Tiny crystals of the iron mineral haematite provide evidence for an oxygenated water body 3,460 million years ago, reports a paper online this week in Nature Geoscience. This evidence indicates that microorganisms capable of producing oxygen through photosynthesis were present at this time, predating the oldest unambiguous fossils of photosynthetic microbes by about 800 million years.

Hiroshi Ohmoto and colleagues analysed the chemical characteristics of grains of haematite found in a jasper formation of Western Australia. They found that the grains, which developed in an early sea that existed over what is now Pilbara, Australia, probably formed as the result of an interaction between hydrothermal fluids and oxygen-rich sea water. The group suggests that organisms capable of photosynthesis must therefore have been present in sufficient numbers to at least occasionally oxygenate this body of water.