The Varieties of Scientific Experience

Three

Once upon a time, the best minds of the human species believed that the planets were attached to crystal spheres, which explained their motion both daily and over longer periods of time. We now know this is not true in several •ways, one of which is that the Copernican theory explains the observed motion to higher precision and with a more modest investment of assumptions. But we also know this is not true, because we have sent spacecraft to the outer solar system with acoustic micro-meteorite detectors-and there was no sound of tinkling crystal as the spacecraft passed the orbits of Mars or Jupiter or Saturn. We have direct evidence that there are no crystal spheres. Now, Copernicus did not have such evidence, of course, but nevertheless his more indirect approach has been thoroughly validated. Now, when they were believed to exist, how was it that these spheres moved? Did they move on their own? They did not. Both in classic and in medieval times, it was prominently speculated that gods or angels propelled them, gave them a twirl every now and then.

The Newtonian gravitational superstructure replaced angels with GMm/r², which is a little more abstract. And in the course of that transformation, the gods and angels were relegated to more remote times and more distant causality skeins. The history of science in the last five centuries has done that repeatedly, a lot of walking away from divine microintervention in earthly affairs. It used to be that the flowering of every plant was due to direct intervention by the Deity. Now we understand something about plant hormones and phototropism, and virtually no one imagines that God directly commands the individual flowers to bloom.

So as science advances, there seems to be less and less for God to do. It's a big universe, of course, so He, She, or It could be profitably employed in many places. But what has clearly been happening is that evolving before our eyes has been a God of the Gaps; that is, whatever it is we cannot explain lately is attributed to God. And then after a while, we explain it, and so that's no longer God's realm. The theologians give that one up, and it walks over onto the science side of the duty roster.

We've seen this happen repeatedly. And so what has happened is that God is moving-if there is a real God of the Western sort, I am, of course, speaking only metaphorically- God has been evolving toward what the French call un roi faineant-a do-nothing king-who gets the universe going, establishes the laws of nature, and then retires or goes somewhere else. This is not far at all from the Aristotelian view of the unmoved prime mover, except that Aristotle had several dozen unmoved prime movers, and he felt that this was an argument for polytheism, something that is often overlooked today.

Well, I want to describe one of the most major gaps that is in the course of being filled in. (We cannot surely say it is fully filled in yet.) And that has to do with the origin of life.

There was, and in some places still is, a very intense controversy about the evolution of life, about the scandalous suggestion that humans are closely related to the other animals and especially to nonhuman primates, that we had an ancestor who would be, if we met it on the street, indistinguishable from a monkey or an ape. A great deal of the attention has been devoted to the evolutionary process, where, as I tried to indicate earlier, the key impediment to its being intuitively obvious is time. The period of time available for the origin and evolution of life is so much vaster than an individual human lifetime that processes that proceed at paces too small to see during an individual lifetime might nevertheless be dominant over 4,000 million years.

One way to think about this, by the way, is the following: Suppose your father or mother-let's say father for the sake of definiteness-walked into this room at the ordinary human pace of walking. And suppose just behind him was his father. And just behind him was his father. How long would we have to wait before the ancestor who enters the now-open door is a creature who normally walked on all fours? The answer is a week. The parade of ancestors moving at the ordinary pace of walking would take only a week before you got to a quadruped. And our quadruped ancestors are, after all, only a few tens of millions of years ago, and that's 1 percent of geological time. So there are many different ways of calibrating this immense vista of time that was necessary to evolve the complexity and beauty of the natural world, and this is one.

Now, the evidence for evolution is ubiquitous, and I will not spend a great deal of time on it here. But just to remind everyone. The centerpiece is, of course, the fossil record. Here we find a correlation of geological strata otherwise identifiable and datable by radioactive dating and other methods-with fossils, the remains, the hard parts-of organisms largely now extinct.

