Molecular Biology: Nucleic Acid. The Origin of Life

The advances made in molecular biology in the mid-twentieth century have brought the mechanist position to an unprecedented pitch of strength. All of genetics can be interpreted chemically, according to the laws that hold for animate and inanimate alike. Even the world of the mind shows signs of giving way before the torrent. It would seem that the process of learning and remembering is not the establishment and retention of nerve pathways, but the synthesis and maintenance of specific RNA molecules. (Indeed, flatworms, a very simple form of life, have been shown capable of learning tasks by eating other flatworms that had already learned the tasks. Presumably, the eater incorporated intact RNA molecules of the eaten into its own body.)

That left the one facet of biology that had represented a clear victory for the nineteenth-century vitalist position —the matter of the disproof of spontaneous generation. With the twentieth century, that disproof had grown less attractive in the absolute sense. If, indeed, life form could never develop from inanimate manner, then how did life begin? The most natural assumption was to suppose that life was created by some supernatural agency, but if one refused to accept that, what then?

In 1908, the Swedish chemist, Svante August Arrhenius (1859-1927), speculated on the origin of life without invoking the supernatural. He suggested that life had begun on earth when spores reached our planet from outer space. The vision arose of particles of life drifting across the vast reaches of emptiness, driven by light pressure from the stars, landing here and there, fertilizing this planet or that. Arrhenius' notion, however, merely pushed back the problem; it didn't solve it. If life did not originate on our own planet, how did it originate wherever it did originate?

It was necessary to consider once again whether life might not possibly originate from nonliving matter. Pasteur had kept his flask sterile for a limited time, but suppose it had remained standing for a billion years? Or suppose not just a flask of solution had remained standing for a billion years, but a whole ocean of solution? And suppose that the ocean might be doing so under conditions far different from those which prevail today?

There is no reason to think that the basic chemicals of life have changed essentially, over the eons. It is quite likely, in fact, that they have not. Thus, small quantities of amino acids persist in some fossils that are tens of millions of years old and those that are isolated are identical to amino acids that occur in living tissue today. Nevertheless, the chemistry of the world generally may have changed.

Growing knowledge of the chemistry of the universe has led men such as the American chemist, Harold Clayton Urey (1893- ), to postulate a primordial earth, in which the atmosphere was a "reducing" one, rich in hydrogen and in hydrogen-containing gases such as methane and ammonia, and with free oxygen absent.

Under such conditions there would be no ozone layer in the upper atmosphere (ozone being a form of oxygen). Such an ozone layer now exists and absorbs most of the sun's ultraviolet radiation. In a reducing atmosphere, this energetic radiation would penetrate to sea level and bring about reactions in the ocean which, at present, do not take place. Complex molecules would slowly form and, with no life already present in the oceans, these molecules would not be consumed but would accumulate. Eventually, nucleic acids complex enough to serve as replicating molecules would be formed and this would be the essential of life.

Through mutation and the effects of natural selection, more and more efficient forms of nucleic acid would be produced. These would eventually develop into cells, of which some would begin to produce chlorophyll. Photosynthesis (with the aid of other processes not involving life, perhaps) would convert the primordial atmosphere into the one with which we are familiar, one rich in free oxygen. In an oxygen atmosphere and in a world already teeming with life, spontaneous generation of the type just described would then no longer be possible.

To a very great extent this is speculation (although carefully reasoned speculation), but, in 1953, one of Urey's pupils, Stanley Lloyd Miller (1930- ), performed what has become a famous experiment. He began with carefully purified and sterilized water and added an "atmosphere" of hydrogen, ammonia, and methane. He circulated this through a sealed apparatus past an electric discharge which represented an energy input designed to mimic the effect of solar ultraviolet. He kept this up for a week, then separated the components of his water solution by paper chromatography. He found simple organic compounds among those components and even a few of the smaller amino acids.

In 1962, a similar experiment was repeated at the University of California, where ethane (a two-carbon compound very similar to the one-carbon methane) was added to the atmosphere. A larger variety of organic compounds was obtained. And in 1963, adenosine triphosphate, one of the key high-energy phosphates was synthesized in similar fashion.

If this can be done in a small apparatus in a matter of a week, what might not have been done in a billion years with a whole ocean and atmosphere to draw upon?

We may yet find out. The course of evolution, pushed back to the dawn of earth's history may seem difficult to work out, but if we reach the moon we may be able more clearly to make out the course of chemical changes prior to the advent of life itself. If we reach Mars, we may even (just possibly) be able to study simple life forms that have developed under conditions quite different from those on earth, and this, too, may be applicable to some of our earthly problems.

Even on our own planet, we are learning more each year about life forms under the alien conditions of the oceanic abysses, for in i960, men penetrated to the very bottom of the deepest of these. It is even possible that in the ocean we may establish communications with nonhuman intelligence in the form of dolphins.

The human mind itself may yield its secrets to the probings of the molecular biologists. Through increasing knowledge of cybernetics and electronics we may be able to develop forms of inanimate intelligence.

But why guess when we need only wait? It is perhaps the most satisfying aspect of scientific work that no matter how great the advances or how startling and smashing the gains of knowledge over the unknown, what remains for the future is always still greater, still more exciting, still more wonderful.

What may not yet be revealed during the very lifetime of those now living?