In Our Time - Ageing the Earth

Episode Date: November 20, 2003

Melvyn Bragg and guests discuss the age of the Earth. It was once thought that the world began in 4004 BC. Lord Kelvin calculated the cooling temperature of a rock the size of our planet and came up with a figure of 20 million years for the age of the Earth. Now, the history of our planet is divided into four great Eons: the Hadean, the Archaen, the Proterozoic and the Phanerozoic. Together, they are taken to encompass an incredible four and a half billion years. How can we begin to make sense of such a huge swathe of time? And can we be sure that we have got the Earth's age right? Geologists use Eras, Periods and Epochs to further punctuate what's known as 'Deep Time', but can we be sure that the classifications we use don't obscure more than they reveal? With Richard Corfield, Research Associate in the Department of Earth Sciences at Oxford University; Hazel Rymer, Senior Lecturer in the Department of Earth Sciences at the Open University; Henry Gee, Senior Editor at Nature.

This BBC podcast is supported by ads outside the UK. 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, in the 17th century, it was thought that the world began in 4004 BC. Lord Kelvin in the 19th century calculated the cooling temperature of a rock the size of our planet and came up with a figure of 20 million years for the age of the earth.

Today, the history of our planet is divided into four great eons, the Hedean, the Archaean, the Proterozoic, and the Phanerozoic. Together, they're taken to encompass four and a half billion years. How can we begin to make sense of such a huge sway of time? And can we be sure that we've got the Earth's age right? Geologists use eras, periods and epochs to further punctuate what's known as deep time. But can we be sure that the classifications we use don't obscure more than they reveal? With me to discuss Aging the Earth, our Hazel Reimer, senior lecturer in the Department of Earth Sciences

at the Open University. Henry G., senior editor at Nature, and author of Deep Time, and Richard Corfield, author of Architects of Eternity, and Research Associate in the Department of Earth Sciences at Oxford University. Richard Corfield, can you give us an overview of the four eons,

what they encompass and what they stand for in the broadest sense, beginning four and a half billion years ago? Four and a half billion years ago, the Earth was formed out of the primordial matter swirling through the solar system. It accreted out of a disk of materials, and this was a time when that primitive disk and the globe which resulted from it was bombarded continuously by a reign of meteorites. It's called the heavy bombardment period. And that lasted from about 4.5 billion years ago. that's an American billion, 1,000 million, to about 3.8 billion years ago. And this is the Hadean period, so called because it was a fairly unpleasant place and time to live.

After the Hadean, starting at about 3.8, broadly billion years before present, you had the Archeon. And that is a time when life started to get going. Very primitive single-celled organisms, blue-green algae started to, form things of fossils called stromatolites. And the Archaean is really the aeon of the stromatolite. And that lasted until about 2.5 billion years ago and was followed by the Proterozoic aeon. Now the Proterozoic was a time when evolution accelerated.

The single-celled algae, which had characterized previous Archaean, were replaced until by progressively more elaborate forms of life until by the end of the Proterozoic, you had forms of life like jellyfish, the so-called Ediacaran fauna of Australia. And then when these became extinct at just before 0.5 billion years before present, then that, in turn, the proto-ozoic was replaced by the Fanaerozoic,

which is technically the era, the aeon of visible life. Okay, and that's when, for whatever reason, around about the time of what's loosely called the pre-Cambrian Cambrian boundary. Fossils started producing hard parts which made them

visible in the fossil record. That's terrific. So that's the four aons for four and a half billion years. So we know we are... Inside the aons are eras. And these areas are... You've mentioned the Cambrian

but by the Ordovician and so on. And we're in this last aon, the current aeon, which is 540 million years old and still just going. Which particular era are we in and which particular... period of that era.

Well, we are in the Cenozoic era. We're at the end of the Cenozoic, obviously, because today is today. What marks the Cenozoic era? The start of the Cenozoic is marked by the death of the dinosaurs, that's 65 million years before present. This obviously was such a catastrophe. It was a major punctuation mark in the history of life. And so the Cenozoic era is

broadly defined by the death of the dinosaurs up to the present day, which, and the particular period we're in at the moment is the Holocene. That only started 10,000 years ago. Why is it caught the Holocene? And why, after these billions and billions, do we get this little bit of 10,000 years justifying itself as part of the map at all? Well, part of the reason is perspective, is that obviously it's very much easier to see things clearly

