Origins of Life on Earth
From: Michael Ragland (ragland37_at_webtv.net)
Date: 09/23/04
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Date: Thu, 23 Sep 2004 17:50:36 +0000 (UTC)
Comment: This is just speculation on the origins of life on "earth". I
read, "On September 28, 1969, a meteorite fell over Murchison,
Australia. While only 100 kilograms were recovered, analysis of the
meteorite has shown that it is rich with amino acids. Over 90 amino
acids have been identified by researchers to date. Nineteen of these
amino acids are found on Earth. (table showing comparison of Murchison
meteorite to Miller/Urey experiment) The early Earth is believed to be
similar to many of the asteroids and comets still roaming the galaxy. If
amino acids are able to survive in outer space under extreme conditions,
then this might suggest that amino acids were present when the Earth was
formed. More importantly, the Murchison meteorite has demonstrated that
the Earth may have acquired some of its amino acids and other organic
compounds by planetary infall."
So of the 90 amino acids found in the Murchison meteorite 19 have been
found to exist on earth. An amino acid is a small molecule [or block of
molecules] used by cells to make proteins. Certainly, however, the
discovery of 90 amino acids on Murchison doesn't constitute "life" or we
would have announced our first meeting with an extraterrestrial. For the
sake of argument let's say Earth did acquire some of its amino acids and
other organic compounds by planetary infall. But given initial
conditions on Earth these amino acids and organic compounds would remain
inactive. It would take different atmospheric conditions to activate
these amino acids and organic compounds.
Apparently the first organisms were the prokaryotic bacteria and algae
which didn't need to rely on oxygen and thrived in an atmosphere mainly
of carbon dioxide. Although it has been discredited by scientists the
Urey-Miller experiment of using reducing gases and electricity to create
amino acids impressed me. But the consensus today is Earth's early
atmosphere was not a reducing atmosphere but one filled mainly with
carbon dioxide and nitrogen. Under such conditions Urey-Miller's
experiment fails. But is it possible, as someone suggested, that in more
local environments there might be more reducing gases conducive to
repeated lightening to create life?
http://www.chem.duke.edu/~jds/ cruise_chem/Exobiology/miller.html
Miller/Urey Experiment
By the 1950s, scientists were in hot pursuit of the origin of life.
Around the world, the scientific community was examining what kind of
environment would be needed to allow life to begin. In 1953, Stanley L.
Miller and Harold C. Urey, working at the University of Chicago,
conducted an experiment which would change the approach of scientific
investigation into the origin of life.
Miller took molecules which were believed to represent the major
components of the early Earth's atmosphere and put them into a closed
system
The gases they used were methane (CH4), ammonia (NH3), hydrogen (H2),
and water (H2O). Next, he ran a continuous electric current through the
system, to simulate lightning storms believed to be common on the early
earth. Analysis of the experiment was done by chromotography. At the end
of one week, Miller observed that as much as 10-15% of the carbon was
now in the form of organic compounds. Two percent of the carbon had
formed some of the amino acids which are used to make proteins. Perhaps
most importantly, Miller's experiment showed that organic compounds such
as amino acids, which are essential to cellular life, could be made
easily under the conditions that scientists believed to be present on
the early earth. This enormous finding inspired a multitude of further
experiments.
In 1961, Juan Oro found that amino acids could be made from hydrogen
cyanide (HCN) and ammonia in an aqueous solution. He also found that his
experiment produced an amazing amount of the nucleotide base, adenine.
Adenine is of tremendous biological significance as an organic compound
because it is one of the four bases in RNA and DNA. It is also a
component of adenosine triphosphate, or ATP, which is a major energy
releasing molecule in cells. Experiments conducted later showed that the
other RNA and DNA bases could be obtained through simulated prebiotic
chemistry with a reducing atmosphere.
These discoveries created a stir within the science community.
Scientists became very optimistic that the questions about the origin of
life would be solved within a few decades. This has not been the case,
however. Instead, the investigation into life's origins seems only to
have just begun.
There has been a recent wave of skepticism concerning Miller's
experiment because it is now believed that the early earth's atmosphere
did not contain predominantly reductant molecules. Another objection is
that this experiment required a tremendous amount of energy. While it is
believed lightning storms were extremely common on the primitive Earth,
they were not continuous as the Miller/Urey experiment portrayed. Thus
it has been argued that while amino acids and other organic compounds
may have been formed, they would not have been formed in the amounts
which this experiment produced.
Many of the compounds made in the Miller/Urey experiment are known to
exist in outer space. On September 28, 1969, a meteorite fell over
Murchison, Australia. While only 100 kilograms were recovered, analysis
of the meteorite has shown that it is rich with amino acids. Over 90
amino acids have been identified by researchers to date. Nineteen of
these amino acids are found on Earth. (table showing comparison of
Murchison meteorite to Miller/Urey experiment) The early Earth is
believed to be similar to many of the asteroids and comets still roaming
the galaxy. If amino acids are able to survive in outer space under
extreme conditions, then this might suggest that amino acids were
present when the Earth was formed. More importantly, the Murchison
meteorite has demonstrated that the Earth may have acquired some of its
amino acids and other organic compounds by planetary infall.
If these compounds were not created in a reducing atmosphere here on
Earth as Miller suggested, then where did they come from? New theories
have recently been offered as alternative sites for the origin of life.
