Gaian Hypotheses
 
When we think of biology, we normally think of individual organisms and how they work, or perhaps molecular biology and genetics. We rarely go up a scale to ecology - the study of the distribution and interactions of many organisms, and few people at all think of biology on a planetary scale, and how the interactions of the trillions or quadrillions of organisms on Earth build up.
 
Those few people, however, did include atmospheric chemist James Lovelock and evolutionary biology Lynn Margulis. Together, these two scientists conceived the Gaia theory, that the interactions of not only all the organisms on Earth but also all of the environments of Earth put together makes the Earth 'alive'.
 
This bold claim is explained by saying that these interactions can control the average temperature of the surface, the salinity of the oceans and the carbon dioxide content of the atmosphere and keep them constant via a intricate network of negative feedback cycles.
 
Most scientists have rejected what is now called the 'strong' Gaian hypothesis, that the Earth is in actuality an organism that, in its regulation of the atmosphere and all other elements of the environment, is undergoing homeostasis (in biological terms, homeostasis is the regulation of a constant internal state of an organism). This has unfortunate religious connotations and many people do not like to feel as if there is some kind of 'purpose' behind Gaia ('Gaia' is used to represent the Earth in totality, including all of its organisms).
 
The moderate Gaia hypothesis is that the Earth behaves as if it were a living organism, but it isn't actually one.
 
Most favoured is the weak Gaia hypothesis which says that all of this 'homeostasis' where the temperature is kept constant and so on is merely a simple processes that is governed by negative feedback - negative feedback essentially means that when something gets too large or too high (like temperature) it is automatically regulated down by another process that 'kicks in' due to the high temperature.