if they're close to you in space and time. And so traditionally, the periods and epochs of the Sinozoic era have been defined on changes in fossil assemblages. And the Holocene was a time when we started to come out of the last major series of glacial ages. We're still in it, but basically the last glaciation was at its maximum 18,000 years before present. And so 10,000 years before present marks the time when that glaciation was terminated, and we have the rise of human life on the planet. Hazel Reim, we've had that outline from Richard Corfield,

which is very clear now, the four eons, the eras, and the periods, the period that we're in now, the Holocene. What are the guiding principles behind the division of time on this massive scale? What makes the basic difference between one eon, and a few billion years later, another eon, and then one era, and then the next? What's working away to say, this is why we divide it? Well, it's simply by looking at the rocks. The answer is there in the rocks. Now, in an ideal situation, you would go to, let's say, a huge quarry face, and at the base of the quarry, you would see the oldest rocks, and at the very top you'd see the youngest rocks, and you could look through a whole sequence and just look at history. And as you do that in a huge quarry face, you can even do this in a road cutting by the side of the road. You can see changes in the rocks as you go up through the sequence. And it's these dramatic.

changes that you see in the rocks that distinguish between the different periods or eras or whatever it happens to be. Now, the trouble is you can't see the whole of history in any one place. And this is what makes geology a little bit complicated because you have to travel around not just the country, but around the world in order to correlate between the different sequences of rocks that you see. But it's a jigsaw. It's a huge, very complicated jigsaw. And some of the pieces have been turned upside down and turned inside out and all sorts of complicated things. But you piece it back together again. And you can see that at certain times of the Earth's history,

there are certain characteristics. Certain fossils, for example, are typically found in particular periods. And so we piece those together and give those a name. And then there's a dramatic change in the environment. And then as another characteristic type of rocks are deposited, and you would find characteristic fossils and other types of life exposed there. And I think it is very much worth making the point here that we're only ever going to be scratching around on the surface of the earth anyway.

All we're looking at is the scum that happens to be on the surface of the earth now. I mean, if you imagine the earth to be an apple, we're not even getting through the skin of it. So it's not really telling us about what's really deep down inside the earth. What we're seeing is what's happened on the surface of the earth. Nevertheless, it's amazing how much we can tell by looking at that. and by moving from quarry to quarry, from road cut to road cut, and to look at the exposed geology in lots of different parts of the world,

that's how we piece the jigsaw together. I think the point here is that time and rock are not synonymous. Time is continuous, rock is not. If you imagine a dual carriageway, two-lane highway, say take the one which goes from, for example, Oxford to Bister, it's the one which I have in my mind. The lane on the left is time. That is continuous.

Imagine the lane on the right as having huge gaps in it, which your car falls through. That is the nature of the rock record. Time is not the same as rock. And we have to try and piece together, continuous time from piecing together, different bits of rock, for example, in your quarry faces. Yeah, yeah.

Which, of course, is what makes it rather more complicated because the gaps in between these different periods, these very different types of rocks, could represent half an hour, or it could represent 10 million years. It could be a huge time gap. Henry G, can I take it on a bit here? Our own neon, 540 million years ago, let us say,

I love the 40, by the way, kicked off with what's known as the Cambrian explosion. Can you tell us what that was and what it means to geologists? And we haven't yet, I don't think, nailed why one layer of rock succeeds another. So if you could do that along the way, that would be helpful. Well, 543, you know, last Thursday, probably. This is one of the most exciting and also perplexing episodes in the whole history of life. When the phanozoic eon began, the eon, which is the one we're currently in,

that means visible life. Now, all of a sudden, rocks had fossils in, all of a sudden, half a billion years ago. This was a puzzle to Darwin, that you could find fossils in rocks, in Roderick Murchison's Silurian system was characterized by the kinds of fossils that were in it and the Cambrian system, all these things we now call periods. They all had lots of rocks and in each period it had its own characteristic kinds of fossil

and that was how you used to age the fossils, age the rocks. But before the Cambrian there was nothing. People looked and looked and looked and they couldn't find anything that they would normally see like your usual shells or trilobites. And this was a great puzzle to Darwin, who assumed that life evolved in a gradual way.

But even in Darwin's day, there seemed to be this junction. All of a sudden, complicated life appeared. And Darwin was so worried about this, he said that if this could not be resolved, it might actually threaten his entire theory. The question really is, is the explosion an artifact of preservation, or did it really happen in evolution?

And what I mean by that is, was evolution really gradual, was the acquisition of hard parts really gradual, but there was something in the earth's history, or the gaps that Hazel has talked about, that means that from our perspective it looks very sudden? Or was the record of the rocks telling you exactly like it was? Was it exactly like it said on the tin that evolution suddenly got on its wing chariot and didn't spare the horses?