If you looked at an undisturbed sedimentary column, the remains of human beings would be found only in the very topmost layers. The farther down you dig, the farther back in time you are going. And no one has ever found any remnant of a human being down in the Jurassic or the Cambrian or any of the geological time periods other than the most recent-the last few million years. And likewise there are many organisms that were absolutely dominant and abundant worldwide for enormous periods of time that became extinct and were never seen again in the higher sedimentary columns. Trilobites are an example. They hunted in herds on the ocean bottoms. They were enormously abundant, and there have not been any of them on the Earth since the Permian. In fact, by far most of the species of life that have ever existed are now extinct. Extinction is the rule. Survival is the exception.

When you look at the fossil record, it is clear that some organisms have powerful anatomical similarities with others. Others are more distinct. There is a kind of taxonomic evolutionary tree that has been painstakingly developed over a century or more. But in recent times it is possible to look for chemical fossils-to examine the biochemistry of organisms that are alive today-and we are even just beginning to know something about the biochemistry of organisms that are extinct, because some of their organic matter can nevertheless be recovered. And here there is a remarkable correlation between what the anatomists say and the molecular biologists say. So the bone structure of chimpanzees and humans is startlingly similar. And then you look at their hemoglobin molecules, and they are startlingly similar. There's only one amino acid difference out of hundreds between the hemoglobins of chimps and humans.

In fact when you look more generally at life on Earth, you find that it is all the same kind of life. There are not many different kinds; there's only one kind. It uses about fifty fundamental biological building blocks, organic molecules. (By the way, when I use the word "organic," there is no necessary implication of biological origin. All I mean when I say organic is a molecule based on carbon that's more complicated than CO and C0₂.)

Now, it turns out that with trivial exceptions all organisms on Earth use a particular kind of molecule called a protein as a catalyst, an enzyme, to control the rate and direction of the chemistry of life. All organisms on Earth use a kind of molecule called a nucleic acid to encode the hereditary information and to reproduce it in the next generation. All organisms on Earth use the identical code book for translating nucleic acid language into protein language. And while there are clearly some differences between, say, me and a slime mold, fundamentally we are tremendously closely related. The lesson is, don't judge a book by its cover. At the molecular level, we are all virtually identical.

This then raises interesting questions about whether we have any idea of the possible range of life, of what could be elsewhere. We are trapped in a single example and have not the imagination to guess even one other way in which life might exist when there might be thousands or millions. Certainly no one deduced from fundamental theoretical chemistry the existence and function of nucleic acids when they were all around us and, in fact, when we ourselves were made of them.

Now, how did it come about that these few particular molecules, out of the enormous range of possible organic molecules, determine all life on Earth? There are two main possibilities and a range of intermediate cases. One possibility is that these molecules were somehow made preferentially in great abundance in the early history of the Earth, and so life just used what was lying around.

The other possibility is that these molecules have some special properties that are not only germane but essential for life, and so they were gradually developed by living systems or preferentially removed from a dilute to a concentrated solution by them. And, as I said, there is a range of intermediate possibilities.

It would be wrong to say that the origin of proteins and nucleic acids is identical with the origin of life. And yet nucleic acids are known in the laboratory to replicate themselves and even to replicate changes in themselves from plausible building blocks in the medium. It is true that an enzyme is needed for this reaction in the laboratory, but this enzyme determines the rate and not the direction of the chemical reaction, so it merely shows us what would happen were we willing to wait long enough. And there was surely plenty of time for the origin of life, which I will come back to as well.

It is certainly conceivable that what we have today is quite different from what was present at the time of the origin of life. We have today a very sophisticated kind of life, evolved by natural selection, that was based upon something much simpler, much earlier. It has been proposed that "much simpler" might in fact be mainly inorganic or it may have been organic; there is no way to be sure. But one thing is undoubtedly of interest for the origin of life-some would say essential-and that is to understand where the molecular building blocks that are present in all living things today came from.

So we now come to the issue of organic molecules. They are found on the Earth, of course, but since the Earth is littered with life, we do not have a clean experiment. We don't know, or at least it's not immediately obvious, which organic molecules we see on the Earth are here because of life and which would be here even if there had not been life. And virtually all the organic molecules that we see in our everyday lives are of biological origin. If you want to know something about organic chemistry on the Earth prior to the origin of life, it is a good idea to look elsewhere.

