Authors: Richard Dawkins
* Which were in turn named after the ‘multitude of dogs of a huge size’ mentioned in Pliny’s Natural History.
* For example, herding in sheepdogs is derived from stalking in wolves, with the killing removed from the end of the sequence.
* It doesn’t affect the point I am making, but this story applies only to female angler fish. The males are usually tiny dwarfs, who attach themselves parasitically to a female’s body, like a little extra fin.
* The popular canard about Hitler being inspired by Darwin comes partly from the fact that both Hitler and Darwin were impressed by something that everybody has known for centuries: you can breed animals for desired qualities. Hitler aspired to turn this common knowledge to the human species. Darwin didn’t. His inspiration took him in a much more interesting and original direction. Darwin’s great insight was that you don’t need a breeding agent at all: nature – raw survival or differential reproductive success – can play the role of breeder. As for Hitler’s ‘Social Darwinism’ – his belief in a struggle between races – that is actually very un-Darwinian. For Darwin, the struggle for existence was a struggle between individuals within a species, not between species, races or other groups. Don’t be misled by the ill-chosen and unfortunate subtitle of Darwin’s great book: The preservation of favoured races in the struggle for life. It is abundantly clear from the text itself that Darwin didn’t mean races in the sense of ‘A group of people, animals, or plants, connected by common descent or origin’ (Oxford English Dictionary, definition 6.I). Rather, he intended something more like the OED’s definition 6.II: ‘A group or class of people, animals, or things, having some common feature or features’. An example of sense 6. II would be ‘All those individuals (regardless of their geographical race) who have blue eyes’. In the technical jargon of modern genetics, which was not available to Darwin, we would express the sense of ‘race’ in his subtitle as ‘All those individuals who possess a certain allele.’ The misunderstanding of the Darwinian struggle for existence as a struggle between groups of individuals – the so-called ‘group selection’ fallacy – is unfortunately not confined to Hitlerian racism. It constantly resurfaces in amateur misinterpretations of Darwinism, and even among some professional biologists who should know better.
* Psychologists have analogous tests of risk-taking among humans, which show interesting differences. Entrepreneurs typically score highly on risk-taking measures, as do pilots, rock-climbers, motorcycle racers and other extreme sports enthusiasts. Women tend to be more risk-averse than men. Feminists will here point out that the causal arrow could go either way: women could become more risk-averse because of the occupations into which society thrusts them.
SILENCE AND SLOW TIME
IF the history-deniers who doubt the fact of evolution are ignorant of biology, those who think the world began less than ten thousand years ago are worse than ignorant, they are deluded to the point of perversity. They are denying not only the facts of biology but those of physics, geology, cosmology, archaeology, history and chemistry as well. This chapter is about how we know the ages of rocks and the fossils embedded in them. It presents the evidence that the timescale on which life has operated on this planet is measured not in thousands of years but in thousands of millions of years.
Remember, evolutionary scientists are in the position of detectives who come late to the scene of a crime. To pinpoint when things happened, we depend upon traces left by time-dependent processes – clocks, in a broad sense. One of the first things a detective does when investigating a murder is ask a doctor or pathologist to estimate the time of death. Much follows from this information, and in detective fiction an almost mystical reverence is accorded to the pathologist’s estimate. The ‘time of death’ is a baseline fact, an inerrant pivot around which more or less far-fetched speculations by the detective revolve. But that estimate is, of course, subject to error, an error that can be measured and can be quite large. The pathologist uses various time-dependent processes to estimate the time of death: the body cools at a characteristic rate, rigor mortis sets in at a particular time, and so on. These are the rather crude ‘clocks’ available to the investigator of a murder. The clocks available to the evolutionary scientist are potentially much more accurate – in proportion to the timescale involved, of course, not more accurate to the nearest hour! The analogy to a precision clock is more persuasive for a Jurassic rock in the hands of a geologist than it is for a cooling corpse in the hands of a pathologist.
