Month: January 2022

There are a lot of human languages that derive their word for “human” from their word for death (mard in the Iranian languages, e.g.) or birth (e.g. tlacatl in Nahuatl). The sense of finitude thus expressed may be taken as establishing a contrast with the nonhuman, i.e. the gods, a contrast that also gets topicalized for instance in the Greco-Roman distinction between mortal humans and immortal divinities. The gods do not die, or else the gods are not born.

So: do gods get to experience the thrill of Darwinian evolution? Presumably not: in Greek myth, for instance, in Hesiod’s Theogony, the gods multiply and expand to fill the universe, then stop. Within that framework, gods compete for power: Cronos overthrows Ouranos, Zeus overthrows Cronos, and various titanomachies or gigantomachies herald broader overturnings. You might also want to talk about Zeus’ differential reproductive success, but that works itself out precisely among mortal humans: because the gods don’t die, there’s no room for Zeus to repopulate heaven in his own image.

In cultures where that set of contrasts operates – human/mortal/born vs divine/immortal – non-Darwinian life hovers at the edge of the human imaginary, as a limit or goal to which certain remarkable humans – Hercules or Augustus, for example – might be able to assimilate themselves. At the same time, however, this form of life finds itself always in the crosshairs of a critical assault that, in the Greek tradition, would run from Xenophanes through Palaiphatus and Euhemerus to early Christian polemicists like Tertullian. Each of these thinkers would, in their own way, attack not the notion of godhood but the image of it which the mythographic tradition presented. They wanted to deny, not that there could be anything undying, but that something undying could count as life. That mythographic form of godhood was an impossibilty even for the gods, let alone for human beings.

This inherited tendency to understand the undying or unborn as an uncrossable threshold, incompatible with the human (and therefore the animal) condition, probably prepared the grounds for the emergence of a science of life in Europe. If we take Darwinian evolution as a biological fact, then, we have been able to integrate its earth-shaking claims about human origins just to the extent that we have already naturalized and universalized a born, dying form of life that could form a substrate for natural selection. Pockets of resistance to Darwinism remain, predictably, among religious groups that see their god or messiah or even their own “immortal souls” as involved in a form of undying life (which is not, by the way, the original position of Christianity or Islam.)

As modern Christian resistance to Darwinism suggests, there are certainly other ways of dividing up the world and thus other ways of conceptualizing life. One example that lies particularly close to my work at the moment is offered by the Manichaeans, who take an atomic approach to life by stipulating that only certain (“light”) particles within apparently living things are actually alive – the rest of the living creature being a kind of clothing of dead matter made by an evil demiurge in imitation of life. These particles of light, living by definition, survive the apparent death of their host and are either reincorporated into another organism or (if they happen to have been eaten by a Manichaean elect) sent up to the moon, whence they can follow a trajectory through the sun and out of the universe. On this model, life is actually indifferent to birth and death, which are events that happen around it but not to it.

This is an alternative bio-ontology that developed alongside mainstream Greco-Roman thinking on life, in the interstices between it and neighboring cultural commonwealths to the East. It perhaps still bears the trace of a “western” obsession with policing the lines between life and nonlife that is less apparent in the case studies collected in Elizabeth Povinelli’s Geontologies, where, among Aboriginal Australians, persons pass in and out of the world without this necessarily corresponding to a birth or a death. Povinelli suggests that some such account of personhood for nonliving things – not as quasi-humans with rights, but as agents with which we can enter into a relationship of care, or not – will be essential to understanding how the anthropocene has placed limits to human “power” at the same time as it seems to represent the extension of that power over the face of the Earth.

There are people, probably including some of the originators of the anthropocene concept, who understand it in mythographic terms as a human “overthrowing” of nature to match Zeus’ triumph over Cronos, except that, in modern, rationalizing terms, we have to take care lest we actually manage to “kill” the life that nature is. The last 2 years of pandemic should have left us no illusions about this: it is rather we who have put ourselves into the power of a range of non-human agents, some living, some not, some – like this virus – uncannily shimmering between living and dead.

