Gaia, which is a concept identified with the planet Earth (all of its living creatures and environment) is the biggest known demonstration of life. Gaia is the entity that represents the union between the rocks, the air, the oceans and the living organisms. The key-elements that characterize Gaia are:
The Planet of the Daisies
The way the organisms can affect the environment aiming its equilibrium condition is demonstrated by the model of "the planet of the daisies" postulated by James Lovelock, the master of the theory of Gaia.
Let’s suppose that the 3 only living species of a planet are white daisies, grey (neutral colour) daisies and black daisies. When the planet becomes colder than the ideal, the black daisies thrive more than the white or grey daisies, because the colour gives them a better capability for absorbing the heat. The planet becomes darker because it covers itself with a flora that is predominantly dark. For that reason it becomes hotter. On the other hand, if the planetary temperature is higher than the ideal, then the white daisies, with a better capability for irradiating the heat and preserve themselves cooler, will get advantage over the other species. The planet will become lighter and, therefore, it will become colder. If the temperature is close to the ideal, the grey daisies would have advantage over the others, because they wouldn’t need to expend any energy in the production of the white or black pigments. If some significative regulation of the temperature was necessary (if the planet was colder or hotter), this advantage would, nevertheless, lose its relevance.
We can add two populations to the 3 daisies’ species: rabbits and foxes. When the number of rabbits increases, the number of daisies decreases (they are eaten by the rabbits), but if the daises became exhausted, the population of rabbits would die from starvation. It is, therefore, necessary to reach an equilibrium at which the number of daisies is exactly enough for feeding the population of rabbits. The same logic is applied to the relation between the populations of rabbits and foxes (the foxes eat the rabbits).
It can be demonstrated that the higher is the variety of species, the higher will be the stability of the system, even following occasional disturbances (like the fall of a meteorite). When the heating of the star provokes the rise of the “inorganic” temperature (under conditions of hypothetical absence of life) up to the limits that are bearable by the organisms, a smaller variety of living beings will populate it (for instance, only the white daisies species) and, therefore, the system will become unstable, subjected to strong fluctuations.
The presence of oxygen in the atmosphere is also a good reflex of the theory of Gaia: The expansion of the forests (producers of this gas) will drive up the percentage of produced oxygen. However, if this percentage was higher than 25%, the forests would completely burn (because the oxygen is a comburent) and, therefore, the quantity of produced oxygen would decrease until an equilibrium would be reached.
On the other hand, the reactions between the free oxygen and other elements like carbon or sulphur release acid substances into the air, which favours the erosion of the rocks of the crust in order to release more nutrients. That would allow that these nutrients could benefit a bigger ecosystem, provoking a rise in the total quantity of organisms that would feed from them, like the photo-synthesizers (producers of oxygen, as the plants, for instance). The growth on the quantity of these organisms causes an increase on the proportion of oxygen that, just as the acids and nutrients, becomes poisonous when it exists in excess. This excess limits, therefore, the number of organisms producers of oxygen and so, an equilibrium is reached - the oxygen composes nowadays 21% of the total atmospheric volume.
Summarizing, we can state that Gaia is a living system that, under its multiple facets, is auto-regulated.
A living being the size of the world (NASA - USGS)