BBCnews has this article detailing new discoveries about the Cambrian explosion. For billions of years, the earth hosted nothing more complex than bacteria and simple algae. Then, in a relatively short space of time, the planet was filled with all sorts of complex life forms, some of which aren't even related to present day families of species. This is the Cambrian explosion, and it is one of the most important but least understood periods in the history of life. New computer models have lead to insight into how, and why, such a thing ocurred.