Key to Lovelock's idea was his observation that the planet is self-regulating. He knew, for example, that the heat of the sun has increased by 25% since life began on Earth, yet the temperature has remained more or less constant.
However he didn't know precisely what mechanisms were behind the regulation. It was when he began to collaborate with the American microbiologist Lynn Margulis that the full theory began to take shape. Margulis was studying the processes by which living organisms produce and remove gases from the atmosphere. In particular she was examining the role of microbes which live in the Earth's soil.
Working together, they managed to uncover a number of feedback loops which could act as regulatory influences.
Here are points which support the Gaia hypothesis;
1.) An example is the carbon dioxide cycle. Volcanoes constantly produce massive quantities of carbon dioxide. Since carbon dioxide is a greenhouse gas, it tends to warm the planet. This eventually would make the Earth too warm to support life.
While plants and animals take in and expel carbon dioxide through life processes such as photosynthesis, respiration and decay, these processes remain more or less in balance and don't affect the net amount of the gas.
Hence there must be other mechanisms which help maintain the level of carbon dioxide in the atmosphere.
One process by which carbon dioxide is removed from the atmosphere is rock weathering, where rainwater and carbon dioxide combine with rocks to form carbonates. Lovelock, Margulis and others discovered that the process is greatly accelerated by the presence of soil bacteria.
The carbonates are washed away into the ocean, where microscopic algae use them to make tiny shells. When the algae die, their shells sink to the bottom of the ocean, forming limestone sediments. Limestone is so heavy that it gradually sinks underneath the Earth's mantle, where it melts. Eventually some of the carbon dioxide contained in the limestone will be fed back into the atmosphere through another volcano.
Since the soil bacteria are more active in high temperatures, the removal of carbon dioxide is accelerated when the planet is hot. This has the effect of cooling the planet. Therefore the whole massive cycle forms a feedback loop similar in principle to feedback loops found in the human body which is living.
2.) A second example is the Oceans. Ocean salinity has been maintained at about 3.4% for billions of years. Cells cannot tolerate salt concentrations much above 5%. Salinity is at least partly controlled by evaporate beds/lagoons where marine life causes limestone deposits, later buried.
This process may be involved in initiating the movement of tectonic plates with consequent removal of salts to land masses.
So what happens when the Earth eventually cools enough so there aren’t any volcanoes and no intercontinental plate movement? Will the earth die?
Lovelock, together with Andrew Watson, developed the Daisyworld model - an imaginary planet, which maintains conditions for its survival simply by following its own natural processes. This simple model has since become an integral part of the debate about the Gaia Hypothesis.
The Daisyworld planet contains only two species of life: light daisies and dark daisies. Light daisies tend to reflect light, which has a cooling effect, while dark ones absorb radiation, and therefore warm the planet. Growth of the daisies depends on the present population, the natural death rate, the available space and the temperature (the equations that Lovelock used to model them were based on the dynamics of real daisy growth). The planet revolves around a sun, from which it absorbs energy at a rate which depends on the sun's luminosity and the albedo of the planet. It also radiates heat out to the universe, at a rate determined by the Stefan-Boltzmann Law.
Interestingly, when the model is run with the sun's luminosity gradually increasing, the population of the light and dark daisies adjust themselves naturally so as to keep the temperature constant at the optimal level for daisy growth. Daisyworld is an example of a self-regulating system.
Daisyworld is only a kind of thought experiment, but demonstrates the principle of self-regulation very convincingly.
Although there appear to be several valid points which support or at least get you thinking about the Gaia hypothesis there are also fairly strong points which cast doubt and speculation on the Gaia hypothesis.
1.) Firstly the concept that Gaia had evolved without any recourse to natural selection – an impossibility, according to the Darwinists. If the Earth is alive, where is its Selfish Gene, and who will it pass it onto?
The theory of evolution developed by Darwin is strongly recognised to be the likely cause of events which has given us the species we see today on the planet. Darwin’s theory is based on the principle that living organisms (as a species) “adapt” to changes in their environment, which is on the contrary to the Gaia hypothesis.
2.) Obviously a fundamental question which puts in to question the Gaia hypothesis is the actual definition of life.
There are many definitions of “life” some if considered correct end up supporting or at least not dismissing the Gaia hypothesis whereas other definitions of live.
For example the “school” definition of life remembered by the famous pneumonic m.r.s.g.r.e.n, states that for something to by classed at a living entity it must have the following characteristics;
Movement
Respiration
Sensitivity
Growth
Reproduction
Excretion
Nutrition
The earth, as referred to in the Gaia hypothesis obviously does not do all of these, hence can’t be considered as a super-organism or living entity.
However, there are other definitions for example; the definition by Chilean neuroscientists Maturana and Varela. In their autopoietic (literally self- making) definition of life there is no single definition of life. Their definition states that living beings produce, by their own rules, the components, including their own boundary, that specify it and realise it as a concrete unit in space and time (Maturana and Varela 1987).
What is important in this definition is not so much the material structure of life as the process, organisation and set of relations between the components. Life is a network which constantly makes itself. The simplest autopoietic system is the living cell. For something to be alive by this definition there is no requirement that it grows or reproduces or passes on DNA.
Since, as the Russian scientist Vernadsky observed, 99.9% of the different molecules on Earth have been created in the life process of Earth, the Earth would seem to qualify as a self-making organism.
Finally, some of the more poetical thoughts of the originator of the theory.
At the end of Chapter 1 in his first book, Lovelock writes:
"If Gaia exists, the relationship between her and man, a dominant animal species in the complex living system, and the possibly shifting balance of power between them, are questions of obvious importance...The Gaia hypothesis is for those who like to walk or simply stand and stare, to wonder about the Earth and the life it bears, and to speculate about the consequences of our own presence here. It is an alternative to that pessimistic view which sees nature as a primitive force to be subdued and conquered. It is also an alternative to that equally depressing picture of our planet as a demented spaceship, forever travelling, driverless and purposeless, around an inner circle of the sun."
