Dating the Dawn of Life

When did life begin on earth? Harvard Paleobotanist Elso S. Barghoorn and his onetime student, J. William Schopf, discovered a possible answer to that intriguing question several years ago, when they found microscopic fossils in ancient South African rocks. These tiny traces of life indicated that single-celled creatures existed as long as 3.1 billion years ago. Now a team headed by Schopf himself has found evidence that could push the dawn of life back at least another 200 million years.

The new evidence was uncovered in South Africa's Transvaal in an unusual formation of sedimentary rock. Once probably part of a shallow ocean bottom, the stratified rock has now been thrust to the surface and scientists can easily examine it. When Schopfs team compared the relative abundance of the two principal isotopes of carbon in each of the formation's many layers, they made an intriguing discovery. In the newer layers, those formed more recently than 3.3 billion years ago, carbon 12 and carbon 13 appeared in approximately the same ratio as they do in more modern deposits known to contain the fossilized remnants of plant life. But in the older layers, the ratios were radically different; they were similar to those in rocks that have not been exposed to living organisms.

Chemical Choice. That sudden change in carbon ratios was highly significant to Schopf and his collaborators, Dorothy Z. Oehler of U.C.L.A. and Keith A. Kvenvolden of NASA'S Ames Research Center. In a recent report in the journal Science, they proceed to explain why:

All organisms capable of photosynthesis--the chemical process by which green plants use the energy of sunlight to convert carbon dioxide and water into food and oxygen--show a marked chemical preference for carbon 12, which is the lighter of the two isotopes. As a result, the carbon in the organic compounds that make up the plants' structure consists largely of carbon 12. What is more, the greater preponderance of that isotope becomes preserved in the earth's geological records when, for example, tiny green sea plants (plankton) die, sink to the ocean bottom, gradually decompose and become part of the sea-floor sediment. Still rich in carbon 12, this sediment is eventually compressed into rock and can be geologically dated with considerable accuracy. Thus, the researchers suggest, the 3.3-billion-year age of the South African rock layers in which the striking change in carbon ratios is recorded may well indicate the time when primitive plants, probably plankton-like blue-green algae, first appeared on the face of the earth.

The implications of their findings could be even more far-reaching. Organisms capable of photosynthesis probably evolved from still simpler forms of life. Thus if tiny plants that used the complex process of photosynthesis were alive at least 3.3 billion years ago, their more rudimentary ancestors, the earth's earliest living things, must be considerably older.

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