Two Harvard biologists have discovered fossils of an algae more than three billion years old -- the oldest fossil organism ever found:

The algae seems to have carried on photosynthesis, the process by which plants change water and carbon dioxide into oxygen and carbohydrates, at least 400 million years before anyone thought the process existed.

"It's surprising first of all that anything was alive that far back," J. William Schopf, Junior Fellow, and co-discover of the fossil along with Elso S. Barghoorn, professor of Botany, said yesterday. "But more important was finding anything so complex as an apparently photosynthetic organism," Schopf added.

In the summer of 1965, Schopf found fossilized remnants of bacteria in rock samples that Barghoorn had brought back from Africa. The fossil organisms -- from a Fig Tree formation in eastern Transvaal, South Africa -- were then the oldest form of life known.

Schopf made the algae discovery in another sample of the Fig Tree rocks about a half year later. The algae was about the same age as the bacteria, but was much more complex, and most startling, apparently lived by photosynthesis.

The discovery of photosynthetic organisms so early in the earth's history has important implications, Schopf and Barghoorn state in an article in the current issue of Science. It means either that life emerged much earlier than suspected -- maybe even as far back as 4 billion years ago -- or that once it did arise complex forms evolved much faster than believed.

The earth is about 4.5 billion years old and, according to Schopf, until around eight years ago, scientists thought life went back no further than 1.5 billion years at most.

The emergence of photosynthetic organisms was a key link in the evolution of life, said Schopf. The oxygen released in the process enabled higher, oxygen-breathing plants and animals to appear.