Man-made clocks work on timescales that are very short by evolutionary standards – hours, minutes, seconds – and the time-dependent processes they use are fast: the swinging of a pendulum, the swivelling of a hairspring, the oscillation of a crystal, the burning of a candle, the draining of a water vessel or an hourglass, the rotation of the earth (registered by a sundial). All clocks exploit some process that occurs at a steady and known rate. A pendulum swings at a very constant rate, which depends upon its length but not, at least in theory, on the amplitude of the swing or the mass of the bob on the end. Grandfather clocks work by linking a pendulum to an escapement which advances a toothed wheel, step by step; the rotation is then geared down to the speed of rotation of an hour hand, a minute hand and a second hand. Watches with hairspring wheels work in a similar way. Digital watches exploit an electronic equivalent of a pendulum, the oscillation of certain kinds of crystals when supplied with energy from a battery. Water clocks and candle clocks are much less accurate, but they were useful before the invention of event-counting clocks. They depend not on counting things, as a pendulum clock or a digital watch does, but on measuring some quantity. Sundials are inaccurate ways of telling the time.* But the rotation of the earth, which is the time-dependent process on which they rely, is accurate on the timescale of the slower clock that we call the calendar. This is because on that timescale it is no longer a measuring clock (a sundial measures the continuously varying angle of the sun) but a counting clock (counting day/night cycles).
Both counting clocks and measuring clocks are available to us on the immensely slow timescale of evolution. But to investigate evolution we don’t need just a clock that tells the present time, as a sundial does, or a watch. We need something more like a stopwatch that can be reset. Our evolutionary clock needs to be zeroed at some point, so that we can calculate the elapsed time since a starting point, to give us, for example, the absolute age of some object such as a rock. Radioactive clocks for dating igneous (volcanic) rocks are conveniently zeroed at the moment the rock is formed by the solidification of molten lava.
Fortunately, a variety of zero-able natural clocks is available. This variety is a good thing, because we can use some clocks to check the accuracy of other clocks. Even more fortunately, they sensitively cover an astonishingly wide range of timescales, and we need this too because evolutionary timescales span seven or eight orders of magnitude. It’s worth spelling out what this means. An order of magnitude means something precise. A change of one order of magnitude is one multiplication (or division) by ten. Since we use a decimal system,* the order of magnitude of a number is a count of the number of zeroes, before or after the decimal point. So a range of eight orders of magnitude constitutes a hundred millionfold. The second hand of a watch rotates 60 times as fast as the minute hand and 720 times as fast as the hour hand, so the three hands cover a range which is less than three orders of magnitude. This is tiny compared to the eight orders of magnitude spanned by our repertoire of geological clocks. Radioactive decay clocks are available for short timescales as well, even down to fractions of a second; but for evolutionary purposes, clocks that can measure centuries or perhaps decades are about the fastest we need. This fast end of the spectrum of natural clocks – tree rings and carbon dating – is useful for archaeological purposes, and for dating specimens on the sort of timescale that covers the domestication of the dog or the cabbage. At the other end of the scale, we need natural clocks that can time hundreds of millions, even billions, of years. And, praise be, nature has provided us with just the wide range of clocks that we need. What’s more, their ranges of sensitivity overlap with each other, so we can use them as checks on each other.
A tree-ring clock can be used to date a piece of wood, say a beam in a Tudor house, with astonishing accuracy, literally to the nearest year. Here’s how it works. First, as most people know, you can age a newly felled tree by counting rings in its trunk, assuming that the outermost ring represents the present. Rings represent differential growth in different seasons of the year – winter or summer, dry season or wet season – and they are especially pronounced at high latitudes, where there is a strong difference between seasons. Fortunately, you don’t actually have to cut the tree down in order to age it. You can peek at its rings without killing it, by boring into the middle of a tree and extracting a core sample. But just counting rings doesn’t tell you in which century your house beam was alive, or your Viking longship’s mast. If you want to pin down the date of old, long-dead wood you need to be more subtle. Don’t just count rings, look at the pattern of thick and thin rings.
Just as the existence of rings signifies seasonal cycles of rich and poor growth, so some years are better than others, because the weather varies from year to year: there are droughts that retard growth, and bumper years that accelerate it; there are cold years and hot years, even years of freak El Niños or Krakatoa-type catastrophes. Good years, from the tree’s point of view, produce wider rings than bad years. And the pattern of wide and narrow rings in any one region, caused by a particular trademark sequence of good years and bad years, is sufficiently characteristic – a fingerprint that labels the exact years in which the rings were laid down – to be recognizable from tree to tree.
