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The Energy Cycle of the Biosphere 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