CHAPTER 9

Life

SPONTANEOUS GENERATION

It is rather breathtaking to decide on the basis of (we hope) strict logic and the best evidence we can find that there are 650 million habitable planets in our Galaxy alone, and therefore over 2 billion billion in the Universe as a whole. And yet, from the standpoint of the subject matter of this book, of what value are habitable planets in themselves? If they lack life, their habitability comes to nothing.

Our calculations concerning extraterrestrial intelligence must therefore come to a halt right here, unless we can say something reasonable about the chance that a habitable planet actually has life on it.

In order to do that, we must again turn to something that is known, and that is the one habitable planet that we know to have life on it—Earth itself. In other words, before we can say anything sensible about life on habitable planets in general, we must be able to say something sensible about how life came to exist on the Earth.

Early speculations about the existence of life on Earth invariably assumed it to have been created through some nonnatural agency, usually through the action of some god or demigod. The best-known story in our Western tradition is that humanity was created in the same series of divine acts that created the Universe generally.

In six days of creation the job was done. God created light on the first day; the land and sea on the second; plant life on the third; the heavenly bodies on the fourth; animal life of the sea and air on the fifth; and animal life on land on the sixth. As the last creative act on the sixth day, humanity was brought into being.

Life, created on three different days, was considered as having come into being in separate species (“after his kind” it says in the King James Bible). Presumably, these were the species that continued to exist into contemporary times. As some believed, no species were added to the first creation and none subtracted.

As to the date of this Divine creation, the Bible is not specific, for the habit of dating with compulsive precision is a rather late development in historical writing. Deductions based on various statements in the Bible, however, place the date of creation only a few thousand years in the past. The precise date usually found in the headings of the King James Bible is 4004 B.C., this date having been worked out by the Irish theologian James Ussher (1581–1656).

Although the creation of the world (or of different worlds) was assumed to be a once-for-all act, it was common in early times to assume that this was not necessarily true for life.

Actually, this is a reasonable attitude. After all, while there was no visual evidence of any creation of worlds in the course of human history, there did seem to be visual evidence for the creation of living things without the intervention of earlier living things.

Field mice may make their nests in holes burrowed into stores of wheat, and these nests may be lined with scraps of scavenged wool. The farmer, coming across nests from which the mother mouse has had to flee, and finding only tiny, naked, blind infant mice, may come to the most natural conclusion in the world: he has interrupted a process in which mice were being formed from musty wheat and rotting wool.

Let meat decay and small wormlike maggots will appear in it. Frogs can seem to arise out of river mud.

If the notion were true for various species of vermin, it might be true for all species of organisms, though perhaps less common for the larger and more complex species such as horses, eagles, lions, and human beings.

In fact, if one were sufficiently daring, one might suppose that the tale in Genesis was a fable; that this sort of “spontaneous generation” of living things from nonliving antecedents might account for the original beginning of life. Little by little each species might have formed, first the simple ones and later the more complex ones, with human beings, naturally enough, last of all.

And in that case, if we were to apply this to habitable planets generally, we would see that they, too, would naturally form life. All of them would bear life.

Provided, that is, the doctrine of spontaneous generation could withstand close examination—and it couldn’t.

The first crack in the doctrine appeared in 1668, thanks to an Italian physician and poet named Francesco Redi (1626–1697). Redi noticed that decaying meat not only produced flies, but also attracted them. He wondered if there were a connection between the flies before and the flies after, and tested the matter.

He did this by allowing samples of meat to decay in small vessels. The wide openings of some vessels he left untouched; others he covered with gauze. Flies were attracted to all the samples, but could land only on the unprotected ones. Those samples of decaying meat on which flies landed produced maggots. The decaying meat behind the gauze, upon which the foot of fly never trod, produced no maggots at all, although it decayed just as rapidly and produced just as powerful a stench.

Redi’s experiments showed plainly that maggots, and flies after them, arose out of eggs laid in decaying meat by an earlier generation of flies. There was no spontaneous generation of flies, just the normal process of birth from eggs (or seed).

Even as Redi was working out his demonstration, a Dutch biologist, Anton van Leeuwenhoek (1632–1723), was riding his hobby and grinding perfect little lenses (primitive microscopes, actually) through which he could look at tiny things and magnify them to easy visibility.

In 1675, he discovered living things in ditch water that were too small to be seen by the naked eye. These were the first “microorganisms” known, and those that van Leeuwenhoek first discovered are now called protozoa, from Greek words meaning first animals. In 1680, van Leeuwenhoek discovered that yeast was made up of tiny living things even smaller than most protozoa, and in 1638 he observed still tinier living things, which we now call bacteria.

Where did these microscopic living things come from?

Broths were invented in which microorganisms could multiply. It turned out not to be necessary to seek microorganisms to place in these broths. A broth might be boiled and filtered until there was nothing in it that the lens of a microscope could detect. If one waited a while and looked again, the broth was inevitably swarming with life. (What’s more, it was microorganisms that caused meat to decay even when no microorganisms were placed in the meat.)

Perhaps spontaneous generation did not take place in the case of those species visible to the unaided eye. In the case of the microorganisms—bits of life far simpler than the familiar plants and animals of everyday life—spontaneous generation might well be possible. In fact, it seemed established.

But then, in 1767, came the work of an Italian biologist, Lazzaro Spallanzani (1729–1799). He not only boiled broth, but he sealed off the neck of the flask containing it. The broth, boiled and sealed, never developed any form of microscopic life. Shortly after the seal was broken, however, life began to swarm.

