When Life Began on Earth

Abstract

More than 23 centuries ago, Aristotle taught that could arise spontaneously from nonliving matter, transformed by a vital force in the elements of earth, air, fire and water. By Aristotle's time this idea, called abiogenesis, had already been around for several hundred years, and it held for 2,000 more. It was accepted dogma that toads and snakes sprang from moist soil and worms from decaying meat. It wasn't until the middle of the last century that the theory crumbled and was supplanted by the realization that comes only from pre-existing life. Yet in a way the proponents of abiogenesis may have been right. Though the condition that only can generate has held for billions of years, there must have been a time early in the earth's history when first began. Many scientists now believe that the ancient precursors of all terrestrial arose spontaneously from inorganic matter on the primeval planet. Paleobotanists and chemists are now finding out how and when this could have occurred. Several recent findings are particularly intriguing. Answers to the question of when began depend on the answer to another question: What is life? The distinction between living and nonliving matter, say some scientists, is arbitrary. In the process of evolution, atoms combined into molecules, molecules formed cells and cells eventually joined together into more and more complex organisms. What point on the continuum from atom to man can we say is the dividing point between living and nonliving matter? There are even a few scientists who talk about thinking neutrons and protons. Most scientists consider life to be a macromolecule which can replicate, such as a nucleic acid. In 1936 the Russian biochemist A. I. Oparin hypothesized that the precursors of life-organic molecules-could have formed spontaneously under the conditions believed to prevail on earth billions of years ago. Energy from sunlight, volcanic activity and lightning could have caused the methane, ammonia, water vapor and hydrogen cyanide then in the atmosphere to combine into amino acids, purines and monosaccharides which in turn would form progressively more complex molecules. In a classic experiment in 1953, Stanley Miller, now of the University of California at San Diego, circulated methane, ammonia and hydrogen past electrical discharges. Within a week, a variety of organic compounds had been synthesized. Since then, a number of researchers have conducted similar experiments. One of them. Cyril Ponnamperuma, now of the University of Maryland, believes that must have started very early in the earth's lifespan. People used to think that the primeval elements had to sit around in the ocean for millions of years before something happened. We now know that once the right molecules accumulated at the right time and in the right arrangement, could begin almost instantaneously. Evolution is what takes time. Until recently, however, it was not known just how valid these experiments were. The confirmation, says Ponnamperuma, has come only recently, with strong evidence that the Murchison meteorite contains not only amino acids of extraterrestrial origin but also hydrocarbons similar to those produced in the laboratory experiments (SN: 12/5/ 70, p. 429), and with the discovery of organic compounds in interstellar space (SN: 10/10/70, p. 299). These findings, says Ponnamperuma, prove, to my satisfaction at least, that chemical evolution can take place elsewhere and that the experiments in the lab are true ones. In the latest issue of the PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, Yecheskel Wolman of the Hebrew University in Jerusalem and William J. Haverland and Stanley Miller of the University of California at San Diego, report that all the nonprotein amino acids found in the Murchison meteorite can be produced by the action of electric discharge on a mixture of methane, nitrogen, ammonia and water. There was also a striking similarity in the relative abundances of the different amino acids in the meteorite and in the lab experiment. There are more than just laboratory experiments and theories, however. Geological evidence also points to an early origin of life. A substantial number of rocks about 2.5 billion years old have fossil microorganisms. The oldest fossils have been found in the Swaziland sequence, a group of sedimentary rocks in the southeastern Transvaal ree ~ ~~~~ os %tl S>