What is life? Some people will say it’s obvious: life is reproduction. But I may never choose to reproduce, and a worker ant couldn’t if it wanted to – does that make us dead?
Others will say life is evolution. But on closer inspection, that doesn’t really stand up either. Evolution is easy enough to implement on a computer. You just store a bunch of random bit strings in memory, evaluate them according to some “fitness function”, and then “mutate” and “recombine” the best ones to produce a new generation. By iterating this process you get what’s called a “genetic algorithm”, and this can be used to design robot controllers and all sorts of other things. These things evolve, but are they alive? Some might say yes, but anyone with any experience in genetic algorithms will say no.
At school I was taught that life was a combination of seven properties, all of which had to be present in order for it to be life. (The creators of our syllabus loved the number seven. There were seven different types of energy as well.) I can’t remember all seven signs of life, but they were things like reproduction, growth, movement and respiration. But this isn’t a very enlightening definition, and one can find exceptions to at least some of those things.
People with a more thermodynamic point of view might say that life has the property of resisting the law of increasing entropy: it stays in an organised state by “feeding on negative entropy” in its environment, expelling all of the positive entropy that gets generated inside its body by exhaling and defecating. This is a wonderful picture that deserves to be understood – but again, if we look, we can find other things besides life that have the same property. Hurricanes are my favorite example, feeding on warm (low entropy) air from the sea surface and expelling that heat as low temperature (high entropy) thermal radiation into space.
Still others will point to the cell membrane as the defining feature, along with the self-maintaining network of chemical reactions we call metabolism. But if we create such membrane-bounded self-maintaining structures in the laboratory (as seems likely soon enough), will we automatically call them alive? Some will say, no, they will only be alive if they have the ability to adapt to their environment as well as having these properties. But again, it’s not so hard to find inanimate structures that adapt to their environment in some sense or other, and the specific combination of membrane-bound chemistry and adaptiveness doesn’t seem any more fundamental than the seven signs of life I was taught at school.
And all of this ignores many other things that all known organisms on Earth have in common. Every single living cell has nucleic acids (in the form of RNA or DNA), they all use ATP to carry energy, they’re all made of proteins, made from the same set of amino acids. They all (with minor variations) use the same ATP-powered molecular machinery to read those nucleic acids, according to the same genetic code. And of course, they’re all (more or less) membrane-bound, they all adapt to their environment, they all feed on negative entropy, they’re all the product of evolution and some individuals of every species are able to reproduce. All of this makes life very easy to differentiate from non-life. All of these things are 100% correlated with each other in living things, so you just have to throw in two or three of these shared properties and you’ve already drawn a sharp distinction between all living things and all non-living things on Earth. The problem with defining life is that it’s too easy.
But why is life so different from non-life in so many specific ways? The answer, at least to the question of specific molecules, is probably quite simple – it’s because all life on the present-day Earth shares a common ancestor. Life in its present form long ago out-competed whatever alternatives existed, and we’re left with only the winning version. The big question, if one cares about the definition of life, is which of these shared properties are fundamental to the very concept of life itself, and which are merely due to all life on Earth sharing the same common ancestor. Of course we have strong intuitions about some of them, but we can never answer them all for sure until we’ve studied many separate examples of trees of life – life that evolved on other worlds and didn’t share a common ancestor with life on Earth. With only one example of life it’s very difficult to say whether (for instance) proteins are the only way it can be achieved, or just the way it happens to have been done on Earth.
Of course, we might well find phenomena on other worlds (or even on our own) that are like life but not like life, in such a way that we’re not sure whether to call them alive or not. I expect this to eventually happen. If we discovered such a thing today it would force us to confront the blurred boundary between the living and the non-living, which only appears sharp to us because of the particulars of the world we inhabit.
Does any of this matter? One reason it might matter to you is if you study the origins of life. In order to know how life began, you might feel you need to know what life is. If you decide that life is evolution you’ll spend your time wondering how evolution could have occurred in a lifeless world; if you decide that life is nucleic acids you’ll wonder what chemistry could have produced them; if you decide life is membrane-bound cells you’ll look for them, and so on.
But life on the present-day Earth isn’t any of those things, it’s all of them, and all of them must have arisen at some point. Perhaps they all arose together, as the first fully formed organism suddenly popped out of an inert substrate (this is extremely unlikely). Perhaps nucleic acids arose first, with proteins, ATP and cell membranes appearing later; perhaps membrane-bound cells came first and nucleic acids came later; perhaps there were proteins and ATP and nucleic acids before there were cells – or perhaps early life was made from some entirely different set of chemicals altogether, which only later evolved into the ones we know today.
It’s fair to say that none of these questions have been answered. People have opinions, but the debates are not over. How each of these things happened, and in what order, is an important question for science, and by focusing on some narrow definition of life we risk ignoring some important parts of the story. Instead we should embrace the strange diversity of specific features shared by life as we know it, and admit to ourselves that in the universe at large the boundary between living and inanimate is probably not as sharp as we might like to think.