Geologists have produced evidence of abundant marine life on the earth from a period when others say a thick layer of ice gripped the entire planet. The find lends considerable support to one side of a scientific controversy that has been widely debated for decades.

The hullabaloo is over a glacial period dating to about 750 million to 600 million years ago. Experts agree about the presence of ice on the planet then--even at the equator--but how much and to what extent is still up in the air. Theories range from a "snowball Earth" hard packed in kilometer-thick ice to a "slush ball Earth" characterized by thin ice and areas of open water. The range of conditions would have impacted the microorganisms present. Thick ice would have made life difficult for plants and animals, one line of reasoning goes, choking oxygen out of the sea and blocking sunlight needed for photosynthesis. Mass extinction would ensue and after the thaw, give rise to an explosion of multicellular life.

The debate has centered around geologic evidence in rocks from places including Libya, Canada and Australia. But geologist Alison Olcott, a Ph.D. student at the University of Southern California, heard rumors that previously unexamined black shale cores drilled by a mining company in Brazil might be worth looking at. The shale was from the Poo Verde Formation in Paracat, Brazil, an area that revealed in its rocky layers the presence of glaciers from the time period in question. Supposedly the rock contained organic matter.

Using a standard laboratory technique that identifies organic chemicals in rocks, Olcott and her team discovered the fatty remains of prehistoric organisms known to photosynthesize. "This means the organisms had to be in the sunlight," says Olcott, who reported her finding in a paper published online yesterday by Science.

Although supporters of the snowball Earth theory do suggest that even during periods of deep freeze, holes of thin ice or open water around the equator may have provided a refuge for marine life, Olcott thinks it is statistically unlikely that her team found a core sample from one of these holes. The sample came from a formation with similar geologic characteristics over a large area, not a small anomalous region.

Olcott admits that the discovery does not settle outright the debate. But "if the Poo Verde strata are representative of more widespread conditions," she writes, "then the hypothesis of thick, global sea icewill need to be reexamined."