No. 120: Nov-Dec 1998
The Gaia hypothesis states that the earth's environment is maintained suitable for life by life itself. Our climate, atmospheric oxygen level, ocean composition, and similar vital conditions are kept livable by life's waste products, such as the oxygen emissions of plants. That something like Gaia is required is seen in the extreme disequilibrium of the earth's atmosphere compared to the near-equilibrium of the atmospheres of apparently lifeless Venus and Mars. For example, our atmosphere's 21% oxygen, a highly reactive gas, is many orders of magnitude higher than one would expect on a lifeless planet. Furthermore, life-friendly conditions have been maintained for billions of years despite large changes in the sun's output and the traumas of asteroid impacts.
T.M. Lenton, writing in Nature, asks a salient question: How has planetary self-regulation (Gaia) been established and maintained by evolution and natural selection which operate on the level of individuals? In other words, evolution tells us that organisms should evolve so as to leave the most progeny not so as to regulate the atmosphere. Lenton answers that there must be feedback loops from the planetary environment that steer the evolution of individuals in the "proper" direction. Lenton goes on to explore some of these many feedback mechanisms; one obscure loop involves the production of dimethyl sulfide by marine phytoplankton. Truly, it is a tangled bank! All of the feedback loops imply that the evolution of life forms is constrained (or dictated) by the need to keep the planet livable and not to simply leave the most progeny, but rather the progeny that will best serve Gaia!
(Lenton, Timothy M.; "Gaia and Natural Selection," Nature, 394:439, 1998.)
Comments. The obvious implication is that all life forms, including humans, are parts of a planet-sized super-superorganism. This leads to the oft-stated and possibly true suspicion that, if a species endangers Gaia by creating ozone holes and undue global warming, the super-superorganism will take appropriate steps -- new diseases, for example.