Geoscience Reference
In-Depth Information
years ago. Then, the prokaryotes were joined by
eukaryotes
. A eukaryote is
any organism whose cells have nuclei, including both plants and animals. Pho-
tosynthesis by eukaryotic plants significantly increased O
2
production rates on
the planet. But the process of building up oxygen levels in the atmosphere was
still slow, because many rocks on the surface were easily oxidized and large
amounts of oxygen were bound into the rocks. About 1 billion years ago these
oxygen sinks on the surface became sufficiently saturated and atmospheric lev-
els of molecular O
2
began to increase more rapidly.
Thus, while life originated on the planet in an anaerobic atmosphere, the
more widespread development of biological activity over long periods of time
created an oxygenated atmosphere. The continuation of biological activity is
essential for maintaining O
2
in the atmosphere.
THE GAIA HYPOTHESIS
The Gaia hypothesis was developed by James Lovelock in his book
Gaia: A
New Look at Life on Earth
. It suggests that the lower atmosphere of the earth
is an integral, self-regulating, and necessary part of life on the planet. The idea
is that, for hundreds of millions of years, life has controlled the temperature,
the chemical composition, the oxidizing ability, and the acidity of the earth's
atmosphere to maintain conditions to sustain, or even optimize, life.
The Gaia hypothesis is named in honor of the Greek goddess of the earth.
The concept embodied by this hypothesis is that the global ecosystem actively
sustains and regulates the environment for its own benefit. The idea is not that
individual species act in this way, as in the Darwinian concept of natural selec-
tion, but that the biosphere as a whole acts to optimize the planet for the exis-
tence of life as a whole. The idea is not literally correct, but it is interesting and
has proved useful for helping earth scientists trained in the physical sciences to
consider an important role for biological activity.
The development of O
2
in the earth's atmosphere discussed previously may
seem to be an example of the Gaia hypothesis at work. As life developed on
the planet, CO
2
levels were drawn down and O
2
levels increased, and this made
the environment more favorable for the presence of life and enabled increased
biological activity.
The Gaia hypothesis ascribes a certain resiliency to life on earth, suggesting
that any changes in climate are controlled to benefit biology as a whole (but
not necessarily individual species). Currently, human activity is changing cli-
mate. Is this process optimizing the environment to benefit life on the planet?
That seems doubtful when we consider the ongoing mass extinction due to
human activity and the threat to ecosystems in rapidly shifting climate zones.
2.6 DATA SOURCES AND REFERENCES
The National Snow and Ice Data Center maintains an excellent website for
general education on the cryosphere as well as more in-depth exploration of
recent findings.