Evolution is a story of life outwitting life, a game that some species – or one species – can win by eliminating all rivals. Yet there are forms of life on Earth, articulated by cultures with different visions than our own, that happen to defy this Darwinian conception by refusing to die or be born. These, and not the impossibility of an immortal god, are what we now face on Earth in the anthropocene – an experience that may better prepare us to encounter life on other planets than do any of the stipulative definitions offered by biology.

In On Inductive Reasoning, the Epicurean philosopher Philodemus is thinking about England. “We don’t know if there are living things on the island of Britain,” he says (pritania in Philodemus’ Greek – the island had become known to the Romans so recently that no single spelling could yet command agreement), “but we can infer that, if there are any living things there, they will be mortal.” Philodemus’ point is that all living things which we know of also die; this unanimity should make us confident that living things unknown to us die too. I suppose that the same logic holds for other planets as for other countries. Yet drawing such universal inferences about life on the basis of the single example known to us may give us a poor conception of the diversity of living things existing in the universe. In particular, as I argued in a previous post, an uncritical commitment to the universality of Darwinian evolution forces us to write birth and death into our definition of life, which will thus exclude apparently living things that neither get born nor die.

The literature on the origins of life is full of examples that trouble this definition – no big surprise, since the mechanisms for information storage and copying that power Darwinian evolution on Earth are probably too complex to have assembled spontaneously from inorganic matter. At its beginnings on Earth, life was probably not capable of evolution; evolution itself had to be developed by non-evolutionary means, the play of chance within an already-living substrate. One way of conceiving this pre-evolutionary life is as a series of lipid membranes within which self-sustaining chemical reactions process chemical species selectively admitted through the membrane, accumulating reaction products that are themselves transformed by further synthetic reactions. An ecology of these bubbles would show, not the uniformity of discrete species, but an almost limitless variety of different chemical complexes. Bubbles could merge, mix together their chemical contents, then bud off again. Would that be death? Then birth? Or are those concepts themselves more modern “evolutionary” developments that make no sense in the context of this primeval micellar world?

One question, I suppose, is whether we’d call such an assemblage “alive” if we encountered it on another world. Would it meet our expectations for living stuff, or would we write it off as a particularly complex and robust non-living system? The individual bubbles making up the system would probably not be self-moving and the system as a whole would probably not give the “appearance” of life. But the chemical reactions it contained and sustained would surely produce artificially high abundances of certain chemical compounds, while others would serve as nexus points between multiple sets of reactions, so that the whole chemical space would end up following a power law distribution – one statistical feature that distinguishes biochemistry from everything else.

Which leads me to raise a practical point, too. For the next couple decades at least, astrobiologists are going to be limited to observing bulk atmospheric characteristics of exoplanets, so most current thinking about biosignatures is focused on figuring out the atmospheric transformations likely to be produced by life. An oxygenated atmosphere in chemical disequilibrium because of the presence of reduced species like methane is one strong candidate, and there’s no reason an ocean full of chemically-active bubbles couldn’t produce such an atmosphere given enough time. A micellar world is one that existing observational techniques could quite possibly flag as “alive,” regardless of whether we’d consider the particular assemblages lying underneath its atmosphere “living” on closer inspection. That gap should make us pause, and should probably make us widen our working definition of life.

We can imagine other possibilities, too. Suppose a living system in which LUCA is a single cell that doesn’t reproduce, but grows – so LUCA lives forever and has no offspring. The first problem such an organism would encounter is that, as it grows, the environmental interface through which it can access materials for survival and further growth – i.e. its surface area – may increase less quickly than its volume. One way to circumvent that issue would be to adopt a flat, spreading habit of growth, a tactic employed by some large unicellular organisms and simple animals on Earth and one dictated, at a certain scale of growth, by the force of gravity. Another would be to develop a core of “dead” material within the cell that serves as a support for future growth, an alternative source of nutrients, and an artificial restriction on runaway increase in volume. Neither of these are per se adaptive responses depending on complex structures that would have to arise out of Darwinian evolution (as, e.g., the large central vacuole of some macroscopic single-celled algae would be).

