The Secret of a Rock.(search for the oldest records of life on earth)

The hunt for the oldest records of life is not a simple task," wrote the eminent paleontologist J. William Schopf in his 1999 book "Cradle of Life: The Discovery of the Earth's Earliest Fossils." "Like trying to solve a really complicated mystery when only a few clues have been revealed, it's easy to make mistakes. These can be worse than embarrassing, major blunders that set back the search for knowledge." To Schopf's eyes, the most serious such blunder of recent times is the claim that a meteorite found in Antarctica, ALH 84001, held the fossils of Martian microbes. From the moment it was unveiled at a NASA press conference in 1996, Schopf has been a trenchant critic of the purported evidence of Martian life. Now a team of scientists is pointing to a similar blunder considerably closer to home. According to a paper in last week's issue of the journal Nature, microscopic fossils found in Australia aren't fossils at all. And to give the whole story a personal irony, the scientist who devoted much of his life to studying the Australian fossils, and who proclaimed them to be the earliest evidence of life on earth, was none other than Schopf himself.

Schopf was a professor at UCLA in the 1980s when he found what he took to be fossilized microbes in a chert--a rock made of grains of quartz-- from a site called Chinaman's Creek in Western Australia. These microscopic fossils lack the drama of dinosaurs, or the charm of trilobites. But in the 4 billion or so years before these creatures arose (the so-called Precambrian era), microbes were the only sort of life earth had to offer. Since the Apex chert was 3.5 billion years old, it appeared that Schopf's fossils came from far earlier in the Precambrian than any other life form ever found.

Not only were the fossils very old, but they were also surprisingly complex. Most scientists, Schopf included, had expected fossils from that early on, if there were any at all, to be very simple, primitive things. But some of Schopf's fossils seemed to be filaments made up of rows of separate cells, "like garden hoses with partitions"; Schopf argued that they were photosynthetic blue-green algae. If so, the Apex fossils were not just evidence that life had evolved early--they were evidence it had evolved quickly, as well. All subsequent stories about the origin and evolution of life have had to take that speed into account.

Now Martin Brasier of the University of Oxford is reinterpreting Schopf's evidence. He starts not with the fossils, but with a careful study of the rocks around them. "People go in and grab the fossils and hope the context will come out later," he explains, "and that's precisely where they go wrong." As a result of these studies, where Schopf sees the chert as a sedimentary rock laid down near a seashore, Brasier and his colleagues have come to see it as something created in a hydrothermal vent. They point to evidence from minerals that no oxygen-producing photosynthesis could have occurred there, and from the makeup of the rock they infer that the temperature at which the chert was formed was above 200 degrees Celsius--too hot for life.

In early versions of their critique, Brasier and his colleagues doubted that there was any organic matter in the fossils at all. Since then Schopf has used a new technique to show that the structures identified as fossils are rich in organic molecules. But "organic," a chemical term that applies more or less to all molecules with carbon in them, isn't the ...