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biosphere 0

In Rational Reconstructions of Time I noted that stellar evolution takes place on a scale of time many orders of magnitude greater than the human scale of time, but that we are able to reconstruct stellar evolution by looking into the cosmos and, among the billions of stars we can see, picking out examples of stars in various stages of their evolution and sequencing these stages in a kind of astrophysical seriation. Similarly, the geology of Earth takes place on a scale of time many orders of magnitude removed from human scales of time, but we have been able to reconstruct the history of our planet through a careful study of those traces of evidence not wiped away by subsequent geological processes. Moreover, our growing knowledge of exoplanetary systems is providing a context in which the geological history of Earth can be understood. We are a long way from understanding planet formation and development, but we know much more than we did prior to exoplanet discoveries.

The evolution of a biosphere, like the evolution of stars, takes place at a scale of time many orders of magnitude beyond the human scale of time, and, as with stellar evolution, it is only relatively recently that human beings have been able to reconstruct the history of the biosphere of their homeworld. This began with the emergence of scientific geology in the eighteenth century with the work of James Hutton, and accelerated considerably with the nineteenth century work of Charles Lyell. Scientific paleontology, starting with Cuvier, also contributed significantly to understanding the natural history of the biosphere. A more detailed understanding of biosphere evolution has begun to emerge with the systematic application of the methods of scientific historiography. The use of varve chronology for dating annual glacial deposits, dendrochronology, and the Blytt–Sernander system for dating the layers in peat bogs, date to the late nineteenth century; carbon-14 dating, and other methods based on nuclear science, date to the middle of the twentieth century. The study of ice cores from Antarctica has proved to be especially valuable in reconstruction past climatology and atmosphere composition.

The only way to understand biospheric evolution is through the reconstruction of that evolution on the basis of evidence available to us in the present. This includes the reconstruction of past geology, climatology, oceanography, etc. — all Earth “systems,” as it were — which, together with life, constitute the biosphere. We have been able to reconstruct the history of life on Earth not from fossils alone, but from the structure of our genome, which carries within itself a history. This genetic historiography has pushed back the history of the origins of life through molecular phylogeny to the very earliest living organisms on Earth. For example, in July 2016 Nature Microbiology published “The physiology and habitat of the last universal common ancestor” by Madeline C. Weiss, Filipa L. Sousa, Natalia Mrnjavac, Sinje Neukirchen, Mayo Roettger, Shijulal Nelson-Sathi, and William F. Martin (cf. the popular exposition “LUCA, the Ancestor of All Life on Earth: A new genetic analysis points to hydrothermal vents as the planet’s first habitat” by Dirk Schulze-Makuch; also We’ve been wrong about the origins of life for 90 years by Arunas L. Radzvilavicius) showing that recent work in molecular phylogeny points to ocean floor hydrothermal vents as the likely point of origin for life on Earth.

This earliest history of life on Earth — that terrestrial life that is the most different from life as we know it today — is of great interest to us in reconstructing the history of the biosphere. If life began on Earth from a single hydrothermal vent at the bottom of an ocean, life would have spread outward from that point, the biosphere spreading and also thickening as it worked its way down in the lithosphere and as it eventually floated free in the atmosphere. If, on the other hand, life originated in an Oparin ocean, or on the surface of the land, or in something like Darwin’s “warm little pond” (an idea which might be extended to tidepools and shallows), the process by which the biosphere spread to assume its present form of “planetary scale life” (a phrase employed by David Grinspoon) would be different in each case. If the evolution of planetary scale life is indeed different in each case, it is entirely possible that life on Earth is an outlier not because it is the only life in the universe (the rare Earth hypothesis), but because life of Earth may have arisen by a distinct process, or attained planetary scale by a distinct mechanism, not to be found among other living worlds in the cosmos. We simply do not know at present.

