In Our Time - The Origins of Life

Episode Date: September 23, 2004

Melvyn Bragg and guests discuss the emergence of the world’s first organic matter nearly four billion years ago. Scientists have named 1.5 million species of living organism on the land, in the skies and in the oceans of planet Earth and a new one is classified every day.  Estimates of how many species remain to be discovered vary wildly, but science accepts one categorical point – all living matter on our planet, from the nematode to the elephant, from the bacterium to the blue whale, is derived from a single common ancestor. What was that ancestor?  Did it really emerge from a ‘primordial soup’? And what, in the explanation of evolutionary science, provided the catalyst to start turning the cycle of life?With Richard Dawkins, Charles Simonyi Professor of the Public Understanding of Science at Oxford University; Richard Corfield, Visiting Senior Lecturer at the Centre for Earth, Planetary, Space and Astronomical Research at the Open University; Linda Partridge, Biology and Biotechnology Research Council Professor at University College London.

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or wherever you get your pods. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the program. Hello. Scientists have named 1.5 million species of living organism on the land, in the skies, and in the ocean of planet Earths. And a new one's classified every day.

Estimates of how many species remain to be discovered vary widely. But science accepts one categorical, point. All living matter on our planet from the nematode to the elephant, from the bacterium to the blue whale is derived from a single common ancestor. What was that ancestor? Did it really emerge from primordial soup? And what did in the explanation of evolutionary science provide the catalyst to start turning the cycle of life? With me to explore the scientific explanation for the origin of life is Richard Dawkins, the Charles Simony, Professor of the Public understanding of science at Oxford University, and author of The Ancestors Tale,

a pilgrimage to the dawn of life. Richard Corfield, visiting senior lecturer at the Centre for Earth, Planetary Space and Astronomical Research at the Open University, an author of the Silent Landscape, and Linda Partridge, Biology and Biotechnology Research Council Professor at University College, London. Richard Corfield, can we start with the time the Earth was formed? What time scale was that, and what were the conditions of Earth then? Well, the Earth accreted out of the primordial solar system disk

about 4.5 billion years ago. The convention in geology is that we call a billion GA, and so you might hear me refer to it as GA from time to time. A more helpful analogy, though, when you think about the span of geological time is to consider one single day, 24 hours, and midnight is taken as 4.5 GA, 4.5 billion years. That's when the Earth formed. From then on, the first aeon of the history of the Earth is called the Heideon,

so-called because the Earth and the other planets which were forming at the same time, solar system accreted at the same time as the Earth, was being continuously pummeled by a reign of asteroids and meteorites. That's where the name Heideon comes from. That's called the heavy bombardment period of Earth history, and that ended at 3.9 billion years before present, 3.9 GA, which if you're thinking about the 24-hour clock, equates to 12 minutes past 3 in the morning.

And then we move on to the time when life may or may not have emerged. Now, what were the conditions on the planet, this planet Earth then, what were the basic conditions? The necessary conditions? Okay. The first thing to say is that the early Earth was quite, quite different to the world we know today. The green planet, which was famously shown, the green and blue planet, which was famously shown from the Apollo 8 mission, is the result of oxygen, photosynthesis,

and the greening of the world. There was no oxygen at the end of the Hadean. Indeed, that oxygen in that time would be toxic to the organisms, rather, which were just beginning to evolve. But there was an atmosphere, wasn't? There was a gravitational field which would hold an atmosphere. It was. Can you tell us how important that was?

Okay, the atmosphere was the so-called reducing atmosphere to distinguish it from an oxidizing atmosphere. It had carbon dioxide, methane, hydrogen, ammonia, nitrogen, carbon monoxide. And this broth of basic chemicals was continuously cooked by lightning discharges and the surface of the earth was pocked with continuous. continuous volcanism. It was a very unpleasant place indeed. So the three things there, if I can, if I

can pick up the word reduce and reduce it to that, there's an atmosphere around with the gravitational pool, which was that. There were volcanoes bringing stuff out, including water, carbon dioxide, methane, and so on, and there were flashes of lightning coming in. This is what was there then. That is the condition of

the Earth at 3.9 billion years before present when the late heavy bombardment period had just finished. Richard Dawkins, could you take up the point made by Richard Corfield, which will surprise a great number of listeners that surprised me when it came across it, was that there was no oxygen then, and oxygen was not necessary for this process, the origin of life, to begin? Can you just develop that?

