Some 3.5 billion years ago, in shallow waters over what is now a desert in northwestern Australia, lived a colony of microscopic, bacteria-like "pond scum" -- a discovery that paleontologists term "very exciting."

These organisms represent the oldest life form ever found, and the discovery pushes the history of life back 400 million years, possibly to the limits of science to trace it, according to Dr. Malcolm Walter of the Australian Bureau of Mineral Resources. "It's dangerous to make predictions, but it's unlikely any well-preserved sedimentary rock older than these will be found anywhere on earth," he said.

The microfossils, which were found in an area known, for its remoteness, as the North Pole, were not those of the bacteria or algae themselves. "There are no preserved cells in the structure," Walter said, just fossilized marks left by the creatures.

Scientists are confident, However, that they know what the organisms were and how they lived because relatives still exist. "These organisms were probably very much the same guys that make pond scum today," said UCLA Prof. J. William Schopf. "They would have been surface creatures with an exterior coat of sticky mucilage that they themselves secreted."

The basketball-sized rock in which the traces were found is known as a stromatolite. It was formed layer by layer as the organisms' sticky coats acquired mineral deposits.

The discovery means that there was life on earth within about a billion years of the planet's formation. Many scientists believe that even more primitive forms existed, but their fossils, left in old rocks that have been subjected to repeated heat, pressure and movement, are unlikely to have survived.

The Australian fossils were analyzed by a 15-member team of scientists from the United States, Australia, Canada and Germany under the direction of Prof. Schopf.