Extraterrestrial Civilizations (24 page)

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 10
¹³⁰
; 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 10
⁹⁵
, 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.

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.

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.