It is a surprising fact that the oxygen that we breathe and which we utterly depend upon is itself a biological product, that it comes from green plants and green bacteria. I think many people are surprised by that, as you say, because they sort of feel there's something fresh and wonderful about the oxygen in our atmosphere. As Richard Caulfield said, it was originally a pollutant. It was toxic, it was poisonous.

When green plants and green bacteria released it into the atmosphere, it then became the condition that life had to adapt to, and we are the end product, not the end product, we are the product of that. We are now the product of natural selection in an oxygen-rich atmosphere. And not only do we tolerate our oxygen, we depend upon oxygen, and we've taken biochemistry much further. And in the 1950s, a man called Stanley Miller performed an extraordinary experiment. Can you tell us about that experiment and why I use the word extraordinary?

He got together the ingredients that were thought to be present in the, early Earth atmosphere. Around the 3.9 billion market, Richard. Yeah, yeah. Right, Richard. And set up an apparatus in which he had a flask that represented the sea and a flask that represented the atmosphere above it and put into the atmosphere the ingredients, the non-oxygenated ingredients of the early earth.

There was a circulation, tube going up, tube going down. there was electric spark which was simulating lightning strikes and he just left it for a couple of weeks and at the end of this time there had accumulated in the sea the lower of the two flasks, the one with water in, a thin brown liquid which when he analysed it it turned out to be pretty much what JBS Haldane had speculated as the hot brown soup

it contained numerous organic compounds, many of which were vital to the origin of life amino acids of various kinds, including several amino acids from the 20 that life actually uses. But we're not talking yet about the origin of life as we're going to come to discuss it further. No, no. I mean, this was... This would not, that in itself would not have led to the development of life. No, there was nothing living there. So what did it prove? It proved that the conditions of the early Earth were ripe for the synthesis

under the ordinary laws of chemistry of many of the basic building blocks of life. It was a precondition for life. It wasn't life itself. Linda Partridge, can you tell us the place that carbon played in all this and why it's so fundamental? Carbon's an absolutely critical atom for this process to occur. The reason is it's structure. Each element consists of a nucleus surrounded by electrons and the interesting thing about carbon

is that its electron space is half-filled so it's very eager to interact with other elements and make more complicated molecules so it can form bonds with oxygen, with nitrogen, with sulphur, all of the elements that make the ingredients of living organisms. So it's bond structure and the fact that it's so keen to interact with other molecules, and that allows it to make large complex polymers and molecules with complicated shapes that make it such a vital building block in living creatures.

And almost certainly, I think, if life has evolved somewhere else in the universe, it will be based on carbon for that reason. There is no other element that has this set of properties that make it such an ideal building block for living things. As a digression, but as we will be coming on to it, as this is the precursor to the great work of Darwin. Darwin wrote a famous letter to Hooker in 1871. Can you tell us about that, Lyndon, why you think it's significant?

It was quite extraordinarily prescient this letter that Darwin wrote. He almost described the conditions that existed on the primitive earth and almost seemed to foresee the experiments that Miller and Yuri eventually did with the gases and the electric discharges and the organic synthesis. He described it in a rather cozy way compared with the picture that we've been given of what the conditions on the early earth were like. They were almost certainly extremely hostile.

Darwin described a nice, warm little pond, and that's almost certainly not the circumstances under which life evolved. But he did foresee the importance of this mixture of organic compounds and the possibility that they would form larger and more complicated molecules and eventually give rise to life. Okay, well, I'd just sort of plan. for my hope really. Now then, Richard Corfield, could you give us a working definition of life before we're talking about the origin of me? What would life be? How would we say, well,

this is life? Right. Well, that of course is a non-trivial question. I was rather hoping you give it to Richard Dawkins, actually. But to be blunt, a definition of life is something that we have to strap on immediately so that we all know what we're talking about. I was looking at a paper recently and I saw that there are 102 criteria to define life, but they can be boiled down into about four, really. And the question is whether even those four are all mutually applicable. Life replicates itself. It makes copies of itself, but that doesn't make life unique. Crystals do the same thing. Life is about metabolism. It builds things up as opposed to letting things break down.