Speaking of adaptive responses, the strongest case for the universality of Darwinian evolution is that it provides a mechanism for population-scale cybernetic responses to a changing environment that are beyond the capacity of all but the most complex individual organisms. If it gets cold, a human can put on a sweater and a ground squirrel can grow out a winter coat; bacteria have to die off, generation after generation, until a cold-tolerant genotype ends up dominant in the population. It has been thought that some capacity for cybernetic response is essential for “not dying” (organisms) or “not going extinct” (populations). Can a single giant cell – perhaps planetary in scale! – still manage that kind of response? Or, if the cell grows so large that it becomes the ecosystem, does it even need to? Would it then not be the environment in which various organelles and chemical species had, themselves, to struggle for survival? We don’t know what to call that, but it’s not exactly the kind of evolution that happens on Earth.

A good general rule in all the observational sciences is that what we can imagine is just a small subset of what’s out there. This is somewhat true of physics, very true of chemistry (would we have been able to predict the intricacies of carbon-based organic chemistry without actual examples of the stuff all around us to study?) and, pari passu, extremely true of biology. If we can imagine a couple of non-Darwinian types of life, how many more are there likely to be out there in the universe? On this point and others, I expect that extraterrestrial life is going to surprise us.

Conversations about astrobiology, the study of life on other planets, depend on our sense of what life is. Some in the field take the definition of life for granted, as though this were a product of another field of study – terrestrial biology – that we could or should port unchanged into the search for life elsewhere in the universe. Yet other scientists regard this move as premature, particularly given the so-called n=1 problem: since all life we can study on earth descends from a last universal common ancestor (LUCA), we know just one token of a type that might end up getting expanded in unpredictable ways if, or when, we find life beyond Earth. All terrestrial life shares certain elements of ribosomal structure that it would be absolutely astonishing to see repeated in an alien life form – but we would hardly want to declare something “not alive” just because it didn’t share this terrestrially universal feature of life. Yet we risk making a similar sort of mistake by demanding that alien life forms meet a rigorous definition of life derived purely from our experience of life on Earth.*

A criterion shared by many stipulative definitions of life is that living things must be capable of Darwinian evolution. This criterion seems to me particularly Earth-parochial and thus particularly likely to pose problems for finding life on other worlds. There are plenty of reasons to think this, not least that practically everyone would judge self-moving things with metabolisms to be alive whether they could undergo evolution or not. But we might also find it useful to reflect on the precise way in which the evolutionary criterion is Earth-parochial, an exercise that could help us to imagine some extraordinary forms of extraterrestrial life.

We may think of evolution in the context of genes, sexual selection and productive mutation as primarily a feature of reproduction, which is to say as associated with birth. In multicellular organisms, evolution does indeed depend on reproduction as a substrate for producing difference – yet this association is far from universal. In bacteria, for example, horizontal gene transfer also serves to create new combinations and configurations on which evolutionary selection can act. Differential reproductive success nevertheless remains the evolutionary mechanism by which a species changes, the next generation of organisms looking like the most successful members of the last one.

This notion of generational turnover, however, also highlights the importance of death as a complement to birth in life capable of Darwinian evolution. Without death as a means of generational turnover, evolutionary change becomes diluted; without death as a means of natural selection, evolution may lose its direction entirely. Violent competition and conflict – “nature red in tooth and claw” – played a much larger role in early evolutionary theory than they do in the modern synthesis, which focuses more directly on reproductive fitness. Yet death remains an essential background feature even of the modern synthesis , especially as it explains evolutionary leaps and structural innovations. The theory of puncuated equilibrium that accounts for these asserts that moments of high mortality are also times of rapid evolutionary change.

Does all life need to reproduce? Does all life need to die? We are not now in any position to say, but we should be willing to entertain negative answers to both these questions. Insofar as the Darwinian criterion limits our thinking on this topic, we should excise it from our definitions of life. Next post, I’ll do some imagineering as to what a non-born, non-dying organism might be like. In the post after that, I’ll try to show how some such organisms already exist after all on Earth.

* Carol Cleland’s The Quest for a Universal Theory of Life is the best modern work on this problem, which has a genealogy stretching back to Aristotle but which astrobiology has made newly urgent.