How did life begin?
What is the likelihood that DNA would turn up on two different planets? And how did it first appear on earth? Dr Matt Powner is from University College London, who studies the origins of life, and he spoke to Chris Smith...
Matt - Essentially, as far back as we can see, we think we can find traces of life. So, as we look back in the rock record of the Earth, we start to run out of actual records somewhere about 3.8 billion years ago, but as far back as that we think we can find traces of carbon isotope effects that suggest life was there even as early as that.
Chris - And suggests that whatever that process was, it happened quick?
Matt - On a geological timescale - quick. But on a chemical timescale - it's still a long period of history. We still have almost half a billion years between the moon forming event when no life could have existed on Earth, the Earth was just too hot, literally molten rock, and 3.8 billion years. There's nearly half a billion years and chemically that's a long time. Geologically, it's quite a short period.
Chris - So would you like to speculate for me and tell me what you think those first life forms, or those first life processes were - what happened?
Matt - Yes. So to think about that, we need to think about what it is to be alive and that's a very difficult thing for us to formulate opinions upon because we only have one example of life and all that life on Earth is all related. It's all part of the same family tree.
But generalistically, there is a working model. This was developed by a panel for NASA of what life is and that is a self-sustaining system of chemicals that can undergo Darwinian evolution. So, to have a system of chemicals that can do this we need a molecule that can pass information from one generation to the next. And what biology uses for this are nucleic acids, so like DNA and it's close cousin RNA. These are polymeric molecules, so a molecule that's a string of individual components. A bit like a string of beads and each bead has an informational unit, or is an informational unit. The order in which we build the string of beads then gives us a message, and that message can talk to itself and to other molecules which then allows it to be copied and allows it to have utility and function. So once you have a molecule that has information and can be copied, then you can start to access the processes of Darwinian evolution.
Chris - So you think the first life was a sequence of chemical reactions which store information in some kind of molecule, maybe like DNA, and it inherently has this ability to copy itself and control what happens to it and it then, ultimately, adds extra complexity that turns into things like the cells that we have that make our bodies?
Matt - That is part of the answer. You need an informational molecules, something that can retain information over many generations, but that molecule doesn't seem to be able to function on it's own. We need other things like a lipid membrane to isolate that information from other forms of information. So realistically, looking for one molecule that is the origin of life is unlikely to give us a fruitful answer. We need many molecules to understand how all of those come together and how they're generationally linked to understand where life could have come from.
Chris - And actually how do we think that happened? Why did this miracle occur?
Matt - Current evidence points to the fact that if we take very simple, geochemically plausible one and two carbon units, they can be reacted together in very simple, very robust reactions to give, specifically, the components that we find in biology. Then if these have energy, if we have the ability to keep recreating these molecules, you can understand how something that can replicate itself can come to dominate a chemical system and start to lead to the processes that we now recognise as life.
Chris - And since we can regard this as capable of being reduced down to chemistry, and given the ubiquity of chemistry in the universe. We know that we've got the same elements here on Earth that we have on Mars and beyond our galaxy and across the universe. Does that mean then, that it's likely that alien life probably will settle on the same sorts of chemistry that we have observed here on Earth and, therefore, we probably do have a high likelihood of meeting aliens that also use DNA as their hereditary material?
Matt - In my opinion, the chemistry that governs why our biology selected the fundamental conserved metabolites that are essential to all living cells, that chemistry is based on some very selective reactions that can assemble these molecules, and these molecules specifically from very, very simple chemical precursors. These chemical precursors we find throughout the universe so, for example, hydrogen cyanide. A very simple compound, we find it everywhere we look in the universe.
Chris - Not terribly good for you though!
Matt - Not for us, but for early life it is thought to be an essential element to building both proteins and nuclear base material. So it's not only the relationship of the individual molecule, so how easy DNA or RNA is to make. It's also the relationship to the other molecules of biology. So the same chemistries that builds nucleotides, build amino acids and can be used to build lipids. So it's the interrelationship of all these molecules and the fact that they are all synthesised, through very simple and similar mechanisms, suggest a universality to that chemistry. There are, obviously, ways you can change it slightly in small ways and you have variations on the theme, but I believe the overarching mechanisms that biology uses at the most fundamental level will likely be universal, or could be replicated many times throughout the universe.
Chris - In conditions as exists here on Earth because we know our own solar system? If you go to Titan, for example, one of moons of Saturn, you will find an ocean there but it's an ocean of ethane. The temperature is -200 and something degrees C. So could we see other chemistries that sustain other interesting life processes? We wouldn't recognise them as our sort of life, but they're still life.
Matt - It is absolutely a possibility that in different conditions we could envision something very, very different as life but it would have to be phenomenally different. For life to have evolved as we know it in an organic environment like ethane, we have to change everything about the cell. So cell membranes are built on the process of excluding water, so the lipophilic interactions of amphiphiles. The same is true of DNA. So DNA has a lipophilic core and it's this exclusion of water that drives so much of life's processes that to envisage life, for example on Titan in a hydrocarbon environment, it would have to be fundamentally different from our life at every level.