Dendrochronologists measure rings on recent trees, where the exact date of every ring is known by counting backwards from the year in which the tree is known to have been felled. From these measurements, they construct a reference collection of ring patterns, to which you can compare the ring patterns of an archaeological sample of wood whose date you want to know. So you might get the report: ‘This Tudor beam contains a signature sequence of rings that matches a sequence from the reference collection, which is known to have been laid down in the years 1541 to 1547. The house was therefore built after AD 1547.’
All very well, but not many of today’s trees were alive in Tudor times, let alone in the stone age or beyond. There are some trees – bristlecone pines, some giant redwoods – that live for millennia, but most trees used for timber are felled when they are younger than a century or so. How, then, do we build up the reference collection of rings for more ancient times? For times so distant that not even the oldest surviving bristlecone pine goes back that far? I think you’ve already guessed the answer. Overlaps. A strong rope may be 100 yards long, yet no single fibre within it reaches more than a fraction of that total. To use the overlap principle in dendrochronology, you take the reference fingerprint patterns whose date is known from modern trees. Then you identify a fingerprint from the old rings of modern trees and seek the same fingerprint from the younger rings of long-dead trees. Then you look at the fingerprints from the older rings of those same long-dead trees, and look for the same pattern in the younger rings of even older trees. And so on. You can daisychain your way back, theoretically for millions of years using petrified forests, although in practice dendrochronology is only used on archaeological timescales over some thousands of years. And the amazing thing about dendrochronology is that, theoretically at least, you can be accurate to the nearest year, even in a petrified forest 100 million years old. You could literally say that this ring in a Jurassic fossil tree was laid down exactly 257 years later than this other ring in another Jurassic tree! If only there were enough petrified forests to daisychain your way back continuously from the present, you could say that this tree is not just of late Jurassic age: it was alive in exactly 151,432,657 BC! Unfortunately, we don’t have an unbroken chain, and dendrochronology in practice takes us back only about 11,500 years. It is nevertheless a tantalizing thought that, if only we could find enough petrified forests, we could date to the nearest year over a timespan of hundreds of millions of years.
How dendrochronology works
Tree rings are not quite the only system that promises total accuracy to the nearest year. Varves are layers of sediment laid down in glacial lakes. Like tree rings, they vary seasonally and from year to year, so theoretically the same principle can be used, with the same degree of accuracy. Coral reefs, too, have annual growth rings, just like trees. Fascinatingly, these have been used to detect the dates of ancient earthquakes. Tree rings too, by the way, tell us the dates of earthquakes. Most of the other dating systems that are available to us, including all the radioactive clocks that we actually use over timescales of tens of millions, hundreds of millions or billions of years, are accurate only within an error range that is approximately proportional to the timescale concerned.
Let’s now turn to radioactive clocks. There are quite a lot of them to choose from, and, as I said, they blessedly cover the gamut from centuries to thousands of millions of years. Each one has its own margin of error, which is usually about 1 per cent. So if you want to date a rock which is billions of years old, you must be satisfied with an error of plus or minus tens of millions of years. If you want to date a rock hundreds of millions of years old, you must be satisfied with an error of millions. To date a rock that is only tens of millions of years old, you must allow for an error of plus or minus hundreds of thousands of years.
To understand how radioactive clocks work, we first need to understand what is meant by a radioactive isotope. All matter is made up of elements, which are usually chemically combined with other elements. There are about 100 elements, slightly more if you count elements that are only ever detected in laboratories, slightly fewer if you count only those elements that are found in nature. Examples of elements are carbon, iron, nitrogen, aluminium, magnesium, fluorine, argon, chlorine, sodium, uranium, lead, oxygen, potassium and tin. The atomic theory, which I think everybody accepts, even creationists, tells us that each element has its own characteristic atom, which is the smallest particle into which you can divide an element without it ceasing to be that element. What does an atom look like, say an atom of lead, or copper, or carbon? Well, it certainly looks nothing like lead or copper or carbon. It doesn’t look like anything, because it is too small to form any kind of image on your retina, even with an ultra-powerful microscope. We can use analogies or models to help us visualize an atom. The most famous model was proposed by the great Danish physicist Niels Bohr. The Bohr model, which is now rather out of date, is a miniature solar system. The role of the sun is played by the nucleus, and around it orbit the electrons, which play the role of planets. As with the solar system, almost all the mass of the atom is contained in the nucleus (‘sun’), and almost all the volume is contained in the empty space that separates the electrons (‘planets’) from the nucleus. Each electron is tiny compared with the nucleus, and the space between them and the nucleus is huge compared with the size of either. A favourite analogy portrays the nucleus as a fly in the middle of a sports stadium. The nearest neighbouring nucleus is another fly, in the middle of an adjacent stadium. The electrons of each atom are buzzing about in orbit around their respective flies, smaller than the tiniest gnats, too small to be seen on the same scale as the flies. When we look at a solid lump of iron or rock, we are ‘really’ looking at what is almost entirely empty space. It looks and feels solid and opaque because our sensory systems and brains find it convenient to treat it as solid and opaque. It is convenient for the brain to represent a rock as solid because we can’t walk through it. ‘Solid’ is our way of experiencing things that we can’t walk through or fall through, because of the electromagnetic forces between atoms. ‘Opaque’ is the experience we have when light bounces off the surface of an object, and none of it goes through.