The idea of extraterrestrial organic matter is important not just for this reason but also because it tells us something relevant at least about the likelihood of extraterrestrial life. If it turns out that there is no sign of organic molecules elsewhere, or they're extremely rare, that might lead you to conclude that life elsewhere was extremely rare. If you found the universe burgeoning and overflowing with organic matter, then at least that prerequisite for extraterrestrial life would be satisfied. So it's an important issue. It's an issue where remarkable progress has been made since the early 1950s, and it speaks to us, I believe, if not centrally at least tangentially, about our origins.

The astronomer Sir William Huggins frightened the world in 1910. He was minding his own business, doing astronomy, but as a result of his astronomy (the work I'm talking about was done in the last third of the nineteenth century) there were national panics in Japan, in Russia, in much of the southern and midwestern United States. A hundred thousand people in their pajamas emerged onto the roofs of Constantinople. The pope issued a statement condemning the hoarding of cylinders of oxygen in Rome. And there were people all over the world who committed suicide. All because of Sir William Huggins's work. Very few scientists can make similar claims. At least until the invention of nuclear weapons. What exactly did he do? Well, Huggins was one of the first astronomical spectro-scopists.

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This is the coma of a comet-the cloud of gas and dust that surrounds the icy comet nucleus when it enters the inner solar system. Huggins used a spectroscope to spread out the light from a comet into its constituent frequencies. Some frequencies of light are preferentially present, from which it is possible to deduce something of the chemistry of the material in the comet. This is an application of stellar spectroscopy that had been going very successfully in the decade or two before Huggins turned his attention to the comets. (Huggins also made major contributions to understanding the chemistry of the stars.)

This image of four spectra is taken from one of Huggins's publications. These are wavelengths of light in the visible part of the spectrum to which the eye is sensitive. At the bottom is the spectrum of an 1868 comet called Brorsen. Above that is the spectrum of another 1868 comet called Winnecke II. And at the top is the spectrum of olive oil.

You can see that Comet Winnecke resembles olive oil more than it does Comet Brorsen. However, nobody deduced the existence of olive oil on the comets. (It would be an important discovery if it could be made.) But instead what this similarity shows is that a molecular fragment, diatomic carbon or C₂-two carbon atoms attached together-is present when you look at the spectrum of the comets and also when you look at natural gas and the vapor from heated olive oil. This is the discovery of an organic molecule, not one very familiar on Earth because of its instability when it collides 'with other molecules. It requires something close to a high vacuum, which does not naturally occur on the surface of the Earth. In the vicinity of a cometary coma, there is a high vacuum sufficient for C₂ not to be destroyed, and so here it is-the first discovery of an extraterrestrial organic molecule. And it turns out not to be one with which we have great familiarity.

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Spectrum of Comet 2001 Q4 (NEAT) on 2004 May 14

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Here is a typical modern cometary spectrum, and we can see the prominent bands of C₂ and other things, too. We see NH, the amino group that is produced by dissociation of ammonia, NH, which is also the defining molecular group of the amino acids, the building blocks of proteins. And we see here the molecular fragment that caused all the trouble, CN, the nitrile or cyanide molecule.

A single grain of potassium cyanide on the tongue will instantly kill a human being. Discovering cyanide in comets worried people.

Especially when it appeared that in 1910 the Earth would pass through the tail of Halley's Comet. Astronomers tried to reassure people. They said it wasn't clear that the Earth would pass through the tail, and even if the Earth did pass through the tail, the density of CN molecules was so low that it 'would be perfectly all right. But nobody believed the astronomers.

Perhaps the Earth did pass through the edge of the tail. In any case the comet came and went, nobody died, and in fact nobody could detect a single additional molecule of CN anywhere on the Earth. William Huggins, however, did die at the time that the comet came by, but not of cyanide poisoning.

Now, when we look closely at a comet, there is a tiny nucleus, the solid body that constitutes the comet everywhere except when it's very close to the Sun. The icy nucleus is typically a few kilometers across-but when it comes close to the Sun, the icy nucleus outgasses mainly water vapor and produces the coma and a long and lovely tail.