As I understand it, this is basically because of the arrival of hard shells. Or an increase in size or both. Now, does this cast doubt on the validity of fossil evidence in a longer term of the dispute over the aging of the earth? Absolutely not. It's just, I mean, it's a very good way, in fact, of realizing the limitations of fossils as a way of dividing time. I mean, that the Fanaerzoic is... basically divided up on the basis of fossils, but how do you divide up the bit from the base of the Phanozoic

half a billion years ago to the formation of the Earth, which is four and a half billion years ago? That's four billion years of time in which you have no signposts. And so basically you need better eyes. And those eyes arrived when people started looking at the micro fossils in the rock.

And nowadays, not only do we look at micro fossils in the rock, We're also looking at the chemistry of the rock, which is one way that we know that life formed, for example, very soon after the heavy bombardment of the Hadean aeon finished. Yes, that's another important point. The problem with fossils is, I'm interested in how you recognise a fossil as the remains of a living organism.

And you can only do that if you have living organisms with which you're familiar that looks something like the fossils. Everyone says a trilobite. Well, a trilobite is an exotic word, but if you say, well, it looks like a sort of large woodlouse. People know exactly what you mean. But if you had no reference,

if you'd never seen a woodlouse before or anything like it, it would be actually quite hard to distinguish between, to say whether a trilobite was an animal or a plant, or even the remains of a living organism at all. And that's part of the problem with the Cambrian explosion. The Cambrian explosion isn't just a sudden efflorescence of fossils. It's a sudden efflorescence of fossils of the kinds of things we're familiar with because people have looked at periods slightly before the Cambrian explosion

and have found these jellyfish like things, these Ediacaran fossils. They're big fossils of things that might be animals or there might be plants. But even now, people have no idea what they were because we have nothing like them today. Hazel, can you address the contention of the late sadly, because he was a great man on this programme for us, Stephen Jay Gould, who developed the idea of punctuated equilibrium. How does that fit into the argument?

Well, again, by looking at the rocks, we see that there seem to be periods of time when things seem relatively stable, and we have all of these fossils, clearly indicating that there was life going on and it was doing its thing and it was very happy. and then something happens and there's a change. And the exact cause of the change, we don't know.

We can make inferences about what it may be. And then things go on again differently. It's a change, but it's a new equilibrium. A new system reigns for a long period of time. And then there's a change again. And I mean, this is a topic that I'm sure we'll keep coming up. The length of these periods is very variable. Sometimes they're a long, long time,

sometimes they're a shorter time. But each time something dramatic happens, something has caused life to not necessarily do a U-turn, but to go off to the right or to the left or to make a big change. Are there changes that we see that you've been indicating the changes? Is the best way to get it, to look at mass extinctions? Is that the best way to get to it? Well, that's the dramatic way to look at it, isn't it?

I mean, those are, I mean, of course, there's the famous one. The dinosaurs were wiped out, as we, as everybody knows, 65 million years ago. And that's a very good example. certainly not the only mass extinction that there has been, and it certainly wasn't the most dramatic one. There have been bigger ones where almost all life on earth was wiped out. Well, let's talk about the dinosaurs, because as you say, everybody has an idea that 65 million years ago,

the dinosaurs were wiped out, and that took us from the Cretaceous to the tertiary period and brought us to our own era. So it was fixed environment in your terms. We're getting warm as to where we are now. Now, what evidence remains for that and how sure is the evidence? What are we talking about here, Henry G. I mean, drive things, an asteroid arrives, bang, Gulf of Mexico, Wallop, they're all wiped out, finish, end of 100 million years of history, and then resurrected.

Well, that's one way looking at it. Well, I declare a rather extreme view here. I think mass extinction. No, no, I'm a creature of moderation in all things, except when it comes to mass extinctions. I think mass extinctions are rather serious. and the reason I say that is because we're looking at it in very human terms. Suddenly we see there are dinosaurs and then there aren't. So we assume that there was some cause that led to the extinction of the dinosaurs

and then modern life could take place. But when you're talking about extinction of a species, let alone a whole group of species, species are not unitary things. They're made of individuals. You have to wait till the last one dies out. And that is an extremely hard thing to estimate. Henry, there was a time when there were large, scaly reptiles running around the earth, and then there was a time when they weren't.

Yes, but you can't get around there. We know in the Cretaceous, in the Jurassic, and in the Triassic, i.e. in the Meso-era era, there were these huge bipedal and quadrupedal reptiles, much larger than anything we have in the reptile clade, which are dominated the earth, and relatively suddenly, in geological terms, they died out.