A sealed neck, keeping out the air, acted like Redi’s gauze, and the conclusions had to be similar to Redi’s conclusions. There are microscopic and unseen creatures all about us in the air that are smaller and harder to observe than even the eggs of flies. These airborne bits of life fall into any broth left open to the air, and there they multiply. (Spallanzani isolated a single bacterium and watched it multiply by simply splitting in two.) If those bits of life are kept out of the broth, no life originates.

In 1836, a German biologist, Theodor Schwann (1810–1882), went even further. He showed that broth remained sterile even when open to air, provided the air to which it was exposed had been heated first in order to kill any forms of life it might contain.

Advocates of the doctrine of spontaneous generation pointed out that heat might kill some “vital principle” essential to the production of life out of inanimate matter. Heating broth and sealing it away would in that case fail to produce life. Exposing heated broth to air that had likewise been heated was no better.

In 1864, however, the French chemist Louis Pasteur (1822–1895) produced the clincher. He boiled a meat broth until it was sterile, and did so in a flask with a long, thin neck that bent down, then up again, like a horizontal 5. Then he neither sealed it off nor stoppered it. He left the broth exposed to cool air.

The cool air could penetrate freely into the vessel and bathe the broth. If it carried a “vital principle” with it, that was welcome. What did not enter, however, was dust and microscopic particles generally. These settled at the bottom of the curve of the flask’s neck.

As a result, the broth did not breed microorganisms and did not show any signs of life. Once Pasteur broke off the swan-neck, however, and allowed dust and particles to reach the broth along with the air, microorganisms made their appearance at once.

With that, the notion of “spontaneous generation” seemed dead, once and for all.

ORIGIN OF LIFE?

Once it was clearly established that spontaneous generation did not take place and that all life (as far as human beings were able to observe) came from previous life, it became very difficult to decide how life originated on Earth—or on any other planet.

The changeover was rather like the one that took place in the theories concerning the origin of planetary systems. As long as one clung to an evolutionary theory such as Laplace’s nebular hypothesis, it was easy to suppose that planetary systems were common and that every star was accompanied by one. The nebular hypothesis, in a way, preached the spontaneous generation of planets.

A catastrophic theory of planetary formation, however, involved an event that was so rare that planets themselves had to be regarded as excessively rare, and it became tempting to think that our own planetary system was not to be duplicated elsewhere.

In the same way, the defeat of spontaneous generation and the new suggestion that life came only from previous life, which came only from still earlier life and so on in an endless chain, made it seem that the original forms of life couldn’t possibly have arisen save through some miraculous event. In that case, even if habitable planets were as plentiful as the stars themselves, Earth might yet be the only one that bore life.

Even as Pasteur was knocking the pins out from under spontaneous generation, however, the situation was being eased a little bit. In 1859, the English biologist Charles Robert Darwin (1809–1882) published a book for which The Origin of Species is the commonly used title.

In it he presented exhaustive evidence in favor of an evolutionary theory in which the various species of living things were not separate and distinct from the beginning. Instead, under the pressure of increasing populations and of natural selection, all living things gradually changed; new, and presumably more suitable, species developed from old. In this way, several different species might have a common ancestor and, if one went back far enough, all life on Earth may have developed from a single very primitive ancestral form of life.

The theory met with much opposition, but biologists came to accept it in time.

What it meant was that one no longer had to account for the separate creation of each of the millions of species of living things known. Instead, it would be sufficient to account for the creation of any form of life, however simple. This original simple form, produced by spontaneous generation, could then by evolutionary processes give rise to all other forms of life, however complex—even human beings.

Of course, if spontaneous generation were really impossible, the production of one form of life was as much a miracle as the production of a million forms.

On the other hand, all that biologists had done was to show that known forms of life could not be generated spontaneously in the short periods of time available in the laboratory. Suppose we dealt with a very simple form of life, much simpler than any known, and suppose we had long periods of time and a whole planet at our disposal; might not that very simple form of life finally be generated?

The key lay in that phrase long periods of time. The hit-or-miss random processes of evolution took a long time (even the evolutionists admitted that) and the question was whether there was time enough for the simple life form to be generated and all the myriad of complex life forms to be developed afterward

In Darwin’s time, scientists had abandoned the notion of a planet that was no more than 6,000 years old and spoke freely of Earth’s age as being in the millions of years, but even that didn’t seem long enough for evolution to do its work.

In the 1890s, however, radioactivity was discovered, and it was found that uranium turned to lead with almost stupefying slowness. Half of any sample of uranium would turn to lead only after 4,500,000,000 years. In 1905, the American chemist Bertram Borden Boltwood (1890–1927) suggested that the extent of the radioactive breakdown in rock would be an indication of the length of time since that rock had solidified.

Radioactive changes of all kinds have been used to measure the age of various parts of the Earth, of meteorites, and, recently, of Moon rocks, and the general consensus now is that the Earth, and the Solar system in general, is about 4,600,000,000 years old.

Hints of this vast age were already available in the early decades of the twentieth century, and it began to appear that there was enough time for evolution to do its work, if life could somehow start spontaneously.

But could that spontaneous start take place?

Unfortunately, by the time the extreme age of the Earth came to be understood, the extreme complexity of life also came to be understood, and the chance of spontaneous generation seemed to shrink further.

Twentieth-century chemists learned that protein molecules, which are molecules peculiarly characteristic of life, were made up of long chains of simpler building blocks called amino acids. They found that every protein had to have every one of thousands of different atoms (even millions in some cases) placed just so if it was to do its work properly. Later on, they discovered that an even more fundamental type of molecule, those of the nucleic acids, were even more complicated than protein molecules. What’s more, different nucleic acids and different proteins, along with smaller molecules of all kinds, intermeshed in complicated chains of reactions.