Three kinds of particle enter into the makeup of an atom, at least as envisaged in the Bohr model. Electrons we have already met. The other two, vastly larger than electrons but still tiny compared with anything we can imagine or experience with our senses, are called protons and neutrons, and they are found in the nucleus. They are almost the same size as each other. The number of protons is fixed for any given element and equal to the number of electrons. This number is called the atomic number. It is uniquely characteristic of an element, and there are no gaps in the list of atomic numbers – the famous periodic table.* Every number in the sequence corresponds to exactly one, and only one, element. The element with 1 for its atomic number is hydrogen, 2 is helium, 3 lithium, 4 beryllium, 5 boron, 6 carbon, 7 nitrogen, 8 oxygen, and so on up to high numbers like 92, which is the atomic number of uranium.
Protons and electrons carry an electric charge, of opposite sign – we call one of them positive and the other negative by arbitrary convention. These charges are important when elements form chemical compounds with each other, mostly mediated by electrons. The neutrons in an atom are bound into the nucleus together with the protons. Unlike protons they carry no charge, and they play no role in chemical reactions. The protons, neutrons and electrons in any one element are exactly the same as those in every other element. There is no such thing as a gold-flavoured proton or a copper-flavoured electron or a potassium-flavoured neutron. A proton is a proton is a proton, and what makes a copper atom copper is that there are exactly 29 protons (and exactly 29 electrons). What we ordinarily think of as the nature of copper is a matter of chemistry. Chemistry is a dance of electrons. It is all about the interactions of atoms via their electrons. Chemical bonds are easily broken and remade, because only electrons are detached or exchanged in chemical reactions. The forces of attraction within atomic nuclei are much harder to break. That’s why ‘splitting the atom’ has such a menacing ring to it – but it can happen, in ‘nuclear’ as opposed to chemical reactions, and radioactive clocks depend upon it.
Electrons have negligible mass, so the total mass of an atom, its ‘mass number’, is equal to the combined number of protons and neutrons. It is usually rather more than double the atomic number, because there are usually a few more neutrons than protons in a nucleus. Unlike the number of protons, the number of neutrons in an atom is not diagnostic of an element. Atoms of any given element can come in different versions called isotopes, which have differing numbers of neutrons, but always the same number of protons. Some elements, such as fluorine, have only one naturally occurring isotope. The atomic number of fluorine is 9 and its mass number is 19, from which you can deduce that it has 9 protons and 10 neutrons. Other elements have lots of isotopes. Lead has five commonly occurring isotopes. All have the same number of protons (and electrons), namely 82, which is the atomic number of lead, but the mass numbers range between 202 and 208. Carbon has three naturally occurring isotopes. Carbon-12 is the common one, with the same number of neutrons as protons: 6. There’s also carbon-13, which is too short-lived to bother with, and carbon-14 which is rare but not too rare to be useful for dating relatively young organic samples, as we shall see.
Now for the next important background fact. Some isotopes are stable, others unstable. Lead-202 is an unstable isotope; lead-204, lead-206, lead-207 and lead-208 are stable isotopes. ‘Unstable’ means that the atoms spontaneously decay into something else, at a predictable rate, though not at predictable moments. The predictability of the rate of decay is the key to all radiometric clocks. Another word for ‘unstable’ is ‘radioactive’. There are several kinds of radioactive decay, which offer possibilities for useful clocks. For our purposes it isn’t important to understand them, but I explain them here to show the magnificent level of detail that physicists have achieved in working out such things. Such detail casts a sardonic light on the desperate attempts of creationists to explain away the evidence of radioactive dating, and keep the Earth young like Peter Pan.