Consider the molecules we have just talked about: CN, C₂, C, NH. What are their parent molecules? Where did they come from? There are some precursors. We are seeing only fragments that have been chopped off of a bigger molecule by ultraviolet light from the Sun and the solar wind. It is clear that there is a repository of much more complex molecules-much more complex organic molecules-that are part of the cometary nucleus but which we have not yet discovered.

Radio astronomical studies have already found HCN (hydrogen cyanide) and CH₃CH (acetonitrile) in at least one comet. And these are interesting organic molecules that in other ways are implicated in the origin of life on Earth.

Imagine the air in front of your nose, highly magnified, say 10 million times. You would see a multitude of molecules, nitrogen and oxygen molecules, and occasional molecules of water

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vapor and carbon dioxide. Air, as you know, is mainly oxygen and nitrogen. Now, if you take some air and cool it, you will progressively condense out the various molecules. Water will condense out first, carbon dioxide next, oxygen and nitrogen much later; that is, at much lower temperatures.

Let's consider the condensation of the water molecule. When condensation happens, it's not just that the water molecules drop out of the air helter-skelter. In fact they form a lovely hexagonal crystal lattice, which stretches off as far as the ice crystal or snowflake or whatever it is goes. Other molecules condense out at much higher temperatures, like silica, for example (silicon dioxide), which also forms a crystal lattice.

Let's go back to the solar nebula from which, as we said earlier, the solar system almost surely formed, with a protosun in the center and the temperature declining the farther we get from the Sun. Now we must imagine this as a mix of cosmically abundant materials, including water (H₂0, which we know through spectroscopic analysis of astronomical images is very abundant), methane (CH₄; we know that's very abundant), silica (Si0₂; we know that's very abundant), and what happens is that at different distances from the Sun, different materials will condense out, because they have different vapor pressures or different melting points. And what we see is (guess what?), water condenses out roughly at the vicinity of the Earth, whereas silicates condense out closer to the Sun, so liquid silicates or gaseous silicates are not to be expected under ordinary planetary conditions, even at the orbit of Mercury Whereas you have to go out to somewhere near the present distance of Saturn before methane condenses. Now, methane is probably the chief carbon-containing molecule in the cosmos, and what this says is that in the early stages of the formation of the solar nebula there should have been a preferential condensation of methane in the outer parts of the solar system, but not in the inner parts. And if that is generally true, then we ought to expect more organic matter in the outer parts and much less in our neck of the cosmic woods.

Well, there is certainly not a huge amount of methane on the Moon or Mercury. But when we do go out to the orbit of Saturn we start finding not only evidence for methane-the planets Jupiter, Saturn, Uranus, and Neptune have lots of methane in their spectra-but we find a set of data that strongly implies the presence of complex organic molecules in the outer solar system.

This is a photograph of Iapetus, one of the outer moons of Saturn. The gray area is not in shadow. There is actually a remarkable division of one hemispheric surface into dark material and the other hemisphere into bright material. And the clear spectral signature of water ice is present in the bright areas.

We did not fly very close with either Voyager 1 or Voyager 2 to Iapetus. We think this is organic matter. It is very dark. At the center of this dark stuff, the albedo, the reflectivity, is something like 5 percent. I can't be sure, but I suspect that there is nothing in the room you are sitting in as dark as 5 percent albedo. Also, it is reddish. That is, it does not reflect much light, but it reflects more light in the red than in the blue part of the visible spectrum. And the values of the albedo and color are inconsistent with a wide range of other materials that you might offhand guess it might be-various of the salts, for example. They are very consistent with complex organic matter of various sorts. We know there is complex organic matter out there. I gave you one argument from the comets. Another argument is a category of meteorites called carbonaceous meteorites that fall to Earth, and they have several percent to as much as 10 percent of complex organic matter in them.

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This is a family portrait of some of the small moons of Saturn. All of them were discovered by the Voyager spacecraft. None of these were known before. The smallest ones are maybe ten kilometers across. The biggest one may be a hundred kilometers. They're little worlds, and all of them are dark and red like Iapetus.

These are rings of Uranus. You may not think it's a very good picture, but it took an awful lot of work to make it. The picture was taken at 2.2 microns, in the infrared part of the spectrum. The rings are known to be quite different from the rings of Saturn. They are thinner, they are wispier, and they are black, again suggesting the prevalence of dark, reddish, presumably organic matter in the outer solar system.

