When life first sprouted on land.

The oceans have teemed with life for 3.5 billion years, but the continents apparently remained sterile for most of Earth's history. The oldest known land fossils date back only a half billion years, a short span compared to the planet's age of 4.6 billion years. The idea of such a delayed colonization of the continents has always troubled some paleontologists; now, they can rest easier.

In the Jan. 28 SCIENCE, Robert J. Horodyski of Tulane University in New Orleans and L. Paul Knauth of Arizona State University in Tempe report the discovery of the earliest known land life --tubular microorganisms that colonized ancient soils. One set of such fossils, found in Arizona, dates to 1.2 billion years ago; a second set, from California, has an age of 800 million years.

The fossils are hollow filaments that stretch as long as 150 micrometers but measure only a micrometer or two in diameter, less than one-tenth the thickness of a human hair. The organisms that made these structures could have been cyanobacteria, other bacteria, or even fungi, says Horodyski.

Knauth suspected that ancient life might have colonized the Arizona and California locations because he had previously found an unusual ratio of carbon isotopes in rocks there. The deposits held less carbon-13 than normal, a feature that develops in modern soils when photosynthetic plants and microbes release carbon dioxide through respiration. The same process must have occurred 1.2 billion years ago, say Horodyski and Knauth.

By pushing back the record of life on land, the new discovery raises important questions about the evolution of animals on the continents. "If we had life on land 1,200 million years ago and these communities were extensive, then there was a source of food on land. There could have been other organisms utilizing that food." If so, these animals. would be more than twice as old as the most ancient land creatures now known.