Solar luminosity has increased from 25% to 70% during the last 4.5 billion years (Earth's age) . By this estimate, Earth's surface temperature should have increased significantly. However, geologic evidence indicates that surface temperatures on Earth have remained remarkably stable for at least 3 billion years. (Otherwise, that much increase in solar luminosity would indicate that the surface should have been frozen during the Archean and Proterozoic eons, since there is only a 10oC difference between contemporary temperate climate and an ice age. No evidence exists for that scenario.)
Ammonia, a greenhouse gas (also important in the origins of life), is estimated to have been about 1 part in 105 in the early atmosphere of Earth. Today, it is maintained at 0.006 ppm (or 6 parts in 109). That's a difference of 4 orders of magnitude, or 10,000 times.
Likewise, CO2, another greenhouse gas, was present at much higher levels on early Earth closer to the levels found on Venus and Mars. Today it is present at only about 300 ppm, or 0.003%.
The reduction of both both ammonia and carbon dioxide by the biota, along with cloud cover (which also involves the biota - see below), has kept Earth's surface cool in spite of increasing solar luminosity.
Here is an example of a negative feedback loop - analagous to negative feedback regulating your own temperature - that is helping to regulate Earth's temperature. As global temperature increases, populations of coccolithophorids like Emelia huxleyi (or "E hux": kingdom Protoctista, phylum Haptomonada) increase. E. hux population explosions cause an increase production of dimethyl sulfide. Dimethyl sulfide acts to increases cloud formation (see last section) which increases albedo (reflection of sunlight by cloud surface), which cools Earth. This reduces coccolithophorid populations, which reduces cloud formation, which contributes to an increase in temperature.
This is but one Gaian feedback process. Just as you have multiple feedback processes regulating your own body temperature, Gaia has multiple processes also.
There are new arguments & evidence emerging from the prestigious Potsdam climate institute about the role of positive feedback processes in the Cambrian explosion These new models support Gaia theory. Here's a snippet from a story published by BBC News:
"...what actually caused the Cambrian Explosion is unknown.
Writing in the journal Geophysical Research Letters, Dr Werner von Bloh and colleagues, from the Potsdam Institute for Climate Impact Research, present a new analysis of happened.
They suggest that "feedback" in the biosphere caused it to jump from one stable state without complex life to one that allowed complicated life to proliferate.
'We believe that there was a change in the environment - a slow cooling of the system - that caused positive feedback that allowed the conditions for complex life,' Dr von Bloh told BBC News Online."
Note that the Potsdam Institute uses Lovelock's model Daisyworld as the platform from which to build its supercomputer models of climate change.
Next section: Evidence: Geology