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In lakes and oceans, organic matter from dead organisms is usually recycled back into CO 2 and water. However, a small fraction of this dead biomass, particularly pigments and lipids, escapes the remineralization process and accumulates in the bottom sediment. Over millions of years, the lipids will turn into hydrocarbon fossils, or biomarkers. Many biomarkers are extremely stable. Preserved in sedimentary rocks over billions of years, they are witnesses to past ecosystem and environmental conditions.

At RSES, we search for the oldest biomarkers on Earth in Precambrian rocks from Australia and other places around the world. The molecules help us reconstruct ancient microbial ecosystems. For instance, we found that the seas around the north of Australia 1.6 billion years ago were anoxic and sulfidic. The waters were inhabited by green and purple sulfur bacteria. Algae and other forms of complex life did not exist or were rare in this toxic broth.