This might sound like a philosophical question, but I intend it more like a scientific question. We’ve discussed this somewhat before, like in our post about the universe’s habitable zone, but I want to focus in a little closer on what life really is, on what makes one thing alive and one thing not alive, how we might go about defining the difference, and whether what we call life deserves the distinction we have hitherto applied.
I read an article recently about our last universal common ancestor (LUCA). It was discussing work by researchers at Heinrich Heine University to reconstruct the genome of the last universal common ancestor – the lifeform from which all other lifeforms evolved. I will leave it to you to read the article and weigh in upon the possible flaws and assumptions in the methods described, and instead focus upon a specific line, in which the LUCA is described as “not a free-living cell…just a very advanced [combination of] chemical reactions.”
Often, I think of science as a hierarchy. Physics is the bottom of the pyramid, the foundation upon which all the other scientific fields are built (mathematics are the rebar that gives that foundation strength in this paradigm). Chemistry is the next level up the pyramid. You could describe everything that happens in chemistry with physics, but the field is separated because you can make certain assumptions and simplifications. In other words, physics can be used to do chemistry, but you can’t really use chemistry to do physics. Biology is then another step up the pyramid. Biological functions are composed of chemical and physical interactions, but we simplify them further and make additional assumptions in biology.
Aside from the methodological concerns that I have with the LUCA work referenced in the article, it really highlights the interaction between the different tiers of science (and it is worth noting that I am not seeking to devalue any particular field of science, just to explicate their relationships and interactions). If we are now calling an uncontained series of semi-self-sustaining chemical reactions “life,” then how does that inform our understanding and definition of life? If the last universal common ancestor of every lifeform on Earth (and I take issue with the assumption that all life on Earth must necessarily have a single common ancestor – it seems perfectly reasonable that more than one lifeform could have developed under similarly favorable conditions) was nothing but some chemical reactions happening in geothermally active rock pores, then how is that different from, say, a self-sustaining nuclear reaction?
There are almost as many definitions of life as there are biologists. Victor and George Tetz proposed in the journal Biomolecular Concepts that “life is an organized matter that provides genetic information metabolism,” which presupposes that life must possess a genetic code. A common definition of life is that “organisms are open systems that maintain homestasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce, and evolve.” This is a slightly more rigorous incorporation of the classic five requirements for life: metabolic action, growth, response to stimulus, reproduction, and death.
It is telling that our definitions of life often refer to life itself. Of course lifeforms will have a life cycle – they’re lifeforms. The fundamental problem here on display is that we don’t really know how to differentiate life from nonlife when we get closer to edge cases. We think that we know what life is and isn’t when it’s obvious – human beings are alive, that rock outside is not, for instance – but the more we examine our assumptions, pare things back to basics, and really dig into these topics, the less obvious the dichotomy becomes. Biologists don’t even agree on whether viruses constitute lifeforms, or recall my argument about stars in our previous post of a similar topic. In that previous post I refrained from proposing alternative definitions for life, but I think the time has come to have that discussion.
Classical physics suggest a deterministic reality, and for many years this paradigm guided research in the fields. Even statistical approaches to certain problems, such as applications of the root mean square law, were considered approximate solutions to complex systems that were still, at a level beyond what contemporary analysis could approach, were obeying deterministic laws. This is the Newtonian physics with which you might be familiar from high school, and according to its thinking, if you account for all of the variables, possess the initial conditions, and know the appropriate equations, you would be able to predict the past, present, and future of every objet in the universe, from the beginning to the end, with accuracy only limited by the information to which you have access.
Under this understanding the question of defining life becomes almost incidental. Biologists might care, but physicists and chemists would consider the question rather irrelevant. If it is possible to predict every event, every action and reaction, every fact about every moment in time from the beginning to the end based on a set of physical laws and the universe’s initial conditions, then life itself is almost incidental, just the inevitable result of the laws of physics propagated from the initial conditions to the present and beyond, a series of chemical and physical interactions that has approximately the significance of a collision between two billiard balls.