All these kinds of instability involve neutrons. In one kind, a neutron turns into a proton. This means that the mass number stays the same (since protons and neutrons have the same mass) but the atomic number goes up by one, so the atom becomes a different element, one step higher in the periodic table. For example, sodium-24 turns itself into magnesium-24. In another kind of radioactive decay, exactly the reverse happens. A proton turns into a neutron. Again, the mass number stays the same, but this time the atomic number decreases by one, and the atom changes into the next element down in the periodic table. A third kind of radioactive decay has the same result. A stray neutron happens to hit a nucleus and knocks out one proton, taking its place. Again, there’s no change in mass number; again, the atomic number goes down by one, and the atom turns into the next element down in the periodic table. There’s also a more complicated kind of decay in which an atom ejects a so-called alpha particle. An alpha particle consists of two protons and two neutrons stuck together. This means that the mass number goes down by four and the atomic number goes down by two. The atom changes to whichever element is two below it in the periodic table. An example of alpha decay is the change of the very radioactive isotope uranium-238 (with 92 protons and 146 neutrons) to thorium-234 (with 90 protons and 144 neutrons).
Now we approach the nub of the whole matter. Every unstable or radioactive isotope decays at its own characteristic rate which is precisely known. Moreover, some of these rates are vastly slower than others. In all cases the decay is exponential. Exponential means that if you start with, say, 100 grams of a radioactive isotope, it is not the case that a fixed amount, say 10 grams, turns into another element in a given time. Rather, a fixed proportion of whatever is left turns into the second element. The favoured measure of decay rate is the ‘half-life’. The half-life of a radioactive isotope is the time taken for half of its atoms to decay. The half-life is the same, no matter how many atoms have already decayed – that is what exponential decay means. You will appreciate that, with such successive halvings, we never really know when there is none left. However, we can say that after a sufficient time has elapsed – say ten half-lives – the number of atoms that remains is so small that, for practical purposes, it has all gone. For example, the half-life of carbon-14 is between 5,000 and 6,000 years. For specimens older than about 50,000–60,000 years, carbon dating is useless, and we need to turn to a slower clock.
The half-life of rubidium-87 is 49 billion years. The half-life of fermium-244 is 3.3 milliseconds. Such startling extremes serve to illustrate the stupendous range of clocks available. Although carbon-15’s half-life of 2.4 seconds is too short for settling evolutionary questions, carbon-14’s half-life of 5,730 years is just right for dating on the archaeological timescale, and we’ll come to it presently. An isotope much used on the evolutionary timescale is potassium-40, with its half-life of 1.26 billion years, and I’m going to use it as my example, to explain the whole idea of a radioactive clock. It is often called the potassium argon clock, because argon-40 (one lower in the periodic table) is one of the elements to which potassium-40 decays (the other, resulting from a different kind of radioactive decay, is calcium-40, one higher in the periodic table). If you start with some quantity of potassium-40, after 1.26 billion years half of the potassium-40 will have decayed to argon-40. That’s what half-life means. After another 1.26 billion years, half of what remains (a quarter of the original) will have decayed, and so on. After a shorter time than 1.26 billion years, a proportionately smaller quantity of the original potassium will have decayed. So, imagine that you start with some quantity of potassium-40 in an enclosed space with no argon-40. After a few hundreds of millions of years have elapsed, a scientist comes upon the same enclosed space and measures the relative proportions of potassium-40 and argon-40. From this proportion – regardless of the absolute quantities involved – knowing the half-life of potassium-40’s decay and assuming there was no argon to begin with, one can estimate the time that has elapsed since the process started – since the clock was ‘zeroed’, in other words. Notice that we must know the ratio of parent (potassium-40) to daughter (argon-40) isotopes. Moreover, as we saw earlier in the chapter, it is necessary that our clock has the facility to be zeroed. But what does it mean to speak of a radioactive clock’s being ‘zeroed’? The process of crystallization gives it meaning.
