The biosphere is the roughly 20 kilometer thick zone inside, on the surface of, and above the earth, including the upper lithosphere, the hydrosphere and the lower atmosphere, in which all life and the products of its activities exist. Life is nearly ubiquitous in the biosphere, although its density and types vary greatly. It even exists on and under the polar ice caps, miles below the surfaces of the oceans, in the harshest of deserts, on tall mountain peaks, and in the air miles above sea level. Much of it is invisible, or not yet even known, including microbes residing deep within the earth's crust to a depth of more than two kilometers, and whose total mass may exceed that of all plant and animal life on the surface.
The biosphere is a virtually closed system with regard to matter, with minimal external inputs and outputs. However, it is an open and self-regulating system with regard to energy, with photosynthesis capturing solar energy on a vast scale. The biosphere is currently thought to have begun roughly 3.7 billion years ago with the emergence of primitive life forms created from naturally occurring organic compounds.