They are not there anymore. And what happened, of course, is that this let a small, insignificant group of furry creatures, which we call mammals, radiate into the vacated ecological space left behind by the dinosaurs. So that is the fact of the matter. The other fact of the matter is that there is an enrichment in this element eridium at the bottom.

boundary between the Cretaceous and the tertiary, the so-called KT boundary. And there's only two places where you find enriched iridium. It's either in the centre of the earth and can be brought up by volcanism, or it's found elsewhere in the solar system. It could be brought in by a meteorite impact. Add to that, the fact that you have a well-dated crater in the Yucatan Peninsula, which is exactly 65 million years old, and I think that you have a very compelling case.

Of course, it's a circumstantial case, but then everything. is in paleontology. Hazel, what would your answer to that? And then, of course, there's the point that there's very good evidence that there was a meteorite impact 65 million years ago. That's absolutely right. And it would have had a devastating

environmental effect. And we think that mass extinctions, which may very well take millions of years to actually occur, they don't happen on a Thursday afternoon when a meteorite happens to impact. They're a much longer process. But it

have been the straw that broke the camels back, or the dinosaurs back, as the case may be. I mean, dinosaurs as a group were around for a very, very long period of time. They came and went, though. I mean, they weren't all there right up until this meteorite impact happened. Many species of dinosaurs had already gone and become extinct long, long, long before this event happened. There were several other events. I think the problem I have with mass extinctions is really one of the assumption of causation. It's a kind of news editor's problem.

It's a kind of elision that says An asteroid killed the dinosaurs. Well, yes, Buster, I won't believe it until you dig up a T-Rex with a bit of meteorite between its teeth. So I always say, well, you know, what kind of causation are you talking about? You're talking about final causes or formal causes or what kind of cause are you talking about?

We're talking about very, very long periods of time that are so long that we as human beings cannot grasp them except in a very raw intellectual way of just writing the numbers, lots of noughts out on a piece of paper. So, but the end of the Cretaceous is a very, very good case in point. Actually, it is a good case, Henry, and I hope you don't mind if I disagree with you. I'm going to agree with you before you have a chance to disagree with.

Go on the Bocciani Gorge and talk about time. That is exactly where I'm going. I'm going... Can we move quickly to the Bottichaeli Goy? Yes, we can. We're taking an awful long time to get... You're there now. You're there now in this gorge where Walter... Can you just tell people what the Botticelli Gorge is and then we'll crack on a bit?

It's a gorge... It's a gorge of... in Italy full of limestone and the limestone is very boring except for this little bit of clay in the middle, which is only a few centimetres thick or maybe a centimetre thick. And this clay actually marks the Cretaceous tertiary boundary. It's the boundary clay. Obviously something happened in the deposition of limestone that interrupted it for a period. The question was how long this period it was and it took physical measurements to show that it did take a few hundred thousand years rather than millions.

Which actually is an amazing discovery because it was very very much. short. However you slice it and dice it, Henry, it was an instant in the geological record. The thing is, getting to that finding that it was short took an awful lot of time and worry and ingenuity. It's not an assumption that you could have immediately made. Hold on, Henry. Let's

have... Hey, what does this signify the body chelagoge? What's important about it? Well, I mean, this whole thing about the junctions between the time periods, which is what you wanted to know about, it really is a big deal whether the boundaries between these different rocks

is a short period or a long period because that helps us to understand about the evolution of the earth and what's been going on. So it does matter. We do care. Yeah, I know that, but what is significant

about the discovery that Henry's outlined in the Botticelli Gorge? That it's a short time period. And that therefore we can take, what can we take from that? That there was a sudden shift, that there was a sudden mass extinction. That something dramatic happened.

You see, there are worries about this looking at fossils, aren't there? I'm just reading, I'm just saying, from what I've read of what you and others of Britain. For instance, were we to be wiped out now, the fossil record of us lot, say for the last 10, 20, 30, 40, 100,000 years, would could be very thin in some areas, totally non-existent in other areas. And very thick in some other areas. And very thick in some other areas in different parts of the world from that in which we are now. Well, that's exactly what we observe. The tectonic plates and so forth.

So what would you make of that? So how we could actually, somebody looking back in say a couple of hundred million, might not even know we'd been around at all. There's this little thin bit somewhere in Portland place and there's nothing else. Well, we should have seen. Yes. But some event that occurs will be the marker horizon above us. There'll be this great thickness of human remains and deposits and so on below.

