Unstable climates may explain the remarkable pace of evolution in the Cambrian period. Oxygen levels began to rise again, supplying some of the energy needed to form multicellular organisms. But carbon dioxide levels rose much faster, reaching levels much higher than today. This was a warm, humid greenhouse world. Whatever the exact changes, violent climatic and geological swings would have increased the evolutionary pace, driving many species to extinction and forcing the evolution of many new types of large organisms.
