CHICAGO, IL.- Variations in the Earth’s atmospheric oxygen levels are thought to be closely linked to the evolution of life, with strong feedbacks between uni- and multicellular life and oxygen. Over the past 400 million years the level of oxygen has varied considerably from the 21% value we have today. Scientists from The Field Museum in Chicago and Royal Holloway University of London publishing their results this week in the journal Nature Geoscience have shown that the amount of charcoal preserved in ancient peat bogs, now coal, gives a measure of how much oxygen there was in the past. Until now scientists have relied on geochemical models to estimate atmospheric oxygen concentrations. However, a number of competing models exist, each with significant discrepancies and no clear way to resolve an answer. All models agree that around 300 million years ago in the Late Paleozoic atmospheric oxygen levels were much higher than