And that leads me to my personal favourite definition of life. The single thing which I happen to think is most important, and I have done since I was a schoolboy, which goes back to Owen Schrodinger's 1944 definition of life, which is that it is reverse entropy. And that is based on Boltzmann's formulation of the second law of thermodynamics, which, simply put, means that systems will tend to run down

to the lowest energy level. I have a glass of water in front of me. Eventually that will assume the same temperature as the rest of the room. That's entropy occurring. Life does the opposite thing. It forms complexity, so it runs uphill against the entropy gradient. If I can give you an example, an analogy, I have the builders in at the moment. I watch them knock down my study.

That's entropy. Now they're building it up again, I hope. And that's negative entropy. and that is my personal favorite definition of life reversal in the entropy gradient okay replication reverse entropy metabolism

which is a kind of part of the reverse entropy thing and the thing which perhaps Richard Dawkins should address which is susceptibility to Darwinian evolution you know visibility to natural selection I think you should three should just take this program over I was about to say

perfect location by me. Richard, could you talk about that, especially with relevance to heredity, because that is a point you make very firmly in your new book. Yes, I think that when you ask what is life, one could treat that as what would life be wherever one found it anywhere in the universe, or it could be what do we happen to observe about life here? And I suspect that the four, which Richard Corfield has mentioned, are universal.

I don't think they're just particular to life on this planet. and I think that the really fundamental one is susceptibility to natural selection or a product of natural selection. Life is what you get when the ordinary laws of physics and chemistry which pervade the entire universe find themselves filtered through this remarkable process of natural selection. And natural selection will arise on any planet in the universe wherever you have true heredity.

And true heredity means that you're... You have entities which are self-replicating with high accuracy, but not perfect accuracy, such that you get a population of such replicating entities, which are not all identical. Therefore, some of them are better at replicating than others. Some of them die away, others increase in the population of replicators. And that is the starting point for natural selection, because once that starts, then everything else follows which we call life. But the real question is, how does that start?

How does the first replicator replicate and how do we know that with any degree of action? That's exactly what we don't know and that's exactly what the whole field of origin of life research is about, in my view. It is how do you get from the ordinary laws of chemistry? To biology. Well, to an entity which is self-replicating in this peculiar sense. And that must have happened.

It did happen because here we are. If it happened anywhere else in the universe, there will be another kind of life. Well, let's just stick to this planet. It's quite hard enough. But at some stage, what you postulate has to be that at some stage, at one time or several times, let's say at one time, there was a spontaneous generation of something that then became susceptible to natural selection, which then became divergent as soon as it became susceptible to natural

and then the whole game was afoot. But what is the scholarship now about what that first spontaneous generation was? We're thinking it's between 3.9 billion and 3.5 billion years. Putting that aside just what happened? Well, when you say it's spontaneous, spontaneous is right, but of course I mean spontaneous things are happening all the time with the ordinary movements of atoms and molecules in chemistry. The particular spontaneous thing that had to happen was that a molecule,

rose, which whatever other properties it had, it had the property of making copies of itself. One might call that what a chemist might call that autocatalysis. A catalyst is a molecule or a chemical agent which facilitates a chemical reaction, which without participating or without being used up. An autocatalyst is a catalyst which facilitates its own production. If I remember correctly, I haven't got that in the note in front of me at the moment, book you say, you use even the word luck. It could have been luck that caused this. It could in... I mean, people, creationists and Christians and religious people,

say this would be the divine spark. One has to say that because that is what a lot of people would believe would happen. But you also use the word luck. I was interested by a use of the word luck. It simply means an improbable event. We know it's an improbable event because if it wasn't, it would be all over the... I mean, there'd be life on Mars, Venus, Jupiter, and it's, it's It's certainly a very improbable event. We don't know quite how improbable. I've speculated that it could be absolutely vanishingly improbable because we don't know there's life on more than one of the billions

and billions of trillions of planets that exist. So it could have been very, very improbable indeed, in which case we're totally wasting our time trying to speculate about it. Oh, that's wonderful waste of time. I don't actually believe that. I mean, I think it's sufficiently improbable to be present here and probably dotted around on isolated islands as well. but it was very improbable and that's another way of saying luck.