Once life originated at some particular point on Earth’s surface, or deep in the oceans, and it expanded to become planetary scale life, there seems to have been a period of time when life consisted primarily of horizontal gene transfer (a synchronic mechanism of life, as it were), before the mechanisms of species individuation with vertical gene transfer and descent with modification (a diachronic mechanism of life). It is now thought the the last universal common ancestor (LUCA) will only be able to be traced back to this moment of transition in the history of life, but this is an area of active research, and we simply do not yet know how it will play out. But if we could visit many different worlds in the earliest stages of the formation of their respective biospheres, we would be able to track this transition, which may occur differently in different biospheres. Or it may not occur at all, and a given biosphere might remain at the level of microbial life, experiencing little or no further development of emergent complexity, until it ceased to be habitable.

While we can be confident that later emergent complexities must wait for earlier emergent complexities to emerge first, no other biosphere is going to experience the same stages of development as Earth’s biosphere, because the development of the biosphere is a function of a confluence of contingent circumstances. The history of a biosphere is the unique fingerprint of life upon its homeworld. Any other planet will have different gravity, different albedo, different axial tilt, axial precession, orbital eccentricity, and orbital precession, and I have explained elsewhere how these cycles function as speciation pumps. The history of life on Earth has also been shaped by catastrophic events like extraterrestrial impacts and episodes of supervolcano eruptions. It was for reasons such as this that Stephen J. Gould said that life on Earth as we know it is, “…the result of a series of highly contingent events that would not happen again if we could rewind the tape.”

Understanding Earth’s biosphere — the particularities of its origins and the sequence of its development — is only the tip of the iceberg of reconstructing biospheres. Ultimately we will need to understand Earth’s biosphere in the context of any possible biosphere, and to do this we will need to understand the different possibilities for the origins of life and for possible sequences of development. There may be several classes of world constituted exclusively with life in the form of microbial mats. Suggestive of this, Abel Mendez wrote on Twitter, “A habitable planet for microbial life is not necessarily habitable too for complex life such as plants and animals.” I responded to this with, “Eventually we will have a taxonomy of biospheres that will distinguish exclusively microbial worlds from others…” And our taxonomy of biospheres will have to go far beyond this, mapping out typical sequences of development from the origins of life to the emergence of intelligence and civilization, when life begins to take control of its own destiny. On our planet, we call this transition the Anthropocene, but we can see from placing the idea in this astrobiological context that the Anthropocene is a kind of threshold event that could have its parallel in any biosphere productive of an intelligent species that becomes the progenitor of a civilization. Thus planetary scale life is, in the case of the Anthropocene, followed by planetary scale intelligence and planetary scale civilization.

levels of biological organization

Ultimately, our taxonomy of the biosphere must transcend the biosphere and consider circumstellar habitable zones (CHZ) and galactic habitable zones (GHZ). In present biological thought, the biosphere is the top level of biological organization; in astrobiological thought, we must become accustomed to yet higher levels of biological organization. We do not yet know if there has been an exchange of life between the bodies of our planetary system (this has been posited, but not yet proved), in the form of lithopanspermia, but whether or not it is instantiated here, it is likely instantiated in some planetary system somewhere in the cosmos, and in such planetary systems the top level of biological organization will be interplanetary. We can go beyond this as well, positing the possibility of an interstellar level of biological organization, whether by lithopanspermia or by some other mechanism (which could include the technological mechanism of a spacefaring civilization; starships may prove to be the ultimate sweepstakes dispersion vector). Given the possibility of multiple distinct interplanetary and interstellar levels of biological organization, we may be able to formulate taxonomies of CHZs for various planetary systems and GHZs for various galaxies.

One can imagine some future interstellar probe that, upon arrival at a planetary system, or at a planet known to possess a biosphere (something we would know long before we arrived), would immediately gather as many microorganisms as possible, perhaps simply by sampling the atmosphere or oceans, and then run the genetic code of these organisms through an onboard supercomputer, and, within hours, or at most days, of arrival, much of the history of the biosphere of that planet would be known through molecular phylogeny. A full understanding of the biospheric evolution (or CHZ evolution) would have to await coring samples from the lithosphere and cryosphere of the planet or planets, and, but the time we have the technology to organize such an endeavor, this may be possible as well. At an ever further future reach of technology, an intergalactic probe arriving at another galaxy might disperse further probes to scatter throughout the galaxy in order to determine if there is any galactic level biological organization.

