What is life?
Will all life in the Universe be like ours? Or something completely different? Nick Lane and Graihagh Jackson ponder the possibilities...
Graihagh - Perhaps then, it’s best left a mystery when it comes to alien visitations… But what about the possibility of life beyond Earth? Well, to that, we probably need to know what life is…
Nick - It is notoriously difficult to do and actually, I think it’s almost pointless to try to define life.
Graihagh - Ok then, maybe not. This is Nick Lane - he’s a biochemist at UCL and is a bit of an authority on the origins of life here on Earth. Perhaps it’s best to think about what life does, rather than is.
Nick - I mean, there’s hundreds of definitions of life out there and they’re all wrong in one way or another. And the problem is that life is really a continuum from a non-living state to a living state and there’s all kinds of intermediate stages. So, is a virus alive or not is a question which is often discussed. It’s really what life does rather than what it is, and in all these cases, life is making copies of itself and it’s using the environment to do so. So, one of the problems with most attempts to define life is that it excludes the environment. All life parasitizes the environment in one way or another. Plants do, they require sunlight, they require carbon dioxide, they require water, and so on, that’s all they require. We parasitize the environment a lot more. We go around eating plants and so on. But essentially, all life is parasitizing an environment which is providing it with its energy needs to make copies of itself, so I think you’d say there are about six different things a cell requires. It requires a carbon source to make more copies of itself, it requires energy to bind things together, to make polymers and to produce more cells, it requires excretion, you’ve got to get rid of the waste products and the end products to drive reactions in a forward direction. There has to be some form of compartmentalization, a cell-like structure that makes the insides different from the outside. There have to be catalysts, the beginnings of biochemical reactions, and then, there has to be some form of replication. Now I think those are the six properties of life that we really need to look for.
Graihagh - Now we have our six things needed for life but it took a long time for it to come about on Earth…
Nick - There’s a lot of arguments about it, a kind of glib answer would be about four billion years ago. There are fractionated isotopes of carbon and so on in ancient rock from about 3.9 billion years ago. There’s a lot of debate about whether that signifies life or not, but I think most people think on balance, it probably does. I think we’ll never know exactly how life started on Earth but what we can know, what are the principles that lead to the origin of life from a non-living environment, and that’s what we’re looking for in trying to understand the origin of life here.
Graihagh - So, if we’re looking for life elsewhere then, it’s likely to be the same.
Nick - I think that’s actually a good argument to say that life could end up, at least at the bacterial level, remarkably similar. I mean, there’s a strong argument to say that carbon is really better than anything else. It’s much better than silicon, for example, at forming, you know, complex bonds between molecules and it’s also available. You know, carbon is far more available in the universe than silicon and also there are gaseous carbon oxides, carbon dioxides, and so on. It’s like a Lego brick, whereas silicon oxides are, you know, sands and so on, you can’t really boot-strap yourself up from the ground with sand. You can’t build on sand.
Graihagh - That’s because the rules of physics and chemistry are universal and, therefore, exactly the same constraints will exist wherever you live in the Milky Way or even the Andromeda galaxy. To Nick then, it’s likely you’re going end up following the same sorts of evolutionary pathways.
Nick - I think, yes, it’s possible. We can conceive that life could’ve operated in different ways but if you think about the probability of finding life, carbon, water, the kind of rocks that are required for hydrothermal systems and so on. They are all very common, so the kind of life that we have here is likely to be the kind of life that we find elsewhere as well.