Like all the radioactive clocks used by geologists, potassium/ argon timing works only with so-called igneous rocks. Named after the Latin for fire, igneous rocks are solidified from molten rock – underground magma in the case of granite, lava from volcanoes in the case of basalt. When molten rock solidifies to form granite or basalt, it does so in the form of crystals. These are normally not big, transparent crystals like those of quartz, but crystals that are too small to look like crystals to the naked eye. The crystals are of various types, and several of these, such as some micas, contain potassium atoms. Among these are atoms of the radioactive isotope potassium-40. When a crystal is newly formed, at the moment when molten rock solidifies, there is potassium-40 but no argon. The clock is ‘zeroed’ in the sense that there are no argon atoms in the crystal. As the millions of years go by, the potassium-40 slowly decays and, one by one, atoms of argon-40 replace potassium-40 atoms in the crystal. The accumulating quantity of argon-40 is a measure of the time that has elapsed since the rock was formed. But, for the reason I have just explained, this quantity is meaningful only if expressed as the ratio of potassium-40 to argon-40. When the clock was zeroed, the ratio was 100 per cent in favour of potassium-40. After 1.26 billion years, the ratio will be 50–50. After another 1.26 billion years, half of the remaining potassium-40 will have been converted to argon-40, and so on. Intermediate proportions signify intermediate times since the crystal clock was zeroed. So geologists, by measuring the ratio between potassium-40 and argon-40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state. Igneous rocks typically contain many different radioactive isotopes, not just potassium-40. A fortunate aspect of the way igneous rocks solidify is that they do so suddenly – so that all the clocks in a given piece of rock are zeroed simultaneously.
Only igneous rocks provide radioactive clocks, but fossils are almost never found in igneous rock. Fossils are formed in sedimentary rocks like limestone and sandstone, which are not solidified lava. They are layers of mud or silt or sand, gradually laid down on the floor of a sea or lake or estuary. The sand or mud becomes compacted over the ages and hardens as rock. Corpses that are trapped in the mud have a chance of fossilizing. Even though only a small proportion of corpses actually do fossilize, sedimentary rocks are the only rocks that contain any fossils worth speaking of.
Sedimentary rocks unfortunately cannot be dated by radioactivity. Presumably the individual particles of silt or sand that go to make sedimentary rocks contain potassium-40 and other radioactive isotopes, and therefore could be said to contain radioactive clocks; but unfortunately these clocks are no use to us because they are not properly zeroed, or are zeroed at different times from each other. The particles of sand that are compacted to make sandstone may originally have been ground down from igneous rocks, but the igneous rocks from which they were ground all solidified at different times. Every grain of sand has a clock zeroed at its own time, and that time was probably long before the sedimentary rock formed and entombed the fossil we are trying to date. So, from a timekeeping point of view, sedimentary rock is a mess. It can’t be used. The best we can do – and it is a pretty good best – is to use the dates of igneous rocks that are found near sedimentary rock, or embedded in it.
To date a fossil, you don’t literally need to find it sandwiched between two slabs of igneous rock, although that is a neat way to illustrate the principle. The actual method used is more refined than that. Recognizably similar layers of sedimentary rock occur all over the world. Long before radioactive dating was discovered, these layers had been identified and given names: names like Cambrian, Ordovician, Devonian, Jurassic, Cretaceous, Eocene, Oligocene, Miocene. Devonian sediments are recognizably Devonian, not only in Devon (the county in south-west England that gave them their name) but in other parts of the world. They are recognizably similar to each other, and they contain similar lists of fossils. Geologists have long known the order in which these named sediments were laid down. It’s just that, before the advent of radioactive clocks, we didn’t know when they were laid down. We could arrange them in order because – obviously – older sediments tend to lie beneath younger sediments. Devonian sediments, for example, are older than Carboniferous (named after the coal which is frequently found in Carboniferous layers) and we know this because, in those parts of the world where the two layers coincide, the Devonian layer lies underneath the Carboniferous layer (the exceptions to this rule occur in places where we can tell, from other evidence, that the rocks have been tilted aslant, or even turned upside down). We aren’t usually fortunate enough to find a complete run of layers, all the way from Cambrian at the bottom up to Recent at the top. But because the layers are so recognizable, you can work out their relative ages by daisychaining and jigsawing your way around the world.
So, long before we knew how old fossils were, we knew the order in which they were laid down, or at least the order in which the named sediments were laid down. We knew that Cambrian fossils, the world over, were older than Ordovician ones, which were older than Silurian; then came Devonian, then Carboniferous, Permian, Triassic, Jurassic, Cretaceous, and so on. And within these major named layers, geologists also distinguish sub-regions: upper Jurassic, middle Jurassic, lower Jurassic, and so on.