Life, even the apparently simple life of bacteria, was enormously more complicated than had been imagined in the days when the matter of spontaneous generation was being squabbled over. Even the simplest form of life imaginable would have to be built up out of proteins and nucleic acids, and how did those come to be formed out of dead matter? The origin of life on Earth, despite evolution, seemed more than ever a near-miraculous event.

Some scientists gave up and, in effect, washed their hands of the problem. The Swedish chemist Svante August Arrhenius (1859–1927) published a book, Worlds in the Making, in 1908, which took up the matter of the origin of life. In the book, Arrhenius upheld the universality of life and suggested that it was a common phenomenon in the Universe.

He went on to suggest that life might be, in effect, contagious. When simple living things on Earth form spores, the wind carries them through the air to burgeon in new places. Some by the blind force of the wind might be wafted upward high into the atmosphere and actually, Arrhenius speculated, out into space. There they might drift for millions of years through vacuum, pushed by the Sun’s radiation, protected by a hard, impervious pellicle and fiercely retaining the spark of life inside. Eventually, a spore would encounter some suitable planet without life and from it life would start on that planet.

In fact, Arrhenius suggested, that was how life on Earth got its start. It was vitalized by spores from space; spores that had originated on some other world that might remain forever unidentified.

Several points can be used to argue against this notion. One can calculate how many spores must leave a world in order that even one might have a reasonable chance to meet another world in the course of the lifetime of the Universe, and the amount is preposterously high.

Then, too, it is unlikely that spores can survive the trip through space. Bacterial spores are highly resistant to cold, even extreme cold; they might also be expected to survive vacuum. It is doubtful that even the hardiest spores could exist for the length of time it would take to drift from planetary system to planetary system, but we could stretch a point and suppose that at least some could. What we do know, though, is that spores are very sensitive to ultraviolet light and other hard radiation.

They are not subjected to this on Earth, where the air forms a blanket that filters out the Sun’s more energetic radiation; nor was Arrhenius, in his time, aware of the extent to which energetic radiation fills the Universe. The radiation from any star anywhere in its ecosphere would be enough to kill wandering spores that were originally adapted to life within a protective atmospheric blanket. Cosmic-ray particles would kill them even in the depths of space.

Arrhenius thought that radiation pressure would propel spores away from a star and through space. We now know the Solar wind is much more likely to do so. In either case, whatever propels the spore away from a star and toward others in the first place would repel the spore as it approached another star and prevent it from landing on a planet within the ecosphere.

All in all, the notion of Earth’s having been seeded by spores from other worlds is exceedingly dubious.

Besides, of what use is it to explain the origin of life on Earth by calling upon life on other planets for help? One would have then to explain the origin of life on the other planet. And if it could form on any planet by some natural and nonmiraculous means, then it could form on Earth in the same fashion.

But how? Even as late as the 1920s, biologists were at a loss for a natural mechanism.

THE PRIMORDIAL EARTH

One objection to the spontaneous generation of life on Earth is this: If life could be formed out of nonlife in the far past, it should happen periodically in later times, even right now. Since no such formation is ever observed in the present day, ought we not to conclude that it did not happen in the far past, either?

The fallacy in this argument is plain. It surely must be that the primordial Earth in the days before life existed upon it had characteristics different from those of today. It follows, if this is so, that we cannot argue from events now to events then. What is not likely now and does not, therefore, take place, might have been quite likely then, and did take place.

One obvious difference between modern Earth and primordial Earth, for instance, is that modern Earth has life and primordial Earth had not. Any chemical substance that arose spontaneously on Earth today and that was approaching the level of complexity where it might be considered as protolife would surely be food for some animal and would be gobbled up. In the primordial and lifeless Earth, such a substance would tend to survive (at least, it would not be eaten) and would have a chance to grow still more complex and to become alive.

Then, too, the primordial Earth might have had an atmosphere that was different from the present one.

This was first suggested in the 1920s by the English biologist John Burdon Sanderson Haldane (1892–1964). It occurred to him that coal was of plant origin, and that plant life obtained its carbon from the carbon dioxide of the air. Therefore, before life came into being, all the carbon in coal must have been in air in the form of carbon dioxide. Furthermore, the oxygen in air is produced by the same plant-mediated reactions that absorb the carbon dioxide and place the carbon atoms within the compounds of plant tissue.

It follows, then, that the primordial atmosphere of the Earth was not nitrogen and oxygen, but nitrogen and carbon dioxide. (This sounds even more logical now than it did when Haldane suggested it, since we now know that the atmospheres of Venus and Mars are made up largely of carbon dioxide.)

Furthermore, Haldane reasoned, if there were no oxygen in the air, there would be no ozone (a highly energetic form of oxygen) in the upper atmosphere. It is this ozone that chiefly blocks the ultraviolet light of the Sun. In the primordial Earth, therefore, energetic ultraviolet radiation from the Sun would be available in much larger quantities than it is now.

Under primordial conditions, then, the energy of ultraviolet light would serve to combine molecules of nitrogen, carbon dioxide, and water into more and more complex compounds that would, finally, develop the attributes of life. Ordinary evolution would then take over, and here we all are.

What could be done on the primordial Earth, with lots of ultraviolet, lots of carbon dioxide, no oxygen to break down the complicated compounds, and no living things to eat them, could not be done on present-day Earth with its dearth of ultraviolet light and carbon dioxide and its overabundance of oxygen and life. We cannot, therefore, use today’s absence of spontaneous generation as a reason to deny its presence on the primordial Earth.

