The prokaryotes are the most abundant organisms on Earth, and their biomass undoubtedly outweighs all the rest of the organisms together. Although they are too small to be seen individually without powerful magnification, they and the results of their activities are everywhere; without them life on Earth would cease. They have persisted for 3.5 billion years exploiting every possible inorganic and organic habitat—the first 2 billion years alone with no other kinds of organisms. In so doing, they have evolved ways to make a living in each. They manage by being metabolically diverse, morphologically small, cellularily simple, and genetically versatile. They are the dispersers and the recyclers of the Earth's materials and great parts of the human economy depend upon either finding ways to make use of the prokaryotes or ways to get rid of them. Table summarizes the basic features that separate the three domains of life.