What is life and where did it come from?

This week, we're looking for life beyond Earth: is there anybody out there? Remarkably, we're making progress in answering that question. In the last few years, we've discovered that our galaxy is teeming with alien planets but the big question remains - do any of them harbour life? Coming up, how do we define life and how did it begin; what can we tell about planets thousands of light years away and how many alien civilisations they might be home to? Now to many people, astrobiology means the hunt for alien life, but a big part of this is understanding what life actually is, and what's needed for it to exist. 

Everywhere we look on earth, there's life in an endless variety of shapes and sizes- from microbes to elephants. But how do we define it? What is life and how is it different from non-life?  Nick Lane is a biochemist and he spoke to Chris Smith about the origins of life.

 Chris - So, how do we tell what's alive like a mouse from a brick? Because chemically they're both made of very similar things.

Nick - It is notoriously difficult to do and actually, I think it's almost pointless to try to define life. 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.

Chris - You said that there has to be a carbon source. To what extent is the life we see here on Earth so unique to this environment that you're not going to find it anywhere else or do you think if another planet Earth-like environment exists out there that life would take exactly the same pathway of evolution that it has here and we will be looking at our mirror image out there, somewhere.

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.

Chris - So, you're sort of saying that because the rules of physics and chemistry are universal throughout the universe, therefore, exactly the same constraints will exist wherever you live - Milky way or even the Andromeda galaxy and therefore, you're gonna' end up following the same sorts of 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.

Chris - The Earth's four and a half billion years old, so how long after the Earth formed, did life first pop up?

Nick - Well, we don't really know. 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.

Chris - Where do you think that life came from? What sorts of theories are out there to explain how life arose? Did it arrive de novo, in other words, from scratch here or is it possible that it could have had some kind of injection of some processes from, say, outer space. 

Nick - Well, we know for sure that there've been plenty of organic molecules delivered from space on meteorites, there's no question about that. Whether that prompted life on Earth in some way, conceptually what it does really is stock a soup, and so conceptually, it's not really any different to say the Miller-Urey experiment from 50-60 years ago, showing that lightning and UV radiation and so on, can also produce organic molecules, so can hydrothermal vent systems. It's actually remarkably easy in some ways to produce organic molecules and remarkably difficult to get beyond a soup.

Chris - So, would you be in favor then of the idea that life just spontaneously started, or do think that actually that there is credence to this idea that there could have been life coming from elsewhere, maybe intact life coming from elsewhere in the universe.

Nick - There's no evidence to suggest that it did, and actually I think it's a pointless theory in the sense that if it did come from somewhere else, well we still don't know any more about how life started elsewhere. 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. And panspermia, the delivery of life from space, it just moves the problem somewhere else so it's pointless.