The energy that sustains all living systems is solar energy, fixed in photosynthesis and held briefly in the biosphere before it is reradiated into space heat. The total amount of solar energy fixed on the earth sets one limit on the total amount of life; the patterns of flow of this energy through the earth's ecosystems set addifional limits on the kinds of life on the earth. Solar energy has been fixed in one form or another on the earth throughout much of earth's 4.5 billion years history. The modern biosphere probably had its beginning about two billion years ago with the evolution of marine organisms that not only could fix solar energy in organic compounds but also did it by splitting the water molecule and releasing free oxygen. .
Marine plant cells accumulated for hundreds of millions of years, gradually building an atmosphere that screened out the most destructive of the sun's rays and opened the land to exploitation by living systems. Evolution fited the new species together in ways that not only conserved energy and the mineral nutrients ultilized nutrients by recycling them, releasing more oxygen and making possible the fixation of more energy and the support of still more life. .
The actual amount of solar energy diverted into living systems is small in relation to the earth's total energy budget. Only about a tenth of 1 percent of the energy received from the sun by the earth is fixed in photosynthesis. More than half of the energy fixed in photosynthesis is used immediately in plant's own respiration. .
Changes accure slowly through a conjoint evolution that is not only biological but also chemical and physical. The time scale for most of changes and evolutions of developments, particulary in the later stages when many of the species have larger bodies and long life cycles, is very long.
Green plants are the "primary producers" of the biosphere, converting solar energy into organic compounds that maintain the plants and other living things.