primitive blue-green algae, or cyanobacteria, survive by taking one of the most abundant gases released by volcanoes-- carbon dioxide-- and combining it with water in the presence of sunlight. the result is carbohydrates used for food and an important waste product called oxygen. so, through time, early life forms helped to change the carbon-dioxide-rich atmosphere to one rich in oxygen... thus permitting the evolution of modern life. in addition to biological activity, another major difference between our planet and most other terrestrial bodies in the solar system is that the earth still experiences a great deal of radioactive decay. mercury, mars, and the moon are essentially dead worlds, too cold inside to create new volcanoes or great crustal movement. it's largely due to internal radioactivity that the earth is a geologically active planet. our earth is alive. i just don't mean biologically alive. it's physically alive. it has an internal heat engine that is fueled by radioactivity. without that, there would be no volcanoes. without that, there woulbe no earthquakes. without that,