Encyclopedia of Evolution.pdf - Online Reading Center

0 Gaia hypothesis
nitrogen oxide (NO), ammonia (NH3), methyl iodide, and
hydrogen (H2) that are produced by microbes.
Temperature. At least two biological processes appear to
affect global temperature.
• Greenhouse gases. When the Earth first cooled off enough
for the oceans to form at the end of the Hadean, the Sun
produced much less radiation. If the Earth had its present
atmospheric composition, the Earth would have frozen.
The Earth apparently did almost completely freeze on more
than one occasion, but this occurred later in Precambrian
times (see Snowball Earth). Apparently the reason the
Earth did not freeze is that the atmosphere contained large
concentrations of greenhouse gases such as carbon dioxide
and methane (see greenhouse effect). These gases
absorbed infrared radiation that would otherwise have
been lost into outer space; the infrared radiation made the
atmosphere, thus the whole Earth, warmer than it would
have been without them. As the Sun became brighter, photosynthesis
of bacteria removed much of the carbon dioxide
and produced oxygen that reacted with much of the
methane. Thus at the very time that the Sun was becoming
brighter, the greenhouse effect was becoming weaker, thus
keeping the average temperature of the Earth relatively constant.
Throughout the history of the Earth, there has been
an approximate balance of photosynthesis (which removes
carbon dioxide from and releases oxygen into the air) and
respiration (which removes oxygen from and releases carbon
dioxide into the air).
• Aerosols. Marine algae, cyanobacteria, and some plants
produce a compound that, when acted upon by bacteria,
becomes dimethyl sulfide gas. Clouds of dimethyl sulfide
gas appear to be an important component of the global
sulfur cycle. Under warm conditions, the algae have more
photosynthesis and bacteria are more active. This results in
more dimethyl sulfide, which produces more clouds, which
causes conditions to become cooler. The molecule from
which dimethyl sulfide is produced serves a protective function
in the algae, but the dimethyl sulfide itself may be just
a waste product. Airborne biological particles and aerosols
(such as hair, skin cells, plant fragments, pollen, spores,
bacteria, viruses, and protein crystals) may also affect the
climate of the Earth.
At the same time that the Sun became brighter, the biological
activities of the Earth altered the atmospheric composition
in such a way that the temperature did not become
too hot. Throughout the last half billion years, atmospheric
oxygen concentration has remained approximately constant.
Apparently atmospheric oxygen was much higher than it is
today during the Devonian period and the Carboniferous
period and very low during the Permian extinction,
which may have contributed to the deaths of large animals
in the ocean and on land. Had the oxygen content of the air
increased too much, spontaneous combustion would have
caused plants all over the world to burn even if they were wet
and alive. All it would take would be a few lightning strikes
to ignite huge conflagrations.
Minerals. Most of the major mineral elements on the
surface of the Earth circulate through the food chain. On a
planet without life, these elements (such as calcium, phosphorus,
and sulfur) would accumulate in their most stable form
and remain in that form.
To a certain extent, the relative stability of atmospheric
carbon dioxide and oxygen can be explained by natural selection
acting upon the populations of the organisms, in their
own interest, rather than a process that operates to maintain
the carbon dioxide and oxygen content of the atmosphere.
When there is a lot of carbon dioxide in the air, photosynthetic
rate increases, and this reduces the amount of carbon
dioxide. Similarly, when oxygen is abundant, chemical reactions
between oxygen and minerals in rocks increases, respiration
in cells increases, and photosynthesis decreases (oxygen
is actually a direct chemical inhibitor of photosynthesis). For
both carbon dioxide and oxygen, photosynthesis and respiration
operate as negative feedback mechanisms. Photosynthesis
and respiration help to maintain a happy medium of
oxygen, carbon dioxide, and temperature on the Earth. The
organisms carry out photosynthesis and respiration to keep
themselves alive, not to take care of the Earth.
The activities of organisms, acting in their own interests,
help to explain the Gaia hypothesis. Lovelock developed
a computer simulation called Daisyworld that makes this
point. Consider a planet populated entirely by daisies that
are either black or white. When the climate on this planet is
cold, the black daisies absorb more sunlight and are warmer,
and their populations increase relative to those of the white
daisies. When the climate on this planet is warm, the white
daisies reflect more sunlight and are cooler, and their populations
increase more than those of the black daisies. Under
cold conditions, when the black daisies spread, the heat from
the daisies may actually cause the climate to become warmer;
and under warm conditions, the reflection of light from
the white daisies may actually cause the climate to become
cooler. This is an example of simple negative feedback: Cold
conditions select for the spread of black daisies, which raise
the temperature of the environment, just as a cold room trips
a thermostat which turns on a heater. Lovelock and Margulis
use Daisyworld as a picture of how individual organisms can
cause a Gaia-like maintenance of stable global conditions.
The Gaia hypothesis indicates that organisms do not
simply live upon the Earth; evolution has not simply caused
organisms to adapt to the physical conditions of the Earth.
Organisms have radically transformed the Earth and have
partly created the very conditions to which evolution has
adapted them. This is the largest scale of symbiosis (see
coevolution); Lynn Margulis says that Gaia is just symbiosis
seen from outer space. It may therefore be not just certain
species interactions but the entire planet that is symbiotic.
Further Reading
Jaenicke, Ruprecht. “Abundance of cellular material and proteins in
the atmosphere.” Science 308 (2005): 73.
Lovelock, James E. Gaia: A New Look at Life on Earth. New York:
Oxford University Press, 1987.