This notion was supported by a Soviet biologist, Aleksandr Ivanovich Oparin (1894–). His book, The Origin of Life, also published in the 1920s but not translated into English till 1937, was the first to be devoted entirely to the subject. Where he differed from Haldane was in supposing that the primordial atmosphere was heavily hydrogenated, containing hydrogen as itself, and some in combination with carbon (methane), nitrogen (ammonia), and oxygen (water).

Oparin’s atmosphere makes sense in the light of what we now know about the composition of the Universe in general, and of the Sun and the outer planets in particular. Indeed, scientists now speculate that life began in Oparin’s atmosphere of ammonia, methane, and water vapor (Atmosphere I). The action of the ultraviolet radiation of the Sun split water molecules, liberating oxygen, which would react with ammonia and methane to produce Haldane’s atmosphere of nitrogen, carbon dioxide, and water vapor (Atmosphere II). Then, finally, the photosynthetic action of green plants produced the present-day atmosphere of nitrogen, oxygen, and water vapor (Atmosphere III).

To be sure, the talk of spontaneous generation of life on a primordial Earth, during the 1920s and 1930s, was purely speculation. There was no real evidence whatever.

Moreover, while Haldane and Oparin (both atheists) could cheerfully divorce life and God, others were offended by this and strove to show that there was no way in which the origin of life could be removed from the miraculous and made the result of the chance collisions of atoms.

A French biophysicist, Pierre Lecomte du Noüy, dealt with this very matter in his book, Human Destiny, which was published in 1947. By then the full complexity of the protein molecule was established, and Lecomte du Noüy attempted to show that if the various atoms of carbon, hydrogen, oxygen, nitrogen, and sulfur arranged themselves in purely random order, the chance of their arriving in this way at even a single protein molecule of the type associated with life was so exceedingly small that the entire lifetime of the Universe would be insufficient to offer it more than an insignificant chance of happening. Chance, he maintained, could not account for life.

As an example of the sort of argument he presented, consider a protein chain made up of 100 amino acids, each one of which could be any of twenty different varieties. The number of different protein chains that could be formed would be 10¹³⁰; that is, a one followed by 130 zeroes.

If you imagine that it took only a millionth of a second to form one of those chains, and that a different chain was being formed at random by each of a trillion scientists every millionth of a second ever since the Universe began, the chance that you would form some one particular chain associated with life would be only one in 10⁹⁵, which is such an infinitesimal chance it isn’t worth considering.

On the primordial Earth, what’s more, you wouldn’t be starting with amino acids, but with simpler compounds like methane and ammonia, and you would have to form a much more complicated compound than a chain made out of 100 amino acids to get life started. The chances of accomplishing something on a single planet in a mere few billion years is just about zero, therefore.

Lecomte du Noüy’s argument seemed exceedingly strong, and many people eagerly let themselves be persuaded by it and still do even today.

—Yet it is wrong.

The fallacy in Lecomte du Noüy’s argument rests in the assumption that pure chance was alone the guiding factor and that atoms can fit together in any fashion at all. Actually, atoms are guided in their combinations by well-known laws of physics and chemistry, so that the formation of complex compounds from simple ones are constrained by severely restrictive rules that sharply limit the number of different ways in which they combine. What’s more, as we approach complex molecules such as those of proteins and nucleic acids, there is no one particular molecule that is associated with life, but innumerable different molecules, all of which are in association.

In other words, we don’t depend on chance alone, but on chance guided by the laws of nature, and that should be quite enough.

Could the matter be checked in the laboratory? The American chemist Harold Clayton Urey encouraged a young student, Stanley Lloyd Miller (1930–), to run the necessary experiment in 1952.

Miller tried to duplicate primordial conditions on Earth, assuming Oparin’s Atmosphere I. He began with a closed and sterile mixture of water, ammonia, methane, and hydrogen, which represented a small and simple version of Earth’s primordial atmosphere and ocean. He then used an electric discharge as an energy source, and that represented a tiny version of the Sun.

He circulated the mixture past the discharge for a week and then analyzed it. The originally colorless mixture had turned pink on the first day, and by the end of the week one sixth of the methane with which Miller had started had been converted into more complex molecules. Among those molecules were glycine and alanine, the two simplest of the amino acids that occur in proteins.

In the years after that key experiment, other similar experiments were conducted, with variations in starting materials and in energy sources. Invariably, more complicated molecules, sometimes identical with those in living tissue, sometimes merely related to them, were formed. An amazing variety of key molecules of living tissue were formed “spontaneously” in this manner, although calculations of the simplistic Lecomte du Noüy type would have given their formation virtually no chance.

If this could be done in small volumes over very short periods of time, what could have been done in an entire ocean over a period of many millions of years?

It was also impressive that all the changes produced in the laboratory by the chance collisions of molecules and the chance absorptions of energy (guided always by the known laws of nature) seemed to move always in the direction of life as we know it now. There seemed no important changes that pointed definitely in some different chemical direction.

That made it seem as though life were an inevitable product of high-probability varieties of chemical reactions, and that the formation of life on the primordial Earth could not have been avoided.

METEORITES

We can’t, of course, be sure that the experiments set up by scientists truly represent primordial conditions. It would be much more impressive if we could somehow study primordial matter itself and find compounds that had been formed by nonlife processes and that were on the way, so to speak, to life.

The only primordial matter we can study here on Earth are the meteorites that occasionally strike the Earth. Studies of radioactive transformations within them show them to be over 4 billion years old and to be dating, therefore, from the infancy of the Solar system.

