In the history of the Earth there have been three extinctions of marine animals, which as yet we cannot explain. Earlier this year, researchers discovered that periods when the oceans had high levels of trace elements, such as zinc, copper, manganese and selenium, seemed to overlap with periods when most groups of living animals first appeared. These minerals are called essential trace elements because without them we die. Now new research has shown that drops in selenium levels in the oceans correlate well with each of the extinction events at the end of the Ordovician, Devonian and Triassic periods. Thus the level of selenium in the oceans ...