 A planet 's life forms inhabit ecosystems , whose total is sometimes said to form a " biosphere " . Earth 's biosphere is thought to have begun evolving about 3 @.@ 5 Gya . The biosphere is divided into a number of biomes , inhabited by broadly similar plants and animals . On land , biomes are separated primarily by differences in latitude , height above sea level and humidity . Terrestrial biomes lying within the Arctic or Antarctic Circles , at high altitudes or in extremely arid areas are relatively barren of plant and animal life ; species diversity reaches a peak in humid lowlands at equatorial latitudes . 
