Since life began on Earth, five major mass extinctions and several minor events have led to large and sudden drops in biodiversity. The Phanerozoic eon (the last 540 milion years) marked a rapid growth in biodiversity via the Cambrain explosion-a period during which nearly every phylum of multicellular organisms first appeared. The next 400 million years included repeated, massive biodiversity losses classified as mass extinction events. In the Carboniferous, rainforest collapse led to a great loss of plant and animal life. The Permian-Triassic extinction event, 251 million years ago, was the worst; vertebrate recovery took 300 million years.