Wired Science [h/t Andrew Sullivan]blurbs this theory that complexity of life took off during the pre-Cambian and Cambian periods after a couple billion years of miniscule change because oxygen levels were finally able to increase in the world's oceans. Carbon was more effectively concentrated and rapidly sequestered by sinking to the ocean floor as poo instead of floating and forming carbonate compounds with oxygen. More free oxygen allows for higher metabolism rates, and higher metabolism allows for more energy to be ordered and directed towards increasing complexity.
A decade ago, he pointed out that the first food consuming, feces-excreting organisms appeared about 40 million years before the Cambrian.Everything comes down to Poo!
At that point, Earth's oceans were rich in carbon and poor in oxygen. Photosynthesizing plankton produced some oxygen, but it was consumed by bacteria and offset by slow-sinking carbon. The arrival of multicellular organisms changed all that: they competed with bacteria for plankton, and their carbon-rich feces dropped quickly to the ocean floor. Bacteria dwindled; oxygen levels rose, especially in sunlit upper waters; and the ecological gates were thrown wide-open to the parade of animals that soon followed.