Encyclopedia of Evolution.pdf - Online Reading Center

Gaia hypothesis The Gaia hypothesis claims that organisms
have transformed the entire Earth and maintain it in a
state very different than it would have without organisms.
“Gaia” is the name of an ancient Greek Earth goddess and
is intended by the scientists who defend the hypothesis as
an image, not referring to the Earth as a literal organism or
person. Biologist Lewis Thomas proposed the concept that
the Earth was like a cell in his 1984 book Lives of a Cell.
However, Thomas’s imagery resulted in no program of scientific
investigation. The scientific predictions based upon the
Gaia hypothesis began with the work of atmospheric scientist
James Lovelock. Its principal proponent within the world of
biological and evolutionary science is evolutionary biologist
Lynn Margulis (see Margulis, Lynn).
The Gaia hypothesis is sometimes worded as “The Earth
is alive,” but this is not correct. The scientists who defend the
Gaia hypothesis claim not that the Earth is an organism but
that the atmosphere and water of the Earth are a nonliving
part of the Earth’s living system of interacting organisms the
same way that a shell is a nonliving part of a mollusk. The
Gaia hypothesis states that organisms transform the Earth in
a way that has resulted in a regulation of the temperature and
chemical composition of the Earth.
Biological activities of organisms have caused the Earth
to be very different from other planets and maintain it in a
chemically unstable condition. Thus, even without visiting a
distant planet, observers should be able to determine whether
life exists on the planet. If all of its atmospheric components
(which can be studied by analyzing the light that they reflect)
are in equilibrium, the planet is either lifeless or nearly so. Life
creates chemical and physical disequilibrium which can be visible
from millions of miles away. This was the basis on which
James Lovelock predicted that no life would be found on
Mars, a prediction that is either entirely or very nearly accurate
(see Mars, life on). Consider the following examples.
Carbon dioxide. The other Earth-like planets (Venus and
Mars) have atmospheres with tremendous amounts of carbon
G
dioxide. In contrast, the Earth’s atmosphere consists of less
than 0.04 percent carbon dioxide. The primordial Earth, during
the Hadean era (see Precambrian time), also had a high
concentration of carbon dioxide, although sediments deposited
at that time indicate that the Earth’s atmosphere never
had as much carbon dioxide as Venus and Mars now have.
The process of photosynthesis (see photosynthesis, evolution
of), carried out by many bacteria during the Archaean
era, reduced the atmospheric carbon dioxide concentration
by at least two orders of magnitude. Where did the carbon
dioxide go? The bacteria made it into organic material,
whether living material (the bacteria themselves) or deposits
of dead material. Eukaryotic photosynthetic organisms, such
as plants, continued this process once they evolved.
Oxygen. Photosynthetic bacteria, from which the chloroplasts
of many protists and of green plants have evolved,
also filled the Earth’s atmosphere with huge amounts of
oxygen gas. The buildup of oxygen made the sky turn blue.
Through most of the late Precambrian time, the Earth’s
atmosphere had approximately the level of oxygen gas that
it now has. The huge amount of oxygen gas in the air is perhaps
the most strikingly unstable result of biological activities.
Oxygen atoms attract electrons from many other kinds
of atoms and are therefore very reactive. All of the oxygen
gas in the air would react with other molecules (such as
nitrogen gas, which is the principal component of the atmosphere;
the ferrous form of iron; and with organic molecules)
and disappear, if it were not continually replenished by photosynthesis.
Other atmospheric components. Methane (CH 4; the
principal form of natural gas) is continually produced by
organisms, mostly by bacteria that live in anaerobic conditions
(for example, mud or animal intestines, away from the
presence of oxygen). While there is not much methane in the
atmosphere, the little that is there quickly reacts with oxygen
to form carbon dioxide. Its continued presence results from
bacterial metabolism. The atmosphere also has nitrogen (N 2),