One for the Ages: Dating of Cambrian Fossils Poses a Precision Problem for Paleontologists

Article excerpt

The search for Earth's earliest life has taken paleontologists to ancient rocks in Africa and Australia - and to the realization that analyzing tiny fossils from eons ago is fraught with imprecision.

Paleobiologists in quest of the earliest bacteria and algae are looking at fossils they date to 3.46 billion years ago.

The search for what caused the Cambrian explosion 345 million years ago - the period when the basic body types of all organisms burst on the scene - involves fossils equally elusive.

At the recent meeting of the North American Paleontological Convention, paleobiologist William J. Schopf reported on cell fossils from Australia, the oldest yet found. The ancient cells match modern cyanobacteria, or blue-green algae.

"They are the guys who invented oxygen-based photosynthesis," Mr. Schopf, a University of California at Los Angeles scientist, said in a keynote lecture. Cyanobacteria "can breathe just as plants breath," he said.

He put the rock dating at such "terrific" precision that it may be off by only 500 years. That places 11 species of cyanobacteria early in the Earth's 4.5 billion years of history, he said.

"We now have direct evidence that within the first billion years of geologic time, during the infancy of the Earth, life originated, evolved and rose to become a flourishing success," an abstract of his findings said.

His dating is not accepted by everyone, however. Biochemists, who calculate dates of origins by mathematically running molecular clocks of protein evolution back in time, date the cells at only 1.9 billion years ago.

While Mr. Schopf said that future work must mesh the contradictory dates, he said fossil finds since the mid-1960s are giving paleontologists the edge.

"I believe there is only one court of last resort, and that is the fossil record," he said. "It's tough for a biochemist to be knowledgeable about the fossil record."

Scientists may have a better grasp of the more recent Cambrian period, a geological time frame of 33 million years that gave rise to the basic structures of today's organisms.

Fossil discoveries since 1995 have suggested that this complex life arose in a more rapid spurt than once thought. …