About 1,700 meteorites have been studied; thirty-five of them weighing over a ton apiece. Almost all of them, however, are either nickel-iron or stone in chemical composition and contain none of the elements primarily associated with life. They therefore give us no useful information concerning the problem of the origin of life.

There remains, however, a rare type of meteorite, black and easily crumbled—the “carbonaceous chondrite.” These actually contain a small percentage of water, carbon compounds, and so on. The trouble is, though, that they are much more fragile than the other types of meteorites, and though they may be common indeed in outer space, few survive the rough journey through the atmosphere and the collision with the solid Earth. Fewer than two dozen such meteorites are known.

Carbonaceous chondrites, to be useful to us, should be studied soon after they have fallen. Any prolonged stay on the ground is sure to result in contamination by Earthly life or its products.

Two such meteorites, fortunately, were seen to fall and were examined almost at once. One fell near Murray, Kentucky, in 1950, and another exploded over Murchison, Australia, in September, 1969.

By 1971, small quantities of eighteen different amino acids were separated out of the Murchison fragments. Six of them were varieties that occur frequently in the protein of living tissue; the other twelve were related to these chemically, but occurred infrequently or not at all in living tissue. Similar results were obtained for the Murray meteorite. Agreements between the two meteorites that fell on opposite sides of the world, nineteen years apart, were impressive.

Toward the end of 1973, fatty acids were also detected. These differ from amino acids in having longer chains of carbon and hydrogen atoms and in lacking nitrogen atoms. They are the building blocks of the fat found in living tissue. Some seventeen different fatty acids were identified.

How did such organic molecules happen to be found in meteorites? Are the meteorites the products of an exploded planet?^* Are the carbonaceous chondrites part of a planetary crust that bore life once and that still carry traces of that life now?

Apparently, this is not likely. There are ways of telling whether the compounds discovered in meteorites are likely to have originated in living things.

Amino acids (all except the simplest, glycine) come in two varieties, one of which is the mirror image of the other. These are labelled L and D. The two varieties are identicial in ordinary chemical properties, so that when chemists prepare the amino acids from their constituent atoms, equal quantities of L and D are always formed.

When amino acids are used to build up protein, however, the results are stable only if one group is used, either the L only or the D only. On Earth, life has developed with the use of L only (probably through nothing more meaningful than chance), so that D-amino acids occur in nature very rarely indeed.

If the amino acids in the meteorites were all L or all D, we would strongly suspect that life processes similar to our own were involved in their production. In actual fact, however, L and D forms are found in equal quantities in the carbonaceous chondrites, and this means that they originated by processes that did not involve life as we know it.

Similarly, the fatty acids formed in living tissues are built up by the addition to each other of varying numbers of 2-carbon-atom compounds. As a result, almost all fatty acids in living tissue have an even number of carbon atoms. Fatty acids with odd numbers are not characteristic of our sort of life, but in chemical reactions that don’t involve life they are as likely to be produced as the even variety. In the Murchison meteorite, there are roughly equal quantities of odd-number and even-number fatty acids.

The compounds in the carbonaceous chondrites are not life; they have formed in the direction of our kind of life—and human experimenters have had nothing to do with their formation. On the whole, then, meteoritic studies tend to support laboratory experiments and make it appear all the more likely that life is a natural, a normal, and even an inevitable phenomenon. Atoms apparently tend to come together to form compounds in the direction of our kind of life whenever they have the least chance to do so.

DUST CLOUDS

Outside the Solar system we can see the stars, but we have eliminated them as breeding grounds of life. Perhaps we could find breeding grounds if we could inspect the cool surfaces of the planets revolving about them.

We can’t do that, but there is cool matter in outer space that we can indeed detect; matter in the form of thin gas and dust that fills interstellar space.

The interstellar material was first detected about the turn of the century because certain wavelengths of light from distant stars were absorbed by the occasional atoms that drift about in the vastness of space. By the 1930s, it was recognized that the interstellar medium contained a wide variety of atoms, probably some of every type of atom cooked in the interiors of stars and broadcast into space during supernova explosions.

The density of the interstellar matter is so low that it seemed natural to suppose that it consisted almost entirely of single atoms and nothing else. After all, in order for two atoms to combine to form a molecule, they must first collide, and the various atoms are so widely spread apart in interstellar space that random motions will bring about collisions only after excessively long periods.

And yet, in 1937, stars shining through dark clouds of gas and dust were found to have particular wavelengths missing that pointed to absorption by a carbon-hydrogen combination (CH) or a carbon-nitrogen combination (CN). For the first time, interstellar molecules were found to exist.

To be sure, CH and CN are the kind of combination that can be formed and maintained only in very low-density material. Such atom combinations are very active and would combine with other atoms at once, if other atoms were easily available. It is because such other atoms are available in quantity on Earth that CH and CN do not exist naturally as such, on the planet.

No other combinations were noted in the interstellar dustclouds through dark lines in the visible spectrum.

After World War II, however, radio astronomy became increasingly important. Interstellar atoms can emit or absorb radio waves of characteristic lengths—something that requires far less energy than emission or absorption of visible light, and therefore takes place more readily. The emission or absorption of radio waves can be detected easily, given the radio telescopes required for the purpose, and the compounds responsible can be identified.

In 1951, for instance, the characteristic radio-wave emission by hydrogen atoms was detected, and the presence of interstellar hydrogen was thus observed directly for the first time and not merely deduced.

