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.