Over billions of years, the land has cooled down and acquired a thick crust. There are different theories of where the water came from on Earth. Water condenses to give us clouds, which create a protective atmosphere and make it habitable for living organisms. About 580 million years ago, multicellular life took hold. Those organisms arose and evolved or died out. Glacial periods, changing climatic conditions, tectonic shifts of plates, volcanic activity, collisions with asteroids altered the planet's appearance and forced living organisms to adapt to changing circumstances.