The first form of life on earth is believed to be a microscopic organism that resembles bacteria, thrives without oxygen and can today be found in such diverse places as sewage treatment plants and the mud of San Francisco Bay.

The organism is technically called methanobacteria thermoautotrophica which is a misnomer because scientists say it is not the bacteria they once thought it was. The organism, which exhales methane gas and can be killed by event a trace of oxygen, is now felt to represent a third form of life, like plants and animals and bacteria and algae represent the two basic life terms.

"People have been locked into a prejudice they didn't recognize," said the University of Illinois' Dr. Carl R. Worse (rhymes with rose) in explaining box it took science so long to identify a third life form. "For all we know, there might be a fourth or even a fifth form of life out there we haven't found."

Dr. Woese is the leader of a team of University of Illinois microbiologists including Dr. Ralph S. Wolfe and Dr. George Fox that made the finding public yesterday. Their research was financed by grants from the National Science Foundation and the National Aeronautics and Space Administration.

"These organisms are a distinct new class, no more related to bacteria than to higher life forms," Woese said. "They are a third from of life on this planet."

Woese said the microorganisms probably evolved in the first billion years of the earth's existence when the atmosphere contained only carbon dioxide and hydrogen. The oxygen that supports most of the life forms on the earth did not begin to accumulate in the atmosphere until the earth was between 1.5 billion and 2 billion years old.

"Our evidence that these are the earth's oldest organisms is only indirect, we have no fossil evidence for that claim," Woose said. "But we think the evidence we have is very good evidence."

The organisms look like bacteria under a microscope and are found in the kinds of places where bacteria flourish. They are found in cow's stomachs, the deepest ocean trenches, sewage treatment plants and the mud of polluted waters.

The organisms thrive on hydrogen and carbon dioxide gas and cannot live with oxygen. They produce methane gas as they metabolize hydrogen and carbon dioxide. They live best in places like the superheated springs of Yellowstone Park which have been stripped clean of all oxygen and whose average temperature is almost 200 degrees Fahrenheit.

"Just like the primitive earth was thought to be," Woese said. "No oxygen and very warm."

What Woese and his colleagues at the University of Illinois did was an exhaustive chemical analysis of the microorganisms, using new diagnostic tools and approaches. They found that the organisms had a different cell wall chemistry than bacteria, contained a unique enzyme system and had their own variation of the ribosomes that make up its genetic code.

"They're not bacteria," Woese emphasized. "They're very, very different.

Woese said he is convinced that the organisms are as old as the oldest organisms found in fossilized rocks and may be even older. The oldest fossil organisms are a primitive form of algae tha have been found in rocks in Swaziland in South Africa that are 3.4 billion years old.

"These organisms love an atmosphere of hydrogen and carbon dioxide," Woese said. "That's just what the earth's atmosphere contained before it contained any oxygen."

Woese said that the finding of a third "line of evolutionary descent" creates new hope that scientists ultimately will piece together how life began.

"This discovery is very important from a biologist's view of evolution," Woese said. "This allows a lot more perspective and choice when there is disagreement on a question that can't be resolved using two lines of evolutionary descent."