Fossils show early diversity of life.

The record of ancient life preserved in Earth's oldest rocks shrinks to a handful of tattered pages as paleontologists struggle back through time to the Archean era the first 2 billion years of our planet's history, Now, recently identified fossil microorganisms add a potentially important chapter to that incomplete record. These fossils suggest that a diverse range of cyanobacteria - creatures that use light as an energy source and produce oxygen - may have thrived about 1 billion years after Earth formed.

Paleobiologist J. William Schopf of the University of California, Los Angeles, has identified 11 distinct species of cyanobacteria-like creatures preserved in 3.465-billion-year-old rock deposits in western Australia. The microscopic creatures, embedded in some sort of sticky substance, probably lived in shallow water, says Schopf. They grew into filaments of connected cells, resembling the cyanobacteria discovered previously in 2.1-billion-year-old Canadian rocks, Schopf reports in the April 30 SCIENCE.

The microorganisms vary significantly in the shapes of their individual cells and in their overall lengths and thicknesses. This diversity demonstrates that primitive life had already seen great evolutionary change by an early point in Earth's history, says Schopf.

Considering the odds against the preservation of such ancient fossils, Schopf comments, "1 feel just enormously pleased that we've finally got something that's nearly 3.5 billion years old, that's diverse and interesting and well-preserved enough to interpret:' Over time, heat and pressure can all but obliterate traces of ancient life from the geological record.

But are Schopf's fossils truly the ancestors of oxygen-making cyanobacteria? Currently, the identity of these somewhat poorly preserved microorganisms remains subject to interpretation. "I personally think it will turn out that these are cyanobacteria, but it's very difficult to nail that at the moment," says Schopf.

The true identity of the creatures in Schopf's fossil menagerie may bear on a controversial question: When did Earth's atmosphere begin to build up significant concentrations of oxygen? Proof of a thriving population of oxygen-generating cyanobacteria 3.5 billion years ago, Schopf maintains, "would show that the current ecological system, with oxygen production and utilization ... may well have been established at a remote time in Earth's history,"

Scientists who accept the conventional wisdom on the "rise of oxygen" might disagree with Schopf. Until about 2.2 billion years ago, according to the standard account, dissolved iron in the oceans combined with any free oxygen in the environment. Thus, aerobic (oxygen-using) creatures - phytoplankton, for instance - could not have existed in the Archean era described in textbooks.

In contrast to this standard view, accumulating evidence indicates that oxygen-producing microbes evolved early in the Archean era and began to enrich the oceans with oxygen, argues Kenneth M. Towe, a paleobiologist at the Smithsonian Institution in Washington, D.C. Towe's research suggests that the amount of iron present in the Archean oceans could not have soaked up all the available oxygen (SN: 12/1/90, p. 347). Without aerobic organisms around, atmospheric oxygen would surely have built up earlier than many believe, he says.

The mystery of when Earth's oxygen levels rose is so complex that proving cyanobacteria existed 3.5 billion years ago will not in itself settle the issue, says paleontologist Andrew H. Knoll of Harvard University "Dealing with almost anything in the first half of Earth's history is not simple, because the rock record stinks," he explains. However, "some glorious insights have been generated, and I think that just the ability to say life was present 3.5 billion years ago is really great."