Gaia Theory and Chemistry
Since the earliest recorded history, philosophers, scientists, and others have described an interplay between life and Earth's environment. The Gaia hypothesis draws on this concept but incorporates modern scientific disciplines such as microbiology, geology, and atmospheric chemistry. The name Gaia is drawn from Greek mythology and means Mother Earth.
James Lovelock, an atmospheric chemist, and Lynn Margulis, a microbiologist, published their first paper on the Gaia hypothesis in the early 1970s. Their hypothesis stated that the biota, which consists of all living organisms, maintain an environment on Earth that is optimal for life. This environment features an atmosphere that has a stable temperature and chemical composition. Because these factors stay within certain limits, other factors such as the climate and the amount of sunlight that reaches Earth's surface are also relatively stable. The Gaia hypothesis was later refined to emphasize that the environment is regulated through feedback between living and nonliving systems. The refined hypothesis, often called the Gaia theory, includes the idea that as biota evolved over millions of years, their ability to regulate their environment also evolved.
The Gaia hypothesis has its roots in Lovelock's work with National Aeronautics and Space Administration (NASA) during the 1960s. Lovelock's team was investigating whether the identity and molecular composition of a planet's atmosphere could be used to predict whether life existed on it. In contrast to Venus and Mars, Earth, the only planet known with life, has an atmosphere that is not at chemical equilibrium. It has too much oxygen and nitrogen and too little carbon dioxide. Because of the high amount of oxygen, other gases such as methane should burn off rapidly. However, this non-equilibrated atmosphere has endured for billions of years, and it is ideal for life. Lovelock concluded that biology must be considered alongside chemistry and physics to answer questions about the formation and regulation of a planet's atmosphere.
The Gaia theory goes a step further and proposes that the biota have an active role in controlling Earth's atmosphere, therefore Earth's environment. The scientific community generally agrees that the biota help form the gases in the atmosphere; however, they do not necessarily agree that the biota help regulate their amounts. One of the first objections to the Gaia hypothesis was that it seemed to imply a purpose behind atmospheric regulation--as though the biota had planned to create a hospitable environment. Another objection was based on Charles Darwin's theory of evolution through natural selection. Natural selection means only individual organisms that can survive in a particular environment will reproduce and pass that ability to the next generation. In stark contrast to organisms evolving in response to the environment, the Gaia hypothesis asserts evolution occurred to affect their environment.
These criticisms prompted Lovelock and Margulis to refine their hypothesis. One refinement was Lovelock's creation of the Daisy World concept as a model for Gaia theory. Daisy World is a simplified Earth system with a plant community of white and black daisies. It incorporates natural selection and shows that foresight is not necessary for regulating an atmospheric quality such as temperature.
Initially, Daisy World is too cold for any daisies to grow. But, as with Earth's sun, solar output increases with time. At a certain point, Daisy World becomes warm enough for the daisies to germinate. The white daisies are at a disadvantage because sunlight is reflected from their petals and the surrounding temperature remains cool. However, the black daisy petals absorb the sun's warmth and increase the surrounding temperature. Since the black daisies are warmer, they grow better and produce more seeds. Eventually, there are so many black daisies that the temperature on Daisy World increases and white daisies also grow well. Because the white daisies are doing well, there is a balance between absorbed and reflected solar radiation and the temperature remains steady.
Daisy World indicates that the phenomena described by Gaia theory could be possible, but it is not proof. A key problem in proving Gaia theory is that Earth is the only planet that has been shown to have life. One positive outgrowth of Gaia theory is increased cooperation between scientists from different fields in the study of Earth. Through their work, they may be able to determine how much humans affect Earth. Of particular interest is the possibility that human activities may be counterbalanced by other parts of the biota. In recent decades, human activities such as deforestation and so-called greenhouse gas emissions (e.g., carbon dioxide and methane) have been blamed for global warming and related climate change. Strikingly, Gaia theory predicts that other parts of the biota would have a counterbalancing effect to prevent dramatic climatic changes such as runaway warming.
Margulis cites several systems to demonstrate the selfregulation proposed by Gaia theory. Two proposed feedback systems are:
Carbon dioxide. Over the last four billion years the luminosity of the sun has increased by about 25% and yet the temperature on earth has remained roughly constant. Decline in atmospheric carbon dioxide levels has been linked to the decline in the ability of the atmosphere to absorb solar radiation. Proponents of the Gaia theory say the decline in atmospheric carbon dioxide was caused by the biota's ability to fix carbon dioxide and form limestone.
Ocean salinity. Normal weathering releases so much salt into the ocean that it would have long ago become too salty for most life if the salt was simply dissolved. However over the course of millions of years, the salt content of seawater has remained below 10% of saturation. Proponents of Gaia theory say the excess salt is sequestered by bacteria to form protective sheaths within which the bacterial colonies can safely live. Thus the bacteria help to regulate the salinity of seawater.
These and other proposed feedback mechanisms have detractors. For example, critics of the Gaia theory believe the drop in carbon dioxide levels can be explained by rain; they propose that rainwater simply reacts with carbon dioxide to form carbonic acid which is in turn neutralized by salt.
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