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Evolution &
the Environment
Evolution
is defined as the gradual change of any object, in our case, the biological
system, through time. We discussed briefly in the previous section some
of the aspects of the complex intricate system of the chemistry of biomolecules.
We now shift to the level of the organism--cells organized into tissues,
organs, and organism--which have persisted and changed through time. Broadly
speaking, the first single-celled organism did not have a nucleus. These
prokaryotes persist today as microbes, such as bacteria and viruses.
Protein
synthesis was an early step in the evolution of organisms who were then
capable of reproducing themselves using their RNA. It is not clear when
the first cells in the form we know now were formed. Somewhere, in the
primordial soups, some molecules got enclosed inside a membrane to form
a cell. RNA molecules within this cell could then facilitate reproduction
of the cell. All we can surmise is that this aggregation of molecules
started about 3.5 billion years ago. It is believed that RNA came after
proteins and enzymes evolved. Once a cell was established, RNA evolved
into DNA, making a subcellular structure we now call the nucleus. The
DNA, which is a coil of two strands of RNA (with the bases A, C, G, and
T, whereas RNA has A, C, G, U), is the molecule that enables protein synthesis.
Some of
the prokaryotic bacteria we know like mycoplasms, as well as different
varieties of bacillus, e-coli, and staphylococcus, still abound on Earth.
Bacteria live in an enormous variety of environments, often symbiotic
with other organisms. Thus benign forms of e-coli live in our guts and
numerous bacteria live in soil and in plants. Species of bacteria can
use a variety of organic molecules such as sugars and polypeptides as
their food. Bacteria are the most abundant type of cell on earth. The
early bacteria seemed to have used ATP to store the energy of sunlight.
Blue-green algae is a type of bacteria that converts CO2 and
N2 into its food.
As plants
developed the capability of photosynthesis, the composition of the early
atmosphere (mostly CO2) began to change over billions of years to this
current composition
of 20% oxygen and only traces of CO2 as the plants "fixed"
the carbon. Many of the early bacteria were used to an oxygen-poor environment
and several of them then became extinct. Others formed a symbiotic association
with oxygen--using (aerobic) types of cells to form the present day cells
with nuclei (eucaryotes). The different organelles in the cell such as
the chloroplasts and mitochondria are now believed to have been separate
organisms that originally associated with each other to form mutually
beneficial colonies we call cells.
Origin
of Life
As we noted
before, life on Earth has evolved around the chemical versatility of a
few atoms, especially carbon. Some special features of chemistry are used
by living systems. These features are:
- the ability
of carbon to combine in so many different ways;
- the unique
properties of water; and
- the ability
of organic molecules to use small amounts of energy efficiently.
A live organism
is an open system, continuously exchanging energy and matter with the
environment. It is "self-organizing," meaning it takes raw material
and reassembles it into complex vital molecules. During this process,
life increases internal order (decreases local entropy). Thus life builds
up information (order) which is then duplicated.
In
the Beginning
The early
environment on Earth is a matter of conjecture. Piecing together evidence,
it is believed now that the environment consisted of high energy events
such as volcanic eruptions, continuous torrents of rain, and large amounts
of lightning. It is believed that there was little if any oxygen in the
atmosphere, and certainly no ozone layer. Therefore, ultraviolet from
the sun could reach all the way to the Earth's surface.
Some of
our models for the early environment of Earth come from the observation
of the atmospheres of Mars and Venus, made by NASA.
Table X
shows the major gases in the atmosphere of Venus, Mars and the Earth.
Note the difference between the estimate of the composition with and without
life on earth.
|
GAS
|
PLANET
|
|
VENUS
|
EARTH
without life
|
MARS
|
EARTH
as it is
|
| CO2 |
98%
|
98%
|
95%
|
0.03%
|
| Nitrogen |
1.9%
|
1.9%
|
2.7%
|
78%
|
| Oxygen |
trace
|
trace
|
0.1%
|
21%
|
| Argon |
0.1%
|
0.1%
|
2%
|
1%
|
Surface
temperature,
° C |
477
|
270
± 50
|
-
53
|
13
|
Pressure
on surface,
bars |
90
|
60
|
.0064
|
1.0
|
|
|
Table
X: Atmospheric compositions of Venus, Mars and Earth (with and
without life)
[from GAIA by James Lovelock , 1995 edition]
|
It has been
shown in laboratory experiments that simple carbon-based (organic) molecules
are formed under these early conditions that prevailed on Earth. In 1953,
Stewart Miller, a graduate student of the famous chemist, Harold Urey,
simulated the early (prebiotic) atmosphere on Earth--a mixture of ammonia
(NH3), water vapor (H2O), hydrogen, and methane
(CH4). He bombarded the mixture with electrical discharges
to simulate lightning. In a week, he saw some spectacular results: alanine
and glycine, two amino acids that form proteins in life forms today (including
humans) were formed in the resulting mixture. Under the conditions provided,
more complex molecules such as formaldehyde (HCHO), formic acid (HCOOH),
and hydrogen cyanide (HCN) had formed. In a water solution these molecules
had then reacted with each other to form more complex organic molecules
such as acetic acid (CH3COOH), glycine (NH2CONH2),
alanine (NH2CHOHCOOH).
The richness
of carbon chemistry and the plethora of carbon compounds form the basis
of life on Earth. Carbon chemistry (called organic chemistry) and the
function of biomolecules are explained in detail in the section on carbon
compounds.
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. 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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