While there is some evidence to suggest that climatic and environmental challenges played a role in the development of hominids (deMenocal, 2001 ), there is also evidence that the growing brains of early Homo sapiens benefited from two important fatty acids, docosahexaenoic acid (DHA) and arachidonic acid (AA) found in fish, shellfish and sea bird nestlings and eggs (Broadhurst, 2001).

According to Broadhurst and his colleagues, "restriction to land based foods as postulated by the savannah and other hypotheses would have led to degeneration of the brain and vascular system as happened without exception in all other land based apes and mammals as they evolved larger bodies."

Some researchers, such as James Lovelock (1995) and Lynn Margulis (1992), suggest that biologic organisms have played an integral role in stabilizing and regulating the Earth's climate system. Their Gaia hypothesis suggests that biologic processes serve as a climatic thermostat, preventing Earth from freezing or a runaway greenhouse effect. Those who favor this controversial theory note that Earth gained an oxygen rich atmosphere about 2.3 billion years ago because of the photosynthesis, which can be seen in the appearance in geologic strata of that time of rocks turned red by the oxidation of iron in them.

Plants play an important role in the global carbon cycle, with land plants drawing carbon out of the atmosphere and into the soil, while shell-bearing ocean plankton extract carbon dioxide from the ocean and store it in their shells. The theory is controversial and unproven, but has provided for fascinating discussions on the biological role in climate processes.

Today's atmosphere is made up primarily of nitrogen (78%), free oxygen (21%) and greenhouse gases which can capture solar radiation: water vapor, which ranges from less than 1% in arid regions to over 3% in moist areas, carbon dioxide (0.035%) and methane (0.00018%).

Images above from Ruddiman, 2001 used by permission of W. H. Freeman & Co.