Work in Progress: Exobiology and the Philosophical Logic of Concepts
Friday 30 September 2022
The Oxford English Dictionary gives the first use of “exobiology” as 1960, with a couple of citations. In 1960 Joshua Lederberg, whose work I have cited in other contexts (Lederberg was an early origins of life researcher), wrote an article that was published in Science, “Exobiology: Approaches to Life Beyond Earth.” This is not one of the two 1960 uses cited in the OED, but my guess (which I cannot substantiate) is that this article is the fons et origo of the concept of exobiology. Lederberg’s article does not attempt to give a definition of exobiology, but one of the cited uses in the OED simply calls it “…the study of life on other planets.”
The OED gives the first use of “astrobiology” as 1898, but the first scientific uses appear in the 1940s, one of which citations characterizes astrobiology as “…the consideration of life in the universe elsewhere than on earth.” Obviously, there is very little difference between this definition of astrobiology and the above definition of exobiology. Moreover, most of what is in Lederberg’s 1960 paper would count as astrobiology today.
In my 2012 presentation in Houston, “the Large Scale Structure of Spacefaring Civilization” (one slide of which referenced Lederberg; this presentation is partially summarized in “The Large Scale Structure of Spacefaring Civilization”) I characterized exobiology as the narrower concept, which comes under the more comprehensive concept of astrobiology. I still think this is a reasonable way to characterize the relationship of these concepts, but given the looseness of their introduction other readings are equally plausible. Insofar as Lederberg spoke of exobiology as “life beyond Earth,” and astrobiology comprehends life beyond Earth together with life on Earth, my reading follows naturally from the early uses of these terms.
However, astrobiology was first introduced as something like a theosophic concept, only coming into scientific usage in the 1940s, and only gaining traction and subsuming exobiology in the 1990s. Now astrobiology is king, and exobiology is little used. Another coinage that had some traction in the 1980s and 1990s was bioastronomy, which has also subsequently been subsumed under the dominant concept of astrobiology. When Lederberg wrote his 1960 article and introduced exobiology, the concept was introduced as a thoroughly scientific concept, in a scientific context, with no distant esoteric ancestors such as astrobiology had. In this sense, exobiology has the better scientific claim to being introduced as a specifically scientific concept. Astrobiology, one could say, was poached from its former esoteric use, but has since become a major scientific discipline.
One could, at this point, write a Foucauldian treatment of power in the marketplace of ideas, observing that dominant ideas simply seem to be dominant without necessarily having vanquished their rivals on the merits. However, I will spare the reader this, and observe instead that it is entirely reasonable that the most comprehensive concept has won out as the umbrella concept under which a variety of forms of research take place, including old exobiology agendas, origins of life, the study of extremophiles, and so on. Army, Navy, Air Force, Marines — they all salute the same flag, and that flag (to shift to an awkward simile) is astrobiology.
From a scientific standpoint, we can be content to let a hundred flowers bloom under the banner of astrobiology, and may the best research program obtain the most funding. But from a philosophical standpoint, these concepts are still quite fuzzy, and that fuzziness invites paradox. Indeed, while I was taking a walk yesterday and dictating into my digital recorder, I noted that, even in the midst of industrialized civilization we still have what are essentially newly introduced “folk” concepts — astrobiology is, in a sense, a folk concept of scientific civilization.
I know that many people find logic to be off-putting, and even among logicians an interest in definition is a specialized niche; many find definitional discussions to be tedious in the extreme, but I find the fascinating, as definitions can be used like a searchlight to look for failures of conceptual clarity. I will acknowledge that some discussions of definition are nearly barren and probably pointless, but the best inquiries into the definitions of imperfectly understood concepts can considerably improve our conceptual framework, and often lead to the formation of a number of subsidiary concepts that can subsequently be used to pick out more specialized uses that formerly jostled each other in the big tent of a folk concept.
The fact that folk concepts continue to be introduced when other areas of scientific thought are exhaustively formalized is interesting, as it demonstrates both that there is a scientific culture that itself undergoes development, and that the progress of science is patchy and fragmentary. Some areas of our thought are pushed to the limit of formalization and the meanings of concepts are defined with a degree of precision not possible at any previous time in human history. Other areas of scientific thought, however, are inchoate and in the process of formation; as such, their concepts are imprecise, and sometimes some early concepts of a discipline have to be abandoned, even if the discipline itself survives. While exobiology and bioastronomy have not been entirely abandoned, and no one feels the need to repudiate their uses of these terms in the 1980s and 1990s, these concepts are on the decline while astrobiology continues to grow as a discipline, so that little use is seen for these alternative concepts.
The concept of exobiology could still be useful as a narrower concept within astrobiology, but we have to constrain several unknown parameters for it to be useful. So say we take exobiology as being life in space exclusive of life on Earth, and astrobiology is the more comprehensive concept that includes both life in space and life on Earth. There still remain some interesting unknowns. We can distinguish biology that originated on Earth from biology that exists on Earth, and this gives us four permutations:
- Biology that originated on Earth and exists on Earth
- Biology that originated on Earth but no longer exists on Earth
- Biology that did not originate on Earth but lives on Earth
- Biology that did not originate on Earth and does not live on Earth
Some of these permutations may seem unlikely, but we can come up with a number of interesting thought experiments in which they obtain, as well as even more unlikely scenarios than these four. However, as the above permutations stand, only 1 is explicitly excluded by the narrow conception of exobiology, though further conventions could also exclude other permutations.
