Life was slow on Earth for the first 3 billion years, Martin says. But, some 500 million years ago, in the Cambrian Period, a riotous array of marine animals in different shapes and anatomical designs appeared. According to Martin, there are two basic hypotheses for why Earth had this burst of new life-forms in the Cambrian, followed by the failure of most of the experimental types. Internal control, or the genome hypothesis, suggests that genetic constraints are set in place in the organisms, keeping them from evolving into very different life-forms through the eons. The ecospace hypothesis suggests external controls were more important, with ecological constraints ranging from predators to environmental changes.
Continents come together and destroy habitats.
Sea levels rise and fall. A modern example occurred in the Great Lakes. When sewage and industrial wastes were dumped into the lakes, there were algal blooms that swamped the lakes. Other aquatic creatures died, and the whole ecosystem was upset. Plentiful food can knock over delicate ecosystems, Martin says.