The Dawn of Life

Some of the world's oldest organisms are identified

Though scientists have established that the earth was born some 4.6 billion years ago, formed from debris orbiting the sun, they are less certain about when --and under what conditions--life began on the planet. Only last month, a Harvard University paleobiologist pushed back the dawn of life by announcing the finding of what appeared to be fossils of single-celled organisms dating back 3.5 billion years. Now biologists working under grants from NASA and the National Science Foundation have identified living creatures that may be little changed from organisms that lived during the first billion years of the earth's existence.

The new candidates for the oldest-form-of-life title are organisms that scientists have dubbed "archaebacteria." They are found in airless recesses like Yellowstone National Park's hot springs, thrive in temperatures ranging from 65DEG to 70DEG C. (150DEG to 170DEG F.), take in carbon dioxide and hydrogen, give off methane gas, and have been known to scientists for years. But it took the efforts of a team led by Geneticist Carl Woese of the University of Illinois in Urbana to demonstrate that the archaebacteria had an extraordinary characteristic. Using enzymes, or chemical catalysts, they broke down and then analyzed the RNA in the archaebacteria's ribosomes, the structures that "read" the message of the master molecule DNA and produce the protein necessary for life. They found that the RNA molecular sequences were distinctly different from those of other bacteria and of plants and animals.

This discovery has far-reaching implications. Scientists have long assumed that all terrestrial life evolved along two lines, one of which gave rise to the "higher" forms of animals and plants, the other to the "lower" forms of bacteria. The identification of the archaebacteria's unique genetic structure suggests that there may be a third line of evolution. It also provides an important clue to the earth's early environment. Scientists have long believed that for about the first billion years after the formation of the earth, the atmosphere consisted largely of hydrogen, carbon dioxide and other gases, but virtually no free oxygen. The life-style and genetic structure of Woese's archaebacteria tend to support the theory; because the strange bugs now live only in remote, airless niches of the environment and die when exposed to free oxygen, they may be little different today from ancestors that evolved in the oxygenless primeval atmosphere.

Then how and when did free oxygen begin appearing in the atmosphere? A clue to the answer has been found in the incredibly old sedimentary rocks of South Africa's eastern Transvaal by Harvard's Elso Barghoorn and Andrew Knoll, now with the Oberlin College department of geology. To the naked eye, the 3.5 billion-year-old rocks Barghoorn and Knoll collected during a visit last year revealed no traces of early life. But the scientists soon uncovered the stones' secrets. Returning to Harvard with samples of the rock, the pair used a diamond cutter's saw to slice several chunks into wafers so thin that light could pass through them. Examining these sections under a microscope, they discovered thousands of microfossils of individual and paired cells, some long and thin, others flat, wrinkled or folded, but all similar to those found in rocks dating from a much later period in the earth's history. The rocks also contained the most unequivocal evidence yet of early biological activity; fully 25% of the microfossils were preserved in the very process of division, or reproduction.

Barghoorn and Knoll believe that their primitive fossils--the oldest direct evidence of terrestrial life--are the ancestors of modern blue-green algae or photosynthetic bacteria, both of which convert carbon dioxide into food and oxygen. If they are correct, these organisms 3.5 billion years ago were already pumping into the atmosphere the oxygen upon which most of today's terrestrial life now depends.

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