Quantum physics, at least as we currently understand it, provides a certain counterpoint to this argument for a deterministic reality with the introduction of probability (although there is a school of thought that claims that our current probabilistic understanding of quantum physics is just evidence of a gap in our understanding of quantum scale effects, and that once we have determined the mechanism by which probability waves are collapsed (for instance) the randomness will be dispelled). Yet even this does not really distinguish life from a complicated system of reactions. There is an argument to be made, therefore, that the entire discussion of a definition of life is somewhat beside the point, and that what we call life is no different from what we don’t call life, that there is no scientific basis for differentiating between life and not-life.
I am not prepared to go that far, however, at least not without a much more convincing argument than I have ever seen presented. Yet no definition or identifier I have encountered seems to me to capture what really separates life from not-life, and I am not content to leave it at “you know it’s alive when you see it.” Of the five classic requirements for life – metabolic action, growth, response to stimulus, reproduction, and death – I would eliminate metabolic action as being just a description of chemical reactions, growth as being irrelevant, and response to stimulus as merely describing action and reaction. That leaves reproduction and death.
My logic for this is something that long confused me about life, which I saw as being in fundamental contradiction with entropy. What seems to most distinguish life from not-life is order, or if you prefer, distinguishability. Recall that entropy is a measure of the uniqueness or uniformity of a system. If your house is well ordered so that you know exactly where everything is and each thing has its own place, that system has a high degree of uniqueness, and is considered low entropy. If, on the other hand, your house is an absolute mess, so that everything is all mixed together and there is no differentiation between, say, the kitchen towels and the bathroom towels, that is a high entropy system. On the scale of cosmology, when the universe can be separated out into planets, stars, interstellar medium, and so forth, that is a low entropy system, since many points in space are distinguishable from others. If all of the matter and energy in the universe were distributed in a uniform cloud, that would be a high entropy system, because there would be nothing to distinguish one point in space from another.
Thermodynamics tells us that entropy will always increase, which has been repeatedly proven and affirmed, and all of the changes in our understanding of physics and the universe have failed to shake the validity of this claim. Yet life, on its surface, appears to be in defiance of entropy. It is an ordered system that propagates itself. Death, of course, is an increase in entropy – I think of it like entropy catching up – but life appears to be in contradiction of ever-increasing entropy.
That is not actually the case. Although entropy does appear to decrease in the microcosm of the arising of a lifeform and its continued existence, growth, development, and so forth, in the larger scale of the universe entropy is still increasing. Human cities may appear to be a bold effort to force order upon the disorder of increasing entropy, the energy required to create this microcosm of lower entropy leads to high entropy overall. In this way, creating an area of lower entropy really is an exchange of entropy, not a reduction. It has even been proposed that life is itself an expression of the universe’s tendency towards increased entropy.
Thus, I would make part of my definition of life that it must exhibit negative entropy within its microcosm of existence – that is, that it makes itself more distinguishable from its surrounding environment. Death would become the cessation of that exhibition of negative entropic tendencies.
Let us examine this definition in practice. It is clear that all of the entities that we currently consider to be lifeforms would meet this definition of decreasing entropy in their locality, even if that locality is only within themselves; the mere fact that we can distinguish them as lifeforms is evidence of this. A rock, however, would not constitute a lifeform, because it does not exhibit a tendency to decrease entropy, and it does not change. Change must be another prerequisite for life, which is how I would encompass the traditional tenets of growth, metabolic action, and response to stimulus. It also does not reproduce.
More interesting and useful, however, are edge cases, and here my definition also encounters the problem that something like a star could be interpreted as being alive. Is it possible, then, that a star is in some sense alive? Or are these new definitions also inadequate?
Rather than looking at this as a new definition of life, although I have made my semi-educated proposal, I would rather that you approach this post as a matter of discussion. It is easy, on the surface, to say what life is and what life is not, but digging deeper into the question becomes more complicated. At what point, for instance, could a robot be considered a lifeform? There will at some point be robots that are capable of a form of reproduction, that degrade eventually, and that meet even the other, classic definitions of life. Yet I suspect that most biologists would argue that such a robot would not be alive. If there is an answer, if this whole idea of life is not just a mistaken assumption about the universe that we humans cannot quite let go of yet, I haven’t yet identified it, but I hope that this has gotten you thinking about just what life really might be.