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Folk Astrobiology

6 August 2016

Saturday


If some alien species had encountered Earth during one of its snowball periods, the planet would have resembled a biosphere with a single biome.

If some alien species had encountered Earth during one of its snowball periods, the planet would have resembled a biosphere with a single biome.

Can there be folk concepts in (and of) recent and sophisticated scientific thought, such as astrobiology? Astrobiology is a recent discipline, and as such is a beneficiary of a long history of the development of scientific disciplines; in other words, astrobiology stands on the shoulders of giants. In From an Astrobiological Point of View I characterized astrobiology as the fourth and latest of four revolutions in the life sciences, preceded by Darwinism, genetics, and evolutionary developmental biology (i.e., evo-devo). Can there be folk concepts that influence such a recent scientific discipline?

In Folk Concepts and Scientific Progress and Folk Concepts of Scientific Civilization I considered the possibility of folk concepts unique to a scientific civilization, and the folk concepts of recent sciences like astrobiology constitute paradigmatic examples of folk concepts unique to scientific civilization. The concepts of folk astrobiology, far being being rare, have proliferated as science fiction has proliferated and made a place for itself in contemporary culture, especially in film and television.

One idea of folk astrobiology that is familiar from countless science fiction films is that of planets the biosphere of which is dominated by a single biome. Both Frank Herbert’s planet Arrakis from the novel Dune and the planets Tatooine and Jakku from Star Wars are primarily desert planets, whereas the Star Wars planet Dagobah is primarily swamp, the planet Kamino is a global ocean, and the planet Hoth is primarily arctic. Two worlds that appear in the Alien films, Zeta Reticuli exomoon LV-426 in Alien and Aliens and LV-223 in Prometheus, are both desolate, rocky, and barren, like the landscapes we have come to expect from the robotic exploration of the other worlds in our own solar system.

The knowledge we have assembled of the long-term history of the biosphere of Earth, that our planet has passed through “hothouse” and “icehouse” stages, suggest it is reasonable to suppose that we will find similar conditions elsewhere in the universe, though Earth today has a wide variety of biomes that make up its biosphere. We should expect to find worlds both with diverse biospheres and with biospheres primarily constituted by a single biome. Perhaps this idea of folk astrobiology will someday be formalized, when we know more about the evolution of biospheres of multiple worlds, and we have the data to plot a bell curve of small, rocky, wet planets in the habitable zone of their star. This bell curve almost certainly exists, we just don’t know as yet where Earth falls on the curve and what kinds of worlds populate the remainder of the curve.

Biosphere diversity is thus a familiar concept of folk astrobiology. But let me backtrack a bit and try to formulate more clearly an explication of folk astrobiology.

In an earlier post I quoted the following definition of folk biology:

Folk biology is the cognitive study of how people classify and reason about the organic world. Humans everywhere classify animals and plants into species-like groups as obvious to a modern scientist as to a Maya Indian. Such groups are primary loci for thinking about biological causes and relations (Mayr 1969). Historically, they provided a transtheoretical base for scientific biology in that different theories — including evolutionary theory — have sought to account for the apparent constancy of “common species” and the organic processes centering on them. In addition, these preferred groups have “from the most remote period… been classed in groups under groups” (Darwin 1859: 431). This taxonomic array provides a natural framework for inference, and an inductive compendium of information, about organic categories and properties. It is not as conventional or arbitrary in structure and content, nor as variable across cultures, as the assembly of entities into cosmologies, materials, or social groups. From the vantage of EVOLUTIONARY PSYCHOLOGY, such natural systems are arguably routine “habits of mind,” in part a natural selection for grasping relevant and recurrent “habits of the world.”

