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Environment and
Life
In the late
1970's, Elso Barghoom of Harvard University was looking for the earliest
evidence of life, and found it eventually in Swaziland, Africa. He found
evidence of bacteria in 3.4 billion year old fossils. This means that
life started very early on our 4.5 billion year old planet. The time it
took to move from inanimate matter to the first forms of life was actually
shorter than that to move from bacteria to larger organisms--the earliest
of which appear to be only 570 million years old, as evidenced by hard-shelled
fossils of that age that appear all over the Earth.
Early life
then probably came from mixtures of materials combining to form biomolecules
with the energy provided by ultraviolet light and lightning. Replication
of DNA and mutation in rapidly dividing bacteria, as well as local variations
in environment, then provided a route to diverse populations of bacteria.
Development of metabolic pathways to store and convert energy--mechanisms
of fermentation to break down sugars--was an early step. Along the way
the bacteria also began to capture atmospheric nitrogen to begin the manufacture
of amino acids and other organic compounds. To this day, we need bacteria
to take the stable nitrogen gas N2 from the atmosphere and
convert it into usable compounds. This "nitrogen-fixing" is
discussed under the nitrogen cycle in the Materials System.
Margulis
and Sagan also state that "the evolution of photosynthesis is undoubtedly
the most important single innovation in the history of life on the planet"
(p. 78). The first photosynthetic organisms were bacteria that used H2S
rather than H2O. H2S must have been plentiful, emitted
from volcanoes. The development of the successive stages of bacterial
development is fascinating as described by Margulis and Sagan. Early adaptations
included developing pigments to protect against ultraviolet, then top
layers protecting the layers below and developing repair enzymes. Repair
enzymes persist in us today. When ultraviolet or other ionizing radiation
damages part of our DNA, these enzymes remove the damaged portion and
replace it with new healthy DNA. Despite the fact that we have had an
ozone layer to filter out the
almost all ultraviolet for over 2 billion years now, we still have this
repair system.
Xeroderma
pigmentosum is a rare genetic defect inhibiting DNA repair mechanisms
against ultraviolet radiation damage. It is characterized by severe sensitivity
to all sources of UV radiation (especially sunlight), and often results
in cutaneous lesions, premature aging of the skin, cataracts, increased
risk of ocular benign and malign tumors, and sometimes neurological disorders
such as mental retardation. To learn more about XP, visit the Xeroderma
Pigmentosum Society, or read its disease description at the Atlas
of Genetics and Cytogenetics in Oncology and Haematology.
Gaia:
Co-Evolution of Climate and Life
Gaia is
the Greek goddess of the Earth. While designing experiments for NASA to
detect life on Mars, the atmospheric chemist James Lovelock developed
the theory called Gaia. Gaia refers to the system of all life on Earth
and the atmosphere which mutually regulates prevailing conditions to continue
life on Earth. (Did you want the notes on atmosphere
of Mars, etc., to be moved here? or did you want them where they are?)
(Note: the
name Gaia was suggested to Lovelock at his request by his neighbor, William
Golding, author of Lord of the Flies.)
The Gaia
hypothesis states that the biota (group of all living organisms) regulate
the temperature and gas composition of the atmosphere. Lovelock came to
this conclusion because the 20-80 composition of O2- N2
in our atmosphere can not be explained by laws of physics and chemistry
alone. If we were to make a simple mixture of these gases in the laboratory
along with some of the other materials on Earth, the gases would react
quickly and become compounds, and not remain as O2 and N2
in the gaseous state. Lovelock therefore postulated that this unlikely
mixture must be aided by the continuous production of these gases by live
organisms! If this were not true, our atmosphere would be a mixture of
N2, NH3, SOx, CH4, methyl
chloride (CH2Cl), and others. These are indeed present but
only in minute quantities.
In addition,
the Earth's average temperature has remained relatively stable (around
22° C) despite the increase in the sun's temperature over the past
4 billion years. This too has been attributed by Lovelock to the feedback
effects of life on the atmosphere.
(Need
to add material)
Several
scientists disagree with Lovelock's hypothesis. Lovelock has actually
modeled a simple system, "Daisy World," a planet with black
and white daisies circling a sun-like star. He and his co-author, Andrew
Watson, have demonstrated the Gaia-like character of this world. The daisies
act as thermostats, stabilizing the temperature. In our world, microbes
can play the role of the daisies. Margulis and Sagan cite the discovery
that about 20,000 years ago there was only two-thirds the amount of CO2
that we have now, and that the rise of CO2 to pre-industrial
levels took place abruptly in a 100 year span. This cannot be explained
by geophysical or chemical processes alone, but could be the result of
a sudden species death of algae.
(Add
stuff from Gaia)
(Find
Gaia or Daisyworld links and add...)
Gaia
Daisyworld
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