Then you have this marker horizon or gap or whatever you want to call it above that. and then you have the post-human era. But coming to a chronological time, Hazel Reimer, in the 1930s Arthur Holmes, as I understand, came up with the current estimate of four and a half billion years. How did he arrive at that figure? Well, he used isotopes. Now, the minerals inside a rock

consist of crystals which have atoms inside them, and each atom is characteristic, and it's characterized by the number of protons inside the nucleus, the tiny, tiny little bit inside the atom, and you can have variable numbers of neutrons inside these nuclei, and the isotopes are not all stable. Some of them are stable and they don't change.

But radioactive isotopes will change through time, and conveniently, they change at exactly a predictable rate. We know exactly what the rate of change, or the rate of radioactive decay of these isotopes is. So some of them decay very rapidly in a few seconds, some of them in a few days, weeks. There's a huge, huge range right up to billions and billions of years. And so by finding isotopes within the rocks with a decay rate that is comparable to the age of the rocks, we can actually date the rocks that way.

So that's why we use carbon isotopes, for example, for looking at things on the basis of a few thousand years, because that's the rate of decay of carbon isotopes. and going back through time using isotopes of uranium and so on that decay, we can date rocks right back the oldest ones on Earth to about 4.5 billion years old. Richard Corfield, can you tell us how this brought together, allowed to be in harness chronological time and geological time? Yes, any core or any rock section represents time in some form. The trick, the difficult thing, is to link that rock section to absolute time.

This gets back to my metaphor of the two-lane highway. You have to cross one lane to the other. Is this partly because sandstone, for instance, is laid down at different times in different areas? And a different right. But it's still sandstone. Yes. So you don't measure it as a sandstone in time. You measure it as a fossil which occurred in different contexts at different times.

It's containing fossils, sorry. Which of you is going to replace? like that. Well, the other interesting thing about it is, of course, if you take a sedimentary rock like that, it consists of grains and particles that have been derived from elsewhere. So the age that you can derive from those grains depends on which grain you're looking at.

And depending on what part of the rocket is you're looking at, you'll end up with all sorts of different ages. And it tells you a lot about the history of how those rocks are formed. That's a very important problem that's affected our knowledge of human evolution. Quite incidentally, I won't digress. So carbonating is, revolutionised the study of palatontology Well there's all kinds of different

carbon, there's all kinds of different dating and because of the some radioactive isotopes decay faster than others. I mean carbon 14 decays relatively quickly so it's quite good for dating things that happened in the archaeological record in the Holocene within the past 10,000

years but not much good beyond about 30 or 40,000 years ago simply because there's too little of it left to measure accurately Richard was talking about uranium and lead. That's down at the other end of the scale,

uranium, the radioactive uranium, I said it decays so slowly that if you were going to look at something recent, there wouldn't have been much of it to have decayed for you to get a very accurate reading. But it's very good if you're looking at things that happened billions of years ago when an appreciable amount has decayed. So there are all kinds of different systems of radioactive decay

going all the way down the fossil record. Can I ask you, we have this neat map, which was very well expressed, by Richard at the start of the program, the Hedean, and so and so forth. Do you think that is set, or do you think these will change when we, are we on the way to getting more knowledge to shuffle all that around to change it? Are we at the beginning of exploring this, or are these eons and eras set? Well, the age of the earth has been getting older and older

and was agreed on in the 1950s, and we haven't changed it since then, and I don't think we're about to. details may change and dates may become more precise with better and better and more accurate dating mechanisms but we're not going to turn it upside down now finally it's a googly for the last minute of the program Richard but when can you be certain that life began in this ageing of the earth life as we might recognise it and understand that's a kind of entire programme in itself

but it goes back to isotopes the different kind of isotope not ones which spontaneously decay, but ones which do not, so-called stable isotopes. Now, the system which we call photosynthesis, fundamental to life on this planet, separates two isotopes of carbon. It preferentially concentrates the light isotope in organically formed tissue. So at about 3.8 billion years before present, soon after the late heavy bombardment finished, we find traces of enrichment of this light isotope carbon 12 in rocks in Greenland. And this seems very good evidence indeed that the life formed about that time. You're absolutely right. It's another program.

Thank you, Richard Coffield and Hazel, Rahman, Henry G. for this program. There's a website if you want to put in your comments. Next week we'll be talking about the St. Bartholomew's Day Massacre. Thank you very much for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programs. is about history, science and philosophy at BBC.com.com.

UK forward slash radio four.