Extraterrestrial Civilizations - Isaac Asimov [81]
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