Linda and Richard want to come in. Linda. Can we just keep ruminating and rummaging around this particular small vital point? Of course. It may have been very improbable but it happened extraordinarily quickly in geological terms. Good point. I mean it was 3.9 million years ago we stopped being bombarded. And 3.6,000 million years ago, the first fossils. So life presumably evolved well before that. So, I mean, one could argue that if the conditions are propitious, it's actually extremely likely to happen.

I agree. One of the interesting things is that the oldest sedimentary rocks we know on this planet are from the issuer complex in Western Greenland. And there is evidence to suggest, and these rocks are 3.8 billion years old. That's 100 million years younger than 3.9 when, you know, the starting gate went up and life could have got started, and that's only half an hour on our 24-hour clock.

Now, if there is evidence for photosynthesis, as some suggest there is, in rocks at 3.8 billion years before present, that means that you got from no life to photosynthesis, i.e. quite complicated life, in 100 million years, which is, it's like Michael Schumacher going down the hangar straight. You know, it's just not hanging around. Shall we then speculate on, well could you please speculate,

on how this might have happened, what formations inside, how did the thing got going in the first place? Richard Dawkins, you've talked about not being DNA, but possibly RNA could have been the cause of it. Can we bring that in? DNA is what Graham Cairns-Smith has called a high-tech replicator. It needs a lot of machinery,

rather like needing a Xerox. machine to actually make a copy of a piece of paper. So it almost certainly wasn't DNA. And there's a catch-22 in the DNA. There is a catch-22. DNA is needed to make protein, and protein in the form of enzymes is needed to make DNA. So it's a closed system in that. So that's a very, that is the catch-22 of the origin of life, and it's very difficult. We have these two fundamental properties of molecules that you need. Replication, which DNA is brilliant at, and catalysis, which protein. are brilliant at. And you seem to need both, and it's not clear how you can get one without the

other. RNA seems to have some of the properties of both. RNA is a good replicator, though not as good as DNA. RNA can act as an enzyme, can act as a catalyst. And so the hope of the RNA world theory is that RNA might have actually done both jobs. And then later, the job of replicating was as it were handed over to the really streamlined high-tech version DNA. Linda, can you spell out RNA for listeners like myself who are not familiar with these letters? Ribos nucleic acid. It's one of the two main nucleic acid molecules

that occur in modern organisms. And as Richard says, it has this extraordinary property. It can act as a genetic template to direct its own replication. But it can also behave like modern day proteins do as a catalyst that makes chemical reactions far more likely to occur easily in cells. And in fact, this is one of the areas where there's been a lot of experimental work on the origin of life, because it's possible to evolve in the test tube RNA molecules that can do

all kinds of chemical jobs that in cells now are done by proteins. So it's very plausible that this whole RNA world existed, I think. Richard Dawkins, can you tell us what might then have happened? Before we move on to complexity, just what might have happened? How, in your view, and it is, I should make it quite clear. How, in your view, the first very early forms came into being which allowed natural selection to begin?

A major step in the subsequent evolution under natural selection was the formation of something like the first cell. Because when you had the first cell, that meant that a membrane of some sort, a wall separated units of self-replication and kept their chemistry. products together, instead of having them streaming around free in the soup. And I think that enabled the possibility of building up complex collections of molecules

which work together with each other, which is what a cell is, as opposed to just having them streaming out into the sea and having a very indirect effect on the replication success of the competing entities. So the first cell wall seems to me to be a very crucial step. Living cells are conventionally divided, nowadays conventionally divided into two main types, eukaryotic and prokaryotic. Prokaryotic cells are bacteria, various kinds. Eukaryotic cells are those of the cells of all the rest of us. They're enormously larger than prokaryotic cells.

They're much more complicated. They have a nucleus within the cell, which contains the genus. genetic material separated off from the rest of the cell. The rest of the cell is filled with complicated systems of membranes, including mitochondria, which we now know are themselves originally derived from bacteria. So at a moment in history, when several different kinds of bacteria, it is now thought, came together to form a kind of social unit, a bigger cell, which was a combination of different kinds of bacteria. Richard Cotter.

