First author

Roughly 2.4 billion years ago, enough molecular oxygen began being produced to support the evolution of oxygen-dependent 'aerobic' organisms. This period has been dubbed the Great Oxidation Event. But how the oxygen was produced remains a mystery. Traces of hydrocarbons in 2.7-billion-year-old shales in Australia are widely accepted as evidence of microorganisms including oxygen-producing cyanobacteria. However, that evidence leaves a 300-million-year gap. On page 1101, Birger Rasmussen, a geologist at Curtin University of Technology in Bentley, Australia, and his colleagues say that their measurements refute the evidence for oxygen-producing microbes existing 2.7 billion years ago. Rasmussen tells Nature that there may not have been a gap at all.

Did you set out to debunk this evidence?

No. We collected black shale samples for fun in 2000 during a separate project. The shales turned out be a goldmine of information: we found a 1-millimetre-thick layer that provided unambiguous evidence for a major asteroid impact. Then, a few years ago, I met co-author Jochen Brocks from the Australian National University in Canberra and mentioned that we had also found organic residues in the shales. When he told me about the controversy over the biomarkers, we came up with a test to authenticate them.

What did you find?

Brocks found that the carbon isotope signatures differed between the fossilized hydrocarbons used as biomarkers and the shale rock. The subsequent discovery of solidified oil droplets, which we knew formed in the shale, provided an additional comparison with rock-originated compounds. We found that the isotopic composition of this oil was the same as that of the rock, but distinctly different from that of the hydrocarbons — making it impossible to explain that the biomarkers came from those rocks.

So was there a time gap?

We can't be sure. Our data eliminate the only evidence for organisms existing 300 million years before the Great Oxidation Event. So the best hypothesis for the emergence of cyanobacteria is the increased oxygen production seen roughly 2.4 billion years ago.

How might the biomarkers that caused the confusion have entered the rocks?

There are many potential sources of contamination — for example, in drilling or sample preparation. Drilling fluid, such as grease, could have been introduced. And, as Brocks recently showed, organic molecules from plastic sample bags can enter rocks.