It was understood that next to hydrogen, helium and oxygen were the most common atoms in the Universe. Helium atoms don’t cling to any other atoms, but oxygen atoms do. Should there not be oxygen-hydrogen combinations (OH) in space? This should emit radio waves in four particular wavelengths, and two of these were detected for the first time in 1963.

Even as late as the beginning of 1968, only three different atom combinations had been detected in outer space: CH, CN, and OH. Each of these were 2-atom combinations that seemed to have arisen from the chance occasional collisions of individual atoms.

No one expected that the far less probable combination of three atoms would build up to detectable level, but in 1968 the characteristic radio-wave emissions of water and ammonia were detected in interstellar clouds. Water has a 3-atom molecule, two of hydrogen and one of oxygen (H₂O) and ammonia has a 4-atom molecule, one of nitrogen and three of hydrogen (NH₃).

This was utterly astonishing, and 1968 witnessed the birth of what we now call astrochemistry.

In fact, once compounds of more than two atoms were detected, the list grew rapidly longer. In 1969, a 4-atom combination involving the carbon atom was discovered. This was formaldehyde (HCHO). In 1970, the first 5-atom combination was discovered, cyanoacetylene (HCCCN). That same year came the first 6-atom combination, methyl alcohol (CH₃OH). In 1971, the first 7-atom combination was discovered, methylacetylene (CH₃CCH).

So it went. Over two dozen different kinds of molecules have now been detected in interstellar space. The exact mechanism by which these atom combinations are formed is not as yet clear, but they are there.

And even in outer space, the direction of formation would seem to be in the direction of life.^* In fact, both in meteorites and in interstellar clouds it is interesting that the carbon chains are forming and that there is no sign of complex molecules that do not involve carbon. This is evidence in favor of our assumption that life (as we know it) always involves carbon compounds.

All of this evidence—in the laboratory, in meteorites, in interstellar clouds—makes it look as though the Haldane-Oparin suggestions are correct. Life did start spontaneously on the primordial Earth, and all indications would seem to be that it must have started readily, that the reactions in that direction were inevitable.

It follows that life would therefore start, sooner or later, on any habitable planet.

WHEN LIFE STARTED

But how much sooner, or later, is “sooner or later”? When did life start on the Earth?

Our knowledge of ancient life forms upon the Earth comes almost entirely from our study of fossils—remnants of shells, bones, teeth, wood, scales, even fecal matter—that have withstood at least some of the ravages of time and have done so sufficiently to tell us something about the structure, appearance, even behavior of the organisms of which they were once part.

Fossils can be dated in various ways, and the oldest ones that we can deal with easily are from the Cambrian period (so called because the rocks from that period were first studied in Wales, which in Roman times was called Cambria).

The oldest Cambrian fossils are 600 million years old, and it was tempting to assume that that was when life on Earth began, more or less. However, since we know Earth is 4,600,000,000 years old, that would mean it lay for 4 billion years without life. Why so long? And if lifelessness continued for that long, why did life suddenly appear? Why is Earth not still lifeless?

Then, too, at the time the fossil record begins in the Cambrian, life is already plentiful, complex, and varied. To be sure, all the life of which we have a record from that period is marine; there is no freshwater life or land life. Then, too, it is all invertebrate. The earliest chordates (the group to which we belong) did not appear for another 100 million years.

Nevertheless, what does exist seems quite advanced. Thousands of species of trilobites are found in the Cambrian period; these are complex arthropods very much like the horseshoe crabs of today. It is impossible to suppose that they sprang out of nothing and split up into many species. Before the Cambrian time, there must stretch long ages of simpler life. In that case, why is there no record of it?

The most likely answer is that the simpler life was not particularly prone to fossilization. It lacked the kind of parts—shells, bones—that survive easily. And yet despite that, traces of earlier life have been found.

The American botanist Elso Sterreberg Barghoorn (1915–), who in the 1960s was working with very ancient rocks, came across faint traces of carbon that, as he could demonstrate, were the remains of microscopic life.

The dim evidence of such microscopic life has now been traced back as far as 3,200,000,000 years, and it probably extended back a few hundred million years before that.

We might conclude, then, that recognizable life forms existed by the time the Earth was one billion years old.

This sounds reasonable intuitively. We can well imagine that during the first half-billion years of Earth’s history the planet may have been in a pretty unsettled state. The crust must have been active and volcanic; the ocean and atmosphere in the process of formation as the planet cooled off from the heat of its initial condensation and its components separated. The second half-billion years may well have been devoted to a slow chemical evolution—the formation of more and more complicated compounds under the lash of the Sun’s ultraviolet light. Finally, a billion years after the Earth’s formation, very simple little bits of life exist here and there.

The Sun’s stay upon the main sequence will be some 12 billion years, and we might consider this average for Sunlike stars. That means that the Earth (and, on the average, habitable planets generally) will last 12 billion years as the abode of life. If, then, life appears on the Earth after one billion years, it does so after only 8 percent of its lifetime has elapsed.

We can assume that (by the principle of mediocrity) habitable planets in general gain life after some 8 percent of their lifetimes as habitable planets has passed.

Suppose we assume that stars have been forming at a steady rate here in the outskirts of the Galaxy, once the first flurry of star formation in the infancy of the Galaxy had passed.

This is not entirely an assumption. There is evidence that stars have been born in recent times, at least. The giant stars of spectral classes O and B must have been formed a billion years ago or less, or they would not still be on the main sequence. And if stars could form in the last billion years, they must have been forming all along and still be forming now. They must at least be doing so in those galactic regions where clouds of dust and gas (the raw material of stars) are plentiful, and those regions are precisely in the outskirts of galaxies, which, we have already decided, are the only places life can exist.

