The biosphere is composed of the parts of Earth where life exists. It includes the dark environment of the ocean's deep trenches, the rainforests and high mountaintops. Bacteria, protozoa and up to 30 million species of animals, plants and fungi are included in the biosphere.
Although the biosphere is a life-supporting global ecosystem, it comprises a relatively thin part of Earth's surface. The biosphere measures approximately 12 miles from top to bottom, but almost all life exists between 1,640 feet below the ocean's surface to 3.75 miles above sea level.
Some ancient prokaryotes developed a way to convert carbon dioxide and water to make simple sugars. Over time, these photosynthetic organisms helped change the biosphere so that new forms of life could exist. The biosphere has existed for 3.5 billion years and is a large ecosystem comprised of many smaller ecosystems.
The biosphere is characterized by the continuous cycling of matter, and water is a major factor. The flow of solar energy helps maintain the structure of organisms, and the elements carbon, hydrogen, nitrogen, oxygen and sulfur combine with proteins, lipids and nucleic acids to form the building blocks of life. Biosphere reserves exist to help people establish and maintain a balanced relationship with the natural world.