Our planet has a long and dynamic history. How narrowly can Gaia be said to have constrained conditions? As we learn more about Earth's history, it is clear that huge changes have occurred in the climate, position of land masses, ocean currents, and other global-scale properties. For example, several times in the planet's history, we believe that it has been largely covered with ice. During the Mesozoic period, it appears that the planet was much warmer than it has been since. The atmosphere has evolved from anaerobic to a high level of free oxygen and many other major chemical changes have occurred. Against the dramatic backdrop of these changes, it is hard to claim that Gaia has held conditions constant and the window of variability seems very large even to qualify as homeorhesis.
Gaia as an organism has foundered on another point. Organisms reproduce. How can an entity the size of a whole planet reproduce? Gaia has not yet done so, but it has been suggested by some that space colonization may be the biosphere's first attempt to reproduce itself on other planetary bodies. The notion of Earth as super-organism may be specious and not central to the idea of global homeorhesis; thus, this may be a fairly trivial semantic criticism.
Because the notion has evoked visions of Gaia as the 'mother goddess', as a benign entity and protector of life, it is appealing to people outside the scientific community. This has also been another point of attack for its critics, who view it as overly romanticized, more philosophical in nature, and scientifically untestable, thus, not of value in the strict scientific sense. If Gaia is the mother goddess, then her first-born were probably bacteria-like organisms who would be poisoned by our current oxygen-containing atmosphere. Her enormous family includes countless species whose individual needs and welfare conflict with each other. This has resulted in the natural extinctions of the bulk of all species that have ever arisen. The interpretation of such a system as benign or nurturing is stretching it too far.
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