Moreover, we need not depend entirely on reason to tell us that stars are still being formed today. It is possible we are actually witnessing the process. In the 1940s, the Dutch-American astronomer Bart Jan Bok (1906–) drew attention to certain dust clouds that were opaque, compact, isolated, and more or less spherical in shape. He suggested that these clouds (now called Bok globules) are in the process of condensing into stars and planetary systems. The evidence since then tends to show he is right. Sagan estimates that in our Galaxy, ten stars are born each year on the average.

Assuming, then, a steady rate of star formation, we can say that x percent of the habitable planets have not yet expended x percent of their lifetime. In other words, 50 percent of the habitable planets have not yet expended 50 percent of their lifetime; 10 percent of the habitable planets have not yet expended 10 percent of their lifetime; and so on.

This means that 8 percent of the habitable planets have not yet expended the 8 percent of their lifetime that they should need to form life; that is, they are less than a billion years old.

The converse is that 92 percent of the habitable planets are old enough to have had life develop upon them. That gives us our ninth figure:

MULTICELLULAR LIFE

Though life may have come to exist on Earth early in its history, its advance was very slow for a long time.

For the first 2 billion years during which life existed on Earth, the dominant forms may have been bacteria and blue-green algae. These were small cells, considerably smaller than the cells that make up our bodies and those of the plants and animals familiar to us. Furthermore, the bacterial cells and the blue-green algae did not have distinct nuclei within which the deoxyribonucleic acid (DNA) molecules that controlled the chemistry and reproduction of the cells were confined.

The difference between these two kinds of cells was that the blue-green algae were capable of photosynthesis (the use of the energy of sunlight to convert carbon dioxide and water into tissue components) and the bacteria were not. Bacteria, without photosynthetic ability, were forced to break down already existing organic compounds for energy (or, in some cases, to take advantage of other types of chemical changes for the purpose).

Although the blue-green algae made use of the energy of sunlight to form their tissue components, they thereafter made use of chemical changes similar to those the bacteria used. These chemical changes did not supply much in the way of energy, so that the growth and multiplication of living things—to say nothing of its evolution into various more advanced species—was extremely slow. The reason for this is that the chemical changes that yield considerable energy to living things on our Earth of today all involve the utilization of molecular oxygen, and in the early days of life on Earth there was virtually no oxygen in the atmosphere.

The blue-green algae did produce small quantities of oxygen in the course of their photosynthesis, but the sparse distribution and feeble activity of the tiny cells made those quantities very small indeed.

But even though evolution progresses very slowly, it progresses. About 1,500,000,000 years ago, when Earth had been the abode of life for over 2 billion years, the first cells with nuclei appeared. These were large cells of the type that exist today, with more efficient chemistries, that were capable of conducting photosynthesis at greater rates than before.

This meant that oxygen began entering the atmosphere in perceptible quantities and carbon dioxide began declining. By 700 million years ago, after Earth had been the abode of life for nearly 3 billion years, the atmosphere was some 5 percent oxygen.

By this time, those forms of burgeoning animal life, still made up of single cells, which like bacteria made use of chemical changes rather than sunlight as a source of energy, began to develop means of making use of the free oxygen of the atmosphere. Combining organic compounds with oxygen releases twenty times as much energy for a given mass of such compounds as does the breakdown of organic compounds without the use of oxygen.

With a flood of energy at their disposal, animal life (and plant life, too) was able to move more quickly, live more briskly and efficiently, reproduce more copiously, evolve in more different directions. It could even make use of energy in what would have been a wasteful fashion by earlier standards. It evolved into organisms in which cells clung together and specialized. Multicellular organisms were developed, and rigid tissues had to be formed to support them and serve as anchors for muscles.

Such hard tissue was easily fossilized and thus by 600 million years ago, it seems (from the fossil record) that out of nowhere multicellular life, advanced and complex, was flourishing.

It was not until Earth was 4 billion years old, with a third of its life span gone, that such complex life forms existed.

If this, by the principle of mediocrity, is characteristic of completely Earthlike planets in general, then one-third of them are too young to have anything more than one-celled life. Conversely, two-thirds of them possess complex and varied multicellular life.

That gives us our tenth figure:

LAND LIFE

However complicated and specialized a life form becomes, it doesn’t interest us in connection with the subject matter of this book unless it is intelligent.

It cannot become intelligent unless it develops a large brain (or the equivalent—except that, on Earth at least, we know of no equivalent) and this, it would appear, cannot be done without the development of manipulative organs of some sort and of elaborate sense organs of considerable variety.

It is the flood of impressions entering the brain from the outside Universe, and the questing manipulative organs that respond to these impressions, which stretch the brain’s resources to its capacity and beyond, and lend survival value to any increase in the brain’s size and complexity. If a small brain is already sufficient to handle the coordinating needs of the information an organism collects, a larger brain is of no advantage; a larger brain would merely require the production of useless and energy-wasting highly complex tissue. If, on the other hand, the brain is being used to capacity, a larger brain can do more and is worth much more.

Viewed from this angle, the sea is ideal as an incubator of life, but is very poor as an incubator of intelligence. The most valuable and information-rich sense that we can imagine life possessing (without veering into fantasy) is that of vision. Under water, vision is limited, for water absorbs light to a far greater extent than air does. In air, vision is a long-distance sense; in water, only a short-distance sense. (To be sure, hearing is even more efficient in water than in air and can perform wonders, but the smallest sound waves used by life forms are still far longer than the tiny light waves, and therefore incapable of transmitting as much information.)

