. A multi-planetary ecosystem bound by rapid lithopanspermia would constitute a speciation pump on an order or magnitude greater than the speciation pumps to be found on a single planet, such as Malenkovitch cycles and supercontinent cycles.
The closely space planets and opportunities for lithopanspermia at TRAPPIST-1 are so interesting because of the possibilities for emergent complexities arising from the sheer number of worlds involved. Numbers matter, especially in relation to emergentism. Emergentism has made great strides in recent years, as the Big History community has been employing emergentism as a periodization for the whole of history. I myself routinely invoke emergent complexity (e.g., in The Apotheosis of Emergent Complexity), and emergence is regularly discussed in the philosophy of science (as in the new paper Emergence and Reductionism: an awkward Baconian alliance by Piers Coleman).
“More is Different” is the title of a scientific paper by P. W. Anderson (Science, New Series, Vol. 177, №4047, Aug. 4, 1972, pp. 393–396) that explicitly took on reductionism in the sciences. Reductionism seems to come naturally to the sciences, and indeed I have written a couple of blog posts (Reduction, Emergence, Supervenience and Scientific Metaphysics and Big History) in which I characterize reduction as the second stage in scientific metaphysics, following eliminationism and followed by emergentism and supervenience. W. V. O. Quine once said that “Philosophy of science is philosophy enough,” and to this I would respond that (to paraphrase Samuel Johnson) if a man would seek to limit philosophy he would also limit science, for there is in science all that philosophy can afford.
If more is different, then more planets may mean different life. Multiple planets located within the habitable zone of a single star is certainly a prospect to inspire the astrobiological imagination. After the TRAPPIST-1 announcement, NASAwatch tweeted:
As a biologist I had to ask Seager if we should consider 3 habitable planets SO close together as 1 possible ecosystem
Seager: What a wonderful question. Sure there is a student out there who will look at that
And Michael Oman-Reagan responded to this in another tweet:
This is an amazing question. A planet-spanning ecosystem.
I was immediately reminded of a series of posts by Sean Raymond in which he sought to “build a planetary system with the most possible habitable worlds.” Here are Raymond’s posts on this idea:
- Building the ultimate Solar System
- Building the ultimate Solar System part 1: choosing the right star
- Building the ultimate Solar System part 2: choosing the right planets
- Building the ultimate Solar System part 3: choosing the planets’ orbits
- Building the ultimate Solar System part 4: two ninja moves — moons and co-orbital planets
- Building the ultimate Solar System part 5: putting the pieces together
- Building the ultimate Solar System part 6: a system with multiple stars
- The biggest tragedy in the history of the Universe
- The Ultimate Trojan 2-star planetary system
Raymond worked his way up to a multiple star system with 480 habitable worlds. I find this thought experiment fascinating, not because I think it is likely that there is a planetary system with 480 habitable worlds actually instantiated in our universe, but rather because, in showing what is scientifically possible, it points beyond our biases rooted in our own planetary system with its single world with a biosphere. It could be argued, and reasonably so, that our solar system has three rocky planets in our sun’s habitable zone, it just happens that two of the three did not develop a biosphere. That may well be true, but there may be planetary systems in which there are multiple inhabited worlds with biospheres.
Now that we know about the planetary system at TRAPPIST-1, we have a concrete example of a planetary system where something like this may be possible, and we may find multiple planets with ecologically coupled biospheres. TRAPPIST-1 isn’t quite an ultimate solar system in this sense, but it is much closer to being an ultimate solar system than our own solar system. And a planetary system like that of TRAPPIST-1 may provide an evolutionary laboratory for biospheres in which biochemically distinct life from multiple origins of life events coevolve into forms of life not possible in the biosphere of a single planet.
It may well prove that more is different when it comes to planets, their biospheres, and ecosystems spanning multiple planets. Multi-planet ecologies (we can’t call them biospheres, because they would be constituted by multiple biospheres) may produce qualitatively distinct emergents based on the greater number of components of the ecosystem so constituted. Emergent complexities not possible in a planetary system like our own, with a single liquid-water world, may be possible where there are multiple such planets ecologically coupled through lithopanspermia, and perhaps through other vectors that we cannot now imagine.