Two fundamental patterns can be distinguished in the fossil record. On one hand, new groups of organisms appear, diversify and generally persist for very long periods of time; on the other, most such groups and their included species eventually cease to exist. Most analyses of the fossil record show an erratic rise in overall biodiversity, increasing through the Mesozoic and Cenozoic and reaching a peak around the end of the Tertiary. However, diversity has been greatly reduced during several periods of radical environmental change during each of which more than half the multicellular species then living became extinct. Such mass extinction phases have provided important new opportunities for diversification in remaining lineages, and the spread of new communities. Evidently, the large number of species existing now on Earth is the result of a modest net excess of originations over extinctions during the 3,800 million year evolution of life.