AN oxygen-rich atmosphere allowed gigantic insects to grow 300 million years ago. Dragonflies with wingspans of up to 70 centimetres flourished in the Carboniferous period. Researchers had long suspected that the giant insects and lush forests needed more oxygen than the level of 21 per cent found in today's atmosphere.

Plants and marine life preferentially take up certain isotopes of carbon and sulphur and their absorption changes as the concentration of oxygen rises (Science, vol 287, p 1630). Analysis of these isotopes in ancient rocks shows that the oxygen content of the atmosphere peaked at 35 per cent between 350 and 250 million years ago, says Robert Berner of Yale University in New Haven, Connecticut.