living per se, but something made by living things to protect against an otherwise hostile world. Air, Lovelock states, is “our protection against the cold depths and harsh radiations of space.”

The most important change in the earth’s environ- ment, the one that allowed current life to exist, was the rise of free oxygen in the atmosphere. Before life on earth, the atmosphere contained almost no free oxy- gen, while today the atmosphere contains 21 per cent. This level has been stable for about 570 million years.

The increase in atmospheric oxygen seems to be due to the presence of bacteria that rely on sunlight and carbon dioxide to produce energy. These bacteria lived in the earth’s early oceans and thrived near the surface where volcanic activity and high carbon atmos- phere provided an ideal living environment. These early bacteria released oxygen as a metabolic waste product. As oxygen levels began to rise, other bacteria, which depended on oxygen, evolved. As atmospheric

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marshes become land, and water travelling through the soil carries some salt back into the ocean. This ongo- ing process works to maintain a balance in ocean salinity.

Another example of a Gaian process is the removal of carbon from the atmosphere. Organisms that use carbon compounds to create shells, take carbon from the atmosphere. As the organisms expire, the remain- ing shells sink to the ocean floor and gather, eventual- ly forming sedimentary rock. Carbon is entered back into the atmosphere from sources such as volcanoes, forest fires, and plants, which use carbon and nitrogen found in soil.

Criticisms of Gaia

Most of the criticism of the Gaia theory comes from scientists who challenge Lovelock to show how organ- isms could unify in seemingly global acts of altruism, when natural selection relies on survival of the fittest.

a feedback loop. Ultimately Lovelock states, “life and the material environment evolved tightly cou- pled together as a single entity. Self regulation is an emergent property of the system.” Lovelock hes- itates to say that Gaia regulates anything.

Gaia Will Die

Life on earth has been around for 2.5 billion years. Lovelock doubts life will last an equal amount of time. Since life on earth began, energy output of the sun has increased about 25 per cent. Throughout this time the average temperature of the earth has remained relatively stable. The aver- age temperature is a balance between periods of warmth and periods of cold. Eventually the sun’s output will become too great for the current envi- ronment to exist, and life will begin to extinguish. As Lovelock says, “It’s part of the cosmic cycle. Gaia is simply a single cell in the universe.”