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Now, this is not in the outer solar system. This is Phobos, the innermost moon of Mars, which may or may not be a captured asteroid from farther out in the solar system, and it too has this dark, reddish composition. Its mean density is known, and it is consistent with organic matter.

Deimos is the outermost Martian moon. Despite its different appearance from Phobos, it is likewise very dark, very red, same story.

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And I should mention that Mars itself, around which Pho-bos and Deimos are orbiting (all that rocky stuff is Mars, and the foreground instrumentation is the Viking 1 Lander), at least in the two places that we landed with Viking 1 and Viking 2, shows not a hint of organic matter. I will return to Martian exploration later, but I want to stress that the limits to the presence of organic matter on Mars are very low. There is not one part in a million of simple organic molecules and not one part in a billion of complex organic molecules. Mars is very dry, denuded in organic matter, and yet there are these two moons that may be made entirely of organic matter orbiting it. It's an interesting dilemma. These are two trenches that were dug by this sample arm in the Martian soil. So we gathered material from the subsurface and withdrew it back into the spacecraft and examined it with a gas chromatograph/mass spectrometer for organic matter, of which there was none.

I want to continue the story about organic matter in the outer solar system. And the best story by far, the one that we have the most information on, although it is still quite limited, is for Titan. Titan is the largest moon in the Saturn system. It is remarkable for many reasons, the most striking of which is that it is the only moon in the solar system with a significant atmosphere. The surface pressure on Titan (we know from Voyager 1) is about 1.6 bars, that is, about 1.6 times what it is in the room I am in as I write this. Since the acceleration due to gravity is about one-sixth on Titan what it is here on Earth, there is ten times more gas in the Titanian atmosphere than in the terrestrial atmosphere, which is a substantial atmosphere.

The organic molecules found in the gas phase in the atmosphere of Titan by the Voyager 1 and 2 spacecraft include hydrogen cyanide (HCN, which we've talked about before), cyanoacetylene, butadiene, cyanogen (which is two CNs glued together), propylene, propane (which we know), acetylene, ethane, ethylene (these are all components of natural gas). Methane, likewise. And the principal constituent of the atmosphere, there as here, is molecular nitrogen.

It is, I think, very interesting that we have a world in the outer solar system that is loaded with the stuff of life. And we can calculate, at the present rate at which these materials are being formed on Titan, how much of this stuff has accumulated during the history of the solar system. The answer is the equivalent of a layer at least hundreds of meters thick all over Titan, and possibly kilometers thick. The difference depends on how long a wavelength of ultraviolet light can be used for such synthetic experiments. And, incidentally, there is also a range of entertaining evidence that there is a surface ocean of liquid

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hydrocarbon at Titan. [3] So just think of that environment. There's land; probably there's ocean. The land is covered with this organic muck that falls from the skies. There is a submarine deposit underneath this ocean of liquid ethane and methane of more of this complex stuff, and then down deep is frozen methane and frozen water and so on.

Now, that's a world worth visiting. What's happened to that stuff over the last 4.6 billion years? How far along has it gotten? How complex are the molecules there? What happens when occasionally there is an external or an internal event that heats things locally and melts some ice and makes some liquid water? Titan is a world crying out for detailed exploration, and it seems to be a planetary-scale experiment on the early steps that here on Earth led to the origin of life but there on Titan were very likely frozen, literally, at the early stages because of the general unavailability of liquid water.

Likewise, there is a very stunning range of studies-mainly in the last two decades-of interstellar organic matter: not just a multiplicity of worlds in our solar system but the cold, dark spaces between the stars are also loaded with organic molecules.

we are looking toward the center of the galaxy in the direction of the constellation Sagittarius. You can see a set of dark clouds, some quite extensive, some much smaller. It is in these giant molecular clouds that well upwards of 50 different kinds of molecules have been found, most of which are organic. And it is precisely in such dark clouds that the collapse of solar nebulae is expected to happen, and therefore the forming solar systems should be composed, in part, of complex organic matter. The conclusion is that complex organic materials are everywhere.

