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,
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."
The Savannah theory suggests
that our hominid ancestors evolved on the dry plains of Africa, and
the theory still has many supporters.
According to scientific
inquiry into the origins of human beings and other organisms, Homo
Sapienshave evolved from a long line of biological organisms,
beginning with single-celled algae that appear in the fossil record
some 3.5 billion years ago.
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
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.