When it comes to manipulative organs, as I mentioned earlier in the book, the necessity for streamlining to allow rapid travel through the viscous medium of water eliminates almost any chance for developing a manipulative organ. What manipulation a sea organism can perform usually involves the mouth, the tail, or the full weight of the body, and it is rarely delicate in its nature.

One exception to this is the octopus and its relatives. The octopus has developed a set of sensitive and limber tentacles with which there can be fine manipulation of the environment, yet when it wishes to travel quickly it can trail them behind and be streamlined. Then, too, the octopus has an excellent eye, the closest approach to the vertebrate eye in any nonvertebrate creature.

But though we may admire the intelligence of the octopus, it is certainly far from intelligent enough to build what we would consider to be a civilization.

There are, of course, sea animals far more intelligent than the octopus, but these—sea otters, seals, penguins—are all land creatures who had secondarily adapted to the water again. Even the whales and dolphins have land animals among their ancestry, and it is undoubtedly in the course of the period during which their ancestors inhabited the land that the cetacean brain developed.

For real intelligence of the level in which this book is interested, then, we must consider land organisms—land organisms who can make use of sight as a long-distance sense of incredible detail and richness; who can develop manipulative organs; and who live surrounded by free oxygen so that they can tame fire and develop a technology.

And yet when all life existed in the sea only, the land was an environment extremely hostile to life; as hostile as space is to us. We, at least, in conquering space can make use of our technology and devise artificial protective devices. Sea life, hundred of millions of years ago, had to develop protection as part of their bodies through the slow course of evolution.

Consider the difficulties they had to overcome:

In the sea, organisms need not fear thirst and drought; they are always surrounded by water, the essential chemical background to life. On the land, on the other hand, life is a continual battle to avoid water loss; water must either be conserved, or it must be replaced by drinking.

In the sea, oxygen is easily absorbed from the water in which it is dissolved. On the land, oxygen must first be dissolved in the fluid lining the lungs and then absorbed, and the lungs must not be allowed to dry out in the process.

In the sea, eggs can be laid in the water and allowed to develop and hatch without care (or with minimal care) in a congenial environment. On the land, eggs must be developed that have a shell that will prevent water loss while allowing gases to pass through freely so that oxygen can reach the developing embryo.

In the sea, temperature scarcely varies. On the land, there are extremes of hot and cold.

In the sea, gravity is almost nil. On the land, it is a powerful force, and organisms must develop sturdy legs that can lift them free of the land, or else they are condemned to crawl.

It is no wonder that even after life in the sea grew energetic and complicated it took hundreds of millions of years to conquer the land.

But the conquest took place. The pressures of competition forced organisms of various sorts to spend more and more time upon the land, until such time as they could live on land more or less permanently.

About 370 million years ago, the first plants invaded the land. The land that had been lying sterile and dead for 4¼ billion years began to turn a faint green about its edges.

Animals followed the plants over the next few tens of millions of years. Insects and spiders appeared as the first true land animals about 325 million years ago. Snails and worms appeared on land. The first vertebrates to be entirely land animals, primitive reptiles, appeared 275 million years ago.

A rich land life appeared when the Earth was about 4.3 billion years old and had passed through 36 percent of its lifetime. By the principle of mediocrity, then, we can say that 64 percent of the habitable planets have a rich land life.

That gives us our eleventh figure:

INTELLIGENCE

Even a land species is not necessarily intelligent. To this day, cattle and other grazing animals are not particularly bright.

Nevertheless, one can see a steady progression of intelligence and a steady elaboration of the brain. Mammals, which first appeared about 180 million years ago, were on the whole an advance in intelligence over the reptiles.

The order of primates, the earliest records of which date back 75 million years, moved toward specialization in eyes and brains. About 35 million years ago, the primates split into the less brainy and smaller monkeys and lemurs on one side, and the more brainy and larger apes on the other.

Some 8 million years ago, a particularly brainy species developed that was the first hominid. About 600,000 years ago, Homo sapiens had developed, and about 5,000 years ago, human beings invented writing, so that written history began and civilization was in full bloom, in some parts of the world at any rate.

By the time civilization appeared, the Earth was 4,600,000,000 years old and had completed roughly 40 percent of its lifetime. That means, if we follow the principle of mediocrity, that 40 percent of the habitable planets in existence are not old enough to have developed a civilization and 60 percent are old enough.

That gives us our twelfth figure:

In other words, one star out of 770 in the Galaxy today has shone down on the development of a technological civilization.

We can go a little bit further. Our own civilization, if we count from the invention of writing to the first venture into space, has lasted 5,000 years. If we want to be glowingly optimistic about it, we can suppose that our civilization will continue to last on Earth as long as the Earth can support life—for another 7.4 billion years—and that our level of technology will advance in all that time.^* Suppose we say, then, that the average duration of a civilization is 7.4 billion years (we’ll have more to say about that later on in the book) and that space flight is reached in the first 5,000 years. That means that only 1/1,500,000 of a civilization passes before space flight is developed, and all the rest of it progresses to technological levels above and beyond that. Or, to put it another way, only 1/1,500,000 of the civilizations in our Galaxy are so unadvanced that they are barely at the brink of spaceflight or have not yet reached it. All the rest are beyond us.

That means that of the 390 million civilizations in our Galaxy, only 260 are as primitive as we are—an inconsiderable number. All the rest (meaning just about all of them) are more advanced than we are.

In short, what we find ourselves to have been doing is to have worked out not merely the chances of extraterrestrial intelligence but the chances of superhuman extraterrestrial intelligence.