Just to give you some perspective on the time, by the time the eukaryotes evolved, you're at effectively 2 billion years before present. You're half of the way pretty much towards where we are now. So the evolution of the eukaryotic cell is actually quite a complicated long-term thing in terms of natural selection. It's a major platform from which life could then step up

on the way to multicellularity. Linda, you wanted to come in. Can you take this forward to the further developments from this eukaryotic cell? What happened, as it were, after that? Well, one of the things about it just before moving on to that and very quickly is that this may really have been one of the very improbable steps in the evolutionary tree, I think,

because it turns out that eukaryotes are almost certainly derived from a very basic fusion of two quite separate bacterial lineages, one of which brought in the information transfer and processing machinery in the cell and the other of which brought in the metabolic bits, the things that actually do things within the cell. And they seem to have had quite separate evolutionary origins,

which makes a lot of sense of why there have been such conflicting results if evolutionary biologists try and ask the question, where did the eukaryotes come from? What are they most closely related to? Well, the answer is they're closely related to several things because essentially the eukaryotic cell is a committee of separate bacteria. lineages. But the next big event, of course, was the evolution of multicellularity, cells coming

together to form more complex organisms. And the evidence on that is that it's happened more than once. It happened separately in the plant, animal and fungal lineages, and also several lines gave rise to multicellular algae from single-cellular forms. So that seems to be quite a common, easy step to take. So by going for the cells, as I were Richard Dawkins, we're finding commonality with right across the planet, whichever way you cut it? Yes, well, the new findings Linda's just been talking about of two separate genomes merging is immensely exciting. That's quite a new thing, which I'm just still trying to digest. But yes, after multicellularity, then you have the evolution of life, as we've known it for a long time,

and life as Darwin would have known it. Big, big life, life that you can see creatures that swim and walk and fly and climb. All that seems to have come about through the modularity, the building up of lots and lots of cells, each cell having fundamentally the same structure, but modified for different particular purposes in a kind of society of cells. A point that has been raised, and it was raised a long time ago by Fred Hoyle, was that life, or the beginnings of life, or the things that could make life begin, came from asteroids came from extraterrestrial sources. What's your view about Richard Colvin?

Well, carbonaceous chondrites, particular type of meteorite, are so called because they have carbon compounds on them. And in fact, the basic building blocks of life amino acids and probably nucleic acids, what came before them, the things that they themselves are formed from, are available in stardust, in outer space. There's nothing particularly unique about the basic chemicals of life. It's just the way they were put together on this planet. If you take that one step further and say, was life seeded from outer space, the pan-spermia theory,

well, it may have been, but that doesn't help you very much because it means that you just have to go somewhere else to figure out how it started in the first place. Linda? Yes, quite agree with all of that. And even if life did in fact evolve here and didn't come from outer space, some of the information that gave rise to it may have come from. elsewhere. There's plenty of evidence that the early Earth received a large amount of organic

input from outside, from more widely in the solar system, and sometimes the sorts of molecules that come have a particular handedness to them, which doesn't tend to happen when they're formed under the conditions that prevailed in the early earth, and that handedness may well have acted as a source of information for modern biological systems. Once it got going, Richard Dawkins, once natural selection, And once there was the ability to enter into Darwinism, as it were, was it inevitable? Was there a sense of inevitability, the massive multiplication of species and so on?

Well, that's very interesting. I think that there may have been more than one very difficult step to get over. It's already been mentioned that the formation of the eukaryocracy, the eukaryotic cell with all that followed from that, could have been a very, very difficult step. and it could be that there's all sorts of beginnings of life elsewhere, none of which reached that point. There could be other major steps.

Multi-cellularity might have been a difficult step. I don't think, in fact, it was because, as Linda says, it happened many times, so we know that multicellularity is not that difficult. But maybe intelligent life, maybe the sort of life that is capable of language, of technology, and of broadcasting itself by radio to other planets, maybe that's a very, very difficult step indeed, and that may be the answer to the riddle of, as Enrico Fermi said,

where are they? Why haven't we received radio communications from outer space? It could be that there's plenty of life out there, but only we have reached the technology threshold. Well, thank you very much. Lynne Appartre, Richard Corfield and Richard Dawkins. And next week we'll be discussing politeness. Thank you for listening. We hope you've enjoyed this Radio 4 podcast.

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