The named strata are usually identified by the fossils they contain. And we are going to use the ordering of the fossils as evidence for evolution! Is that in danger of turning into a circular argument? Certainly not. Think about it. Cambrian fossils are a characteristic assemblage, unmistakably recognizable as Cambrian. For the moment we are using a characteristic assemblage of fossils simply as labels for Cambrian rocks – indicator species – wherever we may find them. This, indeed, is why oil companies employ fossil experts to identify particular strata of rocks, usually by microfossils, tiny creatures called foraminifera, for example, or radiolaria.
A characteristic list of fossils is used to recognize Ordovician rocks, Devonian rocks, and so on. So far, all we are using these fossil assemblages for is to identify whether a slab of rock is, say, Permian or Silurian. Now we move on to use the order in which the named strata were laid down, helped by daisychaining around the world, as evidence of which strata are older or younger than which. Having established these two sets of information, we can then look at the fossils in successively younger strata, to see whether they constitute a sensible evolutionary sequence when compared with each other in sequence. Do they progress in a sensible direction? Do certain kinds of fossils, for example mammals, appear only after a given date, never before? The answer to all such questions is yes. Always yes. No exceptions. That is powerful evidence for evolution, for it was never a necessary fact, never something that had to follow from our method of identifying strata and our method of obtaining a temporal sequence.
It is a fact that literally nothing that you could remotely call a mammal has ever been found in Devonian rock or in any older stratum. They are not just statistically rarer in Devonian than in later rocks. They literally never occur in rocks older than a certain date. But this didn’t have to be so. It could have been the case that, as we dug down lower and lower from the Devonian, through the Silurian and then even older, through the Ordovician, we suddenly found that the Cambrian era – older than any of them – teemed with mammals. That is in fact not what we find, but the possibility demonstrates that you can’t accuse the argument of being circular: at any moment somebody might dig up a mammal in Cambrian rocks, and the theory of evolution would be instantly blown apart if they did. Evolution, in other words, is a falsifiable, and therefore scientific, theory. I shall return to this point in Chapter 6.
Creationist attempts to explain such findings often achieve high comedy. Noah’s flood, we are told, is the key to understanding the order in which we find fossils of the major animal groups. Here’s a direct quotation from a prizewinning creationist website.
Fossil sequence in geological strata shows:(i) INVERTEBRATES (slow moving marine animals) would perish first followed by the more mobile fishes who would be overwhelmed by the flood silt
(ii) AMPHIBIA (close to the sea) would perish next as the waters rose.
(iii) REPTILES (slow moving land animals) next to die.
(iv) MAMMALS could flee from rising water, the larger, faster ones surviving the longest.
(v) MAN would exercise most ingenuity – clinging to logs, etc. to escape the flood.
This sequence is a perfectly satisfactory explanation of the order in which the various fossils are found in the strata. It is NOT the order in which they evolved but the order in which they were inundated at the time of Noah’s flood.
Quite apart from all the other reasons to object to this remarkable explanation, there could only ever be a statistical tendency for mammals, for example, to be on average better at escaping the rising waters than reptiles. Instead, as we should expect on the evolution theory, there literally are no mammals in the lower strata of the geological record. The ‘head for the hills’ theory would be on more solid ground if there were a statistical tailing off of mammals as you move down through the rocks. There are literally no trilobites above Permian strata, literally no dinosaurs (except birds) above Cretaceous strata. Once again, the ‘head for the hills’ theory would predict a statistical tailing off.
Back to dating, and radioactive clocks. Because the relative ordering of the named sedimentary strata is well known, and the same order is found all over the world, we can use igneous rocks that overlie or underlie sedimentary strata, or are embedded in them, to date those named sedimentary strata, and hence the fossils within them. By a refinement of the method, we can date fossils that lie near the top of, say, the Carboniferous or the Cretaceous, as more recent than fossils that lie slightly lower in the same stratum. We don’t need to find an igneous rock in the vicinity of any particular fossil we want to date. We can tell that our fossil is, say, late Devonian, from its position in a Devonian stratum. And we know, from the radioactive dating of igneous rocks found in association with Devonian strata all around the world, that the Devonian ended about 360 million years ago.