Fossilized "organic mats" of the landlubbing microbes were found in soil that has been dated at between 2.6 billion and 2.7 billion years old, more than twice as old as any previously found evidence of life on land.

The fossils represent organisms called cyanobacteria, which use sunlight to generate oxygen from water and atmospheric carbon dioxide.

The finding, if it holds up, indicates that conditions on Earth were ripe for life to crawl from water to land earlier than expected, scientists said. It means an ozone shield would have had to be in place to protect the organisms from deadly solar and cosmic radiation. The atmosphere would have had to be rich in oxygen for an ozone shield to develop.

"The suggestion that an ozone shield existed as early as 2.6 billion years ago boosts our chances in the search for life on planets orbiting other stars," said NASA's Michael Meyer. "Ozone would be easily detectable by the Terrestrial Planet Finder."

The Terrestrial Planet Finder is one of a handful of space-based telescopes planned for the next 10-15 years to search for Earth-like planets around other stars. Scientists say that this and other missions might be able to use ozone as one indicator that a distant planet could harbor life.

The fossils, in the form of seams of organic matter imbedded in soil, were found by a NASA-led team of astrobiologists in the Eastern Transvaal district of South Africa. Researchers said the seams, or "biomats," developed on the soil surface and were trapped while the soil formed. The researchers found chemical elements essential for life -- carbon, hydrogen, nitrogen and phosphorous -- in ratios they say indicate the material was of biological origin.

The discovery parallels the learning process that scientists went through when studying the evolution of marine organisms, says Hiroshi Ohmoto, co-author of a paper on the research, which appears in the November 30 issue of the journal Nature.

"Once scientists thought no living organisms existed in the Earth's oceans before 500 million years ago," said Ohmoto, a Penn State researcher. "Then they studied the carbonaceous matter in ancient sedimentary rocks deposited in the oceans and found that organisms lived in the oceans at least 3.8 billion years ago."

The time frame -- 3.8 billion years ago -- coincides with an era when Earth is thought to have been pummeled frequently by both large and small comets and asteroids. Scientists are currently debating whether life began before, during or after this so-called Late Heavy Bombardment period, which is generally thought to have run from 4.1 to 3.8 billion years ago.

The oldest undisputed remains of terrestrial organisms had been 1.2 billion-year-old microfossils discovered in 1994 in Arizona by Paul Knauth of Arizona State University. However, some scientists think the Earth's land surface was sterile until about 500 million years ago. The new NASA evidence, if it holds up, would refute that notion entirely.