by Carl Sagan
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.
fig. 20
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.
fig. 21
Spectrum of Comet 2001 Q4 (NEAT) on 2004 May 14
fig. 22
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
fig. 23
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 hexag
onal 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.