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.