The oceans and other water bodies contain billions of tons of dissolved uranium. Over the planet's history, some of this uranium was transformed into an insoluble form, causing it to precipitate and accumulate in sediments. There are two ways that uranium can go from a soluble to an insoluble form: either through the action of live organisms - bacteria - or by interacting chemically with certain minerals. Knowing which pathway was taken can provide valuable insight into the evolution and activity of microbial biology over Earth's history. Publishing in the journal PNAS, an international team of researchers led by the Ecole Polytechnique Fédérale de Lausanne in Switzerland describes a new method that uses the isotopic composition of uranium to distinguish between these alternative pathways.