Robert Andrew Wilson and Frank C. Keil, The MIT Encyclopedia of the Cognitive Sciences

And here is a NASA definition of astrobiology that I have previously quoted:

“Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. This multidisciplinary field encompasses the search for habitable environments in our Solar System and habitable planets outside our Solar System, the search for evidence of prebiotic chemistry and life on Mars and other bodies in our Solar System, laboratory and field research into the origins and early evolution of life on Earth, and studies of the potential for life to adapt to challenges on Earth and in space.”

Drawing on both of these definitions — “Folk biology is the cognitive study of how people classify and reason about the organic world” and “Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe” — we can formulate a fairly succinct definition of folk astrobiology:

Folk astrobiology is the cognitive study of how people classify and reason about the origin, evolution, distribution, and future of life in the universe.

I hope that the reader immediately sees how common this exercise is, both in scientific and non-scientific thought. On the scientific side, folk astrobiology is pervasively present in the background assumptions of SETI, while on the non-scientific side, as we have seen above in examples drawn from scientific fiction films, folk astrobiology informs our depiction of other worlds and their inhabitants. These concepts of folk astrobiology are underdetermined by astrobiology, but well grounded in common sense and scientific knowledge as far as it extends today. We will only be able to fully redeem these ideas for science when we have empirical data from many worlds. We will begin to accumulate this data when, in the near future, we are able to get spectroscopic readings from exoplanet atmospheres, but that is only the thin edge of the wedge. Robust data sets for the evolution of multiple independent biospheres will have to await interstellar travel. (This is one reason that I suggested that a starship would be the ultimate scientific instrument; cf. The Interstellar Imperative.)

Folk astrobiology remains “folk” until its concepts are fully formalized as part of a rigorous scientific discipline. As few disciplines ever attain complete rigor (logic and mathematics have come closest to converging on that goal), there is always a trace of folk thought that survives in, and is even propagated along with, scientific thought. Folk concepts and scientific concepts, then, are not mutually exclusive, but rather they overlap and intersect in a Wittgensteinian fashion. However, the legacy of positivism has often encouraged us to see folk concepts and scientific concepts as mutually exclusive, and if one adopts the principle that scientific concepts must be reductionist, therefore no non-reductionist concepts are not scientific, then it follows that most folk concepts are eliminated when a body of knowledge is made scientifically rigorous (I will not further develop this idea at present, but I hope to return to it when I can formulate it with greater precision).

We have a sophisticated contemporary biological science, and thus scientific biological concepts are ready to hand to employ in astrobiology, so that astrobiology has an early advantage in converging upon scientific rigor. But if a science aspires to transcend its origins and to establish itself as a new science co-equal with its progenitors, it must be prepared to go beyond familiar concepts, and in this case this means going beyond the sophisticated concepts of contemporary biology in order to establish truly astrobiological scientific concepts, i.e., uniquely astrobiological concepts, and these distinctive and novel concepts must then, in their turn, converge on scientific rigor. In the case of astrobiology, this may mean formulating a “natural history” where “nature” is construed as to include the whole of the universe, and this idea transcends the familiar idea of natural history, forcing the astrobiologist to account for cosmology as well as biology.

As an example of an uniquely astrobiology concept I above suggested the idea of biosphere diversity. Biosphere diversity, in turn, is related to ideas of biosphere evolution, developmental stages on planets with later emergent complexities, and so on. The several posts I have written to date on planetary endemism (Part I, Part II, Part III, Part IV, Part V, and more to come) may be considered expositions of the folk astrobiological idea of planetary endemism. Similarly, the homeworld concept is both a folk concept of astrobiology and scientific civilization (cf. The Homeworld Effect and the Hunter-Gatherer Weltanschauung, Hunter-Gatherers in Outer Space, and The Martian Standpoint).

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The idea of a homeworld is a folk concept of astrobiology and scientific civilization.

The idea of a homeworld is a folk concept of astrobiology and scientific civilization.

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