Researchers taking a second look at ancient sediments have concluded that they do not represent the earliest traces of the rise of oxygen-producing organisms on Earth as previously thought. The findings come as a relief to some scientists, whose data had been in conflict with an earlier study with the same sediments that suggested oxygen first emerged in the planet's atmosphere some 300 million years later.
According to the geological record, oxygen first rose in Earth's atmosphere about 2.4 billion years ago. The problem with using fossils to track down the organisms that gave rise to the oxygen is that fossils can take you back only about 2 billion years out of the planet's 4.5 billion. To figure out what was stirring before that, scientists must rely on traces of chemicals produced by biological processes and, by inference, in certain environmental conditions. That's what happened in 1999, when researchers examined samples of 2.7-billion-year-old shale from Western Australia (ScienceNOW , 12 August 1999). They found certain types of hydrocarbons that indicated oxygen-producing bacteria had existed at the time. But the discovery also created a conundrum, because it placed the rise of photosynthesis some 300 million years before chemical studies of ancient rocks indicated oxygen began to spread in Earth's atmosphere. This discrepancy continues to bedevil researchers, who have struggled to mesh their data with evolutionary models.
An Australian team reports a possible solution in tomorrow's issue of Nature. Analyzing microscopic, solidified oil droplets contained in the shale, the researchers inferred the composition of the organic materials present as the shale formed. The analysis clearly shows the hydrocarbons identified in the 1999 study could not have been derived from bacteria contained in the sediments, says geochemist and lead author Birger Rasmussen of the Curtin University of Technology in Perth, Australia. Instead, they probably represent contaminants introduced from younger sedimentary rocks that somehow got mixed in with the shale layer or by the drills the researchers used to extract the samples, he says.
The conclusions are "pretty strong and a lot of eyes will be focused on how this shakes out," says geobiologist Woodward Fischer of the California Institute of Technology in Pasadena. He says the previous findings had been regarded as a standard time measurement for both the rise of oxygen and for photosynthetic organisms, and, therefore, any conflicting data produced by other researchers had been regarded as suspect. But he's convinced that the new results ultimately "will free up people's minds again" to find the other sources of early oxygen.