Now let's return to the question of the origin of life on Earth. The organic stuff could have fallen in during the formation of the Earth, or it could have been generated in situ from simpler materials on the Earth in the same way as on Titan. At the present time there is no way of assessing the relative contributions from these two sources. What seems clear is that either source would be sufficient-adequate.

The Earth formed from the collapse of lumps of matter of the sort we talked about earlier, condensing from the solar nebula. Therefore in its final stages of formation, it was collecting objects that collided at high velocity and produced a set of catastrophic events, including the melting of much of the surface. This, it turns out, was not a good environment for the origin of life, as you might have suspected. But after a while, when the final sweeping up of the debris in the solar system was more or less completed, water delivered from the outside or outgassed from the inside started forming on the surface, filling in the ancient impact craters. And a trickle of material was still falling in from space. At the same time, electrical discharges and ultraviolet light from the Sun and other energy sources produced in-

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digenous organic matter. The amount of organic matter that could have been produced in the first few hundred million years of Earth history was sufficient to have produced in the present ocean a several-percent solution of organic matter. That is just about the dilution of Knorr's chicken soup, and not all that different from the composition either. And chicken soup is widely known to be good for life. In fact, it is just this warm, dilute soup, in the words of J. B. S. Haldane, who was one of the first two people to realize that this sequence of events was likely, in which the standard scenario for the origin of life occurs.

In the laboratory -we can take molecules of water, ammonia, and methane-rather like the ones we've been talking about for Titan-and dissociate them by ultraviolet light. The fragments make a set of precursor molecules, including hydrogen cyanide, which then combine and, in water, form the amino acids. In such experiments not just the building blocks of the proteins but the building blocks of the nucleic acids are routinely produced. There is a range of subsequent experiments, in which the smaller molecular building blocks join together to form large and complex molecules.

If we look at the fossil record, we find that there is a range of evidence for microfossils dating back not just to the beginning of the Cambrian but dating back to as much as 3,500 million years ago.

Now, just think about these numbers. The Earth itself forms about 4,600 million years ago. Because of the final stages of accretion, we know that the Earth environment was not suitable for the origin of life back then. From studies of the late crater-ing on the Moon, it looks-since the Earth and the Moon were presumably in the same part of the solar system then as now- as if the Earth was not in a suitable state for the origin of life until perhaps 4,000 million years ago. So if the Earth is not appropriate to the origin of life until 4,000 million years ago and the first fossils are around 3,500 million years ago, then there are only about 500 million years for the origin of life. But those earliest fossils are by no means extremely simple organisms. They are, in fact, colonial algal stromatolites, and a great deal of evolution had to precede them. And that therefore says that the origin of life happened in significantly less than 500 million years. We don't know how much less. Six days was once a popular hypothesis. It's not excluded by these data, but at least it cannot be as long as 500 million years. It must have happened very fast. A process that happens quickly is a process that in some sense is likely. The faster it happens, the more likely it is. There is a difficulty in extrapolating from a single case; nevertheless this evidence suggests that the origin of life was in some sense easy, in some sense sitting in the laws of physics and chemistry. And if that's true, that is a very important fact for the consideration of extraterrestrial life.

There is a classic objection to this kind of argument about the origin of life. As far as I know, this objection was first posed by Pierre Lecompte du Noiiy in a 1947 book called Human Destiny and is regularly rediscovered about once every half decade. It goes something like this: Consider some biological molecules. Not all of them. We'll give the evolutionists the benefit of the doubt. Let's just take a small, simple one, not something thousands of amino acids long. Let's pick an enzyme with a hundred amino acids. That's a very modest enzyme. Now, a way to think of it is as a kind of necklace on which there are a hundred beads. There are twenty different kinds of beads, any one of which could be in any one of these positions. To reproduce the molecule precisely, you have to put all the right beads-all the right amino acids-in the molecule in the right order. If you were blindfolded while assembling a necklace from equally abundant beads, the chance of getting the right bead in the first slot is 1 chance in 20. The chance of getting the right bead in the second slot is also 1 chance in 20, so the chance of getting the right bead in the first and second slots simultaneously is 1 chance in 20². Getting the first three correct is 1 chance in 20³, and getting all hundred correct is 1 chance in 20¹⁰⁰. Well, you can see 20¹⁰⁰ is 2¹⁰⁰ x 10¹⁰⁰. And since 2¹⁰ is a thousand, which is 10³, then 2¹⁰⁰ is 10³⁰, so this is the same as 10¹³⁰. One chance in 10¹³⁰ of assembling the right molecules the first time. Ten to the hundred-thirtieth power, or 1 followed by 130 zeros, is vastly more than the total number of elementary particles in the entire universe, which is only about ten to the eightieth (10⁸⁰).