For 2 above, it is possible that a kind of life originated on Earth, was transferred elsewhere by some panspermia mechanism, and subsequently went (regionally) extinct on Earth but continues in existence elsewhere. Suppose that the earliest Earth was as clement a place for life to begin as later in Earth’s history, and further suppose that life arose on Earth through abiogenesis prior to the hypothesized Theia impact (believed to be the event that resulted in the formation of the moon). With the Theia impact, Earth was made molten and so was sterilized, but the power of the impact threw some of the pre-Theia life clear, which now continues to live elsewhere in the solar system, but no longer lives on Earth. Perhaps a deep cave on the moon is the final refugia for pre-Theian Earth life. Given that we know that life on Earth can and has arisen, it makes a certain kind of sense to suppose that Earth in its long history might throw off several varieties of life before settling down into a routine biosphere regime.
For 3 above, any panspermatological explanation could have life emerge elsewhere — on Mars, on Venus, in comets, or what-have-you — and be brought to Earth, where it lives, but where it did not originate. In regard to 4 above, this is the strongest sense of exobiology, and perhaps the intuitive concept of exobiology that is held by those who have intuitions on these kinds of questions (not everyone does). However, we can see from the above examples that life on Earth has not necessarily originated on Earth, while life beyond Earth could have originated on Earth, both of which latter concepts are weaker senses of exobiology that might be included within exobiology in a more comprehensive rational reconstruction of the concept, or excluded from it in a less comprehensive rational reconstruction of the concept.
All of these formulations considered so far are geocentric; they all refer to life either originating on Earth or existing on Earth. Only a terrestrially endemic species would insist on formulating a scientific concept in this way, and in a non-anthropocentric, non-geocentric astrobiology, our formulations would be liberated from this anthropic and geocentric bias. A fully formalized astrobiology would have no place for any particular planet, nor for our relationship to Earth — and as soon as we understand this, as soon as we realize our relationship to Earth, we realize that we have never really fully internalized the Copernican revolution. Our conceptual framework is still built on our anthropocentrism and our terrestrial geocentrism. Sometimes I say that life on Earth exemplifies planetary endemism, but, more specifically, our experience of life on Earth, our experience as terrestrial species in the terrestrial biosphere, is more accurately termed terrestrial endemism (or, if you prefer, Tellurian endemism).
Terrestrial is already a partially formalized concept, since we sometimes speak of “terrestrial planets” when we mean any dwarf planet with a rocky (silicate) crust and mantle, and we do not exclusively mean Earth when we use “terrestrial” as an adjective, and this is the proper direction for astrobiology to take if it is ever to be a proper science, rather than a self-congratulatory exercise in Tellurian particularism. Some of our folk concepts hold us back, but some of our folk concepts, like terrestrial, have scientific promise, and it is part of the evolution of science to select and to extrapolate the most intrinsically scientific concepts. Science is one long rational reconstruction of folk concepts, and we make progress in this rational reconstruction when we make the right selections, which is never a foregone conclusion.
Taking “terrestrial” as a proto-formal concept, we can reformulate the above four permutations something like this:
1a. Biology that originated on a given terrestrial planet, Pn, and exists on Pn.
2a. Biology that originated on Pn but no longer exists on Pn.
3a. Biology that did not originate on Pn but lives on Pn.
4a. Biology that did not originate on Pn and does not live on Pn.
This is a bit awkward, but it represents an advance on the above (most scientific advances are counterintuitive until we frame new intuitions and then accustom ourselves to them). In this conceptual schema, exobiology is always relative to some Pn, but astrobiology, as the more comprehensive concept, remains valid, categorically, as it were. However, we still stand in need of this relativized concept of exobiology, as this sort of situation is commonplace in evolutionary biology and especially in island biogeography. I once wrote that astrobiology is island biogeography writ large, and I still endorse this formulation.
In a biologically productive planetary system, we will want to know how many times life arose in a given context, i.e., on a given astronomical body, how many kinds of life arose, where it spread to, how it spread, to what extent this spread was an adaptive radiation, whether the life from the source in question has gone regionally extinct in any of the places where it survived for a time (including the astronomical body of the origins of life event in question), and so on. We cannot even do this yet for our own solar system, but we can clearly envision an astrobiological science that would take this task as its mandate.
I could keep going on in this vein for quite some time. As I noted above, I find definitional discussions as fascinating as others find them barren. For example, in 1a above, we can see that there is an important difference between life that originates on a given terrestrial planet, Pn, and exists only on Pn, and life that originates on a given terrestrial planet, Pn, and exists on Pn but may also exist elsewhere in addition to Pn. Our formulations need to be even narrower in order to preclude that ambiguity.