Re-Creating the Pre-Life Earth

Adenine is an undistinguished-looking chemical with a molecule made of two ordinary carbon-nitrogen rings. But to biochemists, it is one of the keys of life. It takes part in the formation of a long list of vital substances, and it is one of the five "bases" that are built into DNA and RNA, the magic nucleic acids that control the reproduction and heredity of all living organisms. Since the first life probably appeared on earth when chemicals already dissolved in sea water formed a giant molecule that had the power to reproduce itself, it is likely that this ancestral molecule was a nucleic acid. If it was, reasoned Ceylonese Biochemist Cyril Ponnamperuma, the pregnant primitive ocean must have contained such bases as adenine.

To check his theory, Dr. Ponnamperuma had to find out whether adenine could be created by nonliving chemical reactions. He and his associates at the University of California's Lawrence Radiation Laboratory set up a small-scale replica of the prebiotic, pre-life earth. Their model did not look much like the earth; it was only a glass tube dressed up with valves and various bulges. But it contained a mixture of methane, ammonia and water to simulate the earth's atmosphere before the start of life, and it was built so that a beam of high-energy electrons could be shot through the tube. The electrons represented the cosmic rays that scientists believe assailed the primitive atmosphere.

Dr. Ponnamperuma allowed the electron beam to squirt through his model earth for 45 minutes. Then, with the aid of radioactive carbon 14, he made an extremely delicate analysis of the tube's contents. One of the chemicals that had been formed by the electron bombardment was indeed adenine.

When Dr. Ponnamperuma added hydrogen to the mix in the tube, less adenine was formed, and he concluded that life could not have developed on earth until most of the free hydrogen in the earth's primitive atmosphere had escaped into space. Only then could adenine and similar chemicals have been made out of methane, ammonia and water. Gradually, those chemicals accumulated in the ocean where the first life appeared, and at last they formed nucleic acid, life's key substance.

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