So let's imagine that every star in the universe has a planetary system like ours. Let's say one planet has oceans. Let's suppose that the oceans are just as thick as ours. Let us suppose that there is a few-percent solution of organic matter in every one of those oceans and that in every tiny volume of the ocean that has enough molecules there is an experiment happening once every microsecond to construct this particular hundred-amino-acid-long protein. So in the ocean every microsecond an enormous number of these little experiments are going on. And identical things are happening in the next star system and the next star system, filling an entire galaxy. And then not just in that galaxy but in every galaxy in the universe. It turns out that if that sequence of experiments had gone on for the entire history of the universe, you could never produce one enzyme molecule of predetermined structure. And in fact it's much worse than that.

If you did that same experiment once every Planck time, the shortest unit of time that is permissible in physics, you still couldn't generate a single hemoglobin molecule, from which many people have decided that God exists, because how else do you make these molecules? If you haven't heard this before, doesn't this seem like a pretty compelling argument? Strong argument, right? A whole universe of experiments once every Planck time. Can't beat that.

Now let's take another look. Does it matter if I have a hemoglobin molecule here and I pull out this aspartic acid and I put in a glutamic? Does that make the molecule function less well? In most cases it doesn't. In most cases an enzyme has a so-called active site, which is generally about five amino acids long. And it's the active site that does the stuff. And the rest of the molecule is involved in folding and turning the molecule on or turning it off. And it's not a hundred places you have to explain, it's only five to get going. And 20⁵ is an absurdly small number, only about 3 million. Those experiments are done in one ocean between now and next Tuesday. Now, remember what it is we're trying to do: We're not trying to make a human being from scratch, to have all the molecules of a human being fall simultaneously together in a primitive ocean and then have someone swim out of the water. That's not what we're asking for. What we're asking for is something that gets life going, so this enormously powerful sieve of Darwinian natural selection can start pulling out the natural experiments that work and encouraging them, and neglecting the cases that don't work.

So it turns out here, as in some of the arguments I was talking about yesterday, there is an important point that is left out in these apparent deductions of divine intervention by looking at the natural world. A very dramatic, strong statement of this sort has been made by the astronomers Fred Hoyle and N. C. Wick-ramasinghe. And their phrase, after a calculation in this spirit, goes something like this:

They say it is no more likely that the origin of life could occur spontaneously by molecular interaction in the primitive ocean than that a Boeing 747 would be spontaneously assembled when a whirlwind passed over a junkyard. That's a vivid image. It's also a very useful image, because, of course, the Boeing 747 did not spring full-blown into the world of aviation; it is the end product of a long evolutionary sequence, which, as you know, goes back to the DC-3 and so on until you get to the Wright biplane. Now, the Wright biplane does look as if it were spontaneously assembled by a whirlwind in a junkyard. And while I don't mean to criticize the brilliant achievement of the Wright brothers, as long as you remember that there is this evolutionary history, it's a lot easier to understand the origin of the first example.

I want to close on a beautiful little piece of poetry written by a woman in rural Arkansas. Her name is Lillie Emery, and she is not a professional poet, but she writes for herself and she has written to me. And one of her poems has the following lines in it:

My kind didn't really slither out of a tidal pool, did we? God, I need to believe you created me: we are so small down here.

I think there is a very general truth that Lillie Emery expresses in this poem. I believe everyone on some level recognizes that feeling. And yet, and yet, if we are merely matter intricately assembled, is this really demeaning? If there's nothing in here but atoms, does that make us less or does that make matter more?