Over the past 4 billion years, life has evolved from simple single-celled organisms into the tremendous variety of plants and animals that exist today. As scientists learn more about the Earth's history, they are realizing that the forces which have shaped the planet have also had a profound effect on the course of evolution. The movement of the tectonic plates has rearranged the continents, providing ever-changing conditions for living organisms, stimulating the evolution of new life-forms. Violent volcanic eruptions, meteorite impacts and drastic climatic changes have triggered mass extinctions, causing setbacks to life on Earth. But the same events have provided new opportunities for the survivors.