____________________________________
http://www.fortunecity.com/emachines/e11/86/comefrom.html
Where Do We Come From?
Robert Shapiro
My son had many things to play with during his childhood, but one was
special. Her name was Frizzle. She was a ball of inquisitive fur several
inches long that the pet store classified as a gerbil. Frizzle spent
much of her time in the brief few years of her life in exploring the
multichambered residence that we had prepared for her, and then in
trying to escape from it. When she died, we missed her.
For various reasons, I had no animal pet when I was young, but for a
time, I attempted to nourish a small cactus plant. Its activities were
less interesting than those of a gerbil; it grew slightly, but made no
effort to escape. Yet I was saddened when it turned gray and drooped,
and I realized that it had lost the struggle for survival.
We all learn at an early age how profoundly living things change when
they die. We also recognize that the living things we know are a small
part of a larger surrounding universe of things like water, rocks, and
the moon that are not, and have never been, alive. This wisdom belongs
to our own time, however. For centuries, many observers, including
skilled scientists, did not recognize that dead things do not become
alive. They felt, for example, that river mud could give rise to
serpents, and raw meat could give birth to worms in a process called
spontaneous generation. Only through many carefully controlled
experiments, culminating in a brilliant series carried out by Louis
Pasteur in the nineteenth century, was this theory disproven. We now
recognize that life comes only from previously existing life, like a
flame that can be divided, and spread, but once extinguished, can never
be rekindled.
How then did life first come into existence, on this planet or anywhere
else that life may exist in the universe? Many religions and some
philosophies avoid the problem by presuming that life, in the form of a
deity or other immortal being, has existed eternally. An alternative
view can be found in science, where we look for natural answers in
preference to supernatural ones and turn to another possibility: that
life arose from nonlife at least once, sometime after the creation of
the universe.
To learn about life's origins, we obviously cannot rely upon human
records or memories, and must turn instead to the evidence stored in the
earth itself. Those data are left as fossils in sediments whose age can
be deduced from the amount of radioactivity remaining in surrounding
rocks. For example, an unstable isotope of potassium is sealed into a
volcanic rock when it solidifies from lava. Half of this isotope decays
every 1.3 billion years, with part of it converted to the stable gas
argon, which remains trapped in the rocks. By measuring the amounts of
the remaining potassium isotope and the trapped argon, and performing a
simple calculation, we can determine the age of the rock.
The fossil record tells a marvellous tale of the rise of life. It starts
three and a half billion years ago with single-celled forms that
resemble bacteria and algae, and leads up to the overwhelming variety of
living creatures, including ourselves, that are present today.
Earth itself is only one billion years older than the earliest record of
life discovered thus far. If we were to compare Earth's age to that of a
sixty-year-old man, then the fraction of Earth's existence needed for
life to appear could be compared to the time needed for the man to reach
puberty. The geological time occupied by all recorded human history
would be equivalent to the last half- hour or so of the man's existence.
Unfortunately, the record of geology peters out beyond three and a half
billion years ago. No rocks remain that tell us anything about the way
in which the first bacteria-like creatures came into existence. This
process remains a profound puzzle.
A human being is so much more than a bacterium, though. If we can
understand the evolutionary process that produced ourselves from
one-celled creatures, and the developmental changes that convert a
one-celled fertilized egg into a multicelled adult, then why is it
difficult to understand how a bacteria could form from nonliving matter?
To appreciate this problem, we must explore the structure of life as it
exists today. Obviously, this topic can fill many volumes, but here I
wish to draw out a single thread that serves to distinguish life from
nonlife: Living things are highly organized. I have chosen a word that
is familiar; some scientists prefer terms like negative entropy, but
basically, the idea is the same. By using organized, I mean to describe
the quality that distinguishes the complete works of William Shakespeare
from a series of letters struck at random on a typewriter, or a symphony
from the sound one gets by dropping dishes on the floor. Clearly, things
produced by the activities of life, for example, Shakespeare's works,
can also be organized, but things unconnected with life, like a rock on
the surface of the moon, are much less so. A bacterium, when compared
with nonliving matter, is still very organized. The comparison between a
dust particle and a bacterium would then be similar to the match of a
random string of letters to a Shakespeare play.
Some scientists who have thought about life's origins have believed that
this gap in organization between living and nonliving matter could be
closed by random chance alone, if enough tries were made. They have
drawn great encouragement from a famous experiment devised by Stanley
Miller and Harold Urey. Miller and Urey showed in 1953 that certain
amino acids could be formed very easily when electrical energy is passed
through a simple mixture of gases. These amino acids are building blocks
of proteins, one of life's key components. If key chemicals can arise so
readily, can the rest of life be far behind?
To support the theory that pre-biotic methane may have played an
important role in the development of life during the Hadean, the
Miller-Urey experiments were conducted in the 1950s. These experiments
tried to replicate the environmental conditions of the primitive Earth
to see if organic compounds would form. When methane was included, the
experiments showed that complex reduced carbon compounds, such as amino
acids, were formed by non-biological processes. This seemed to prove
that complex organic molecules can build up from very simple organic
molecules, such as methane. The precise gas mixture used in the
Miller-Urey experiments has since fallen out of favor with scientists as
the most likely pre-biotic atmosphere. Still, methane could have been a
component of the Earth's early atmosphere. Unfortunately, life is vastly
more organized than the "prebiotic" chemical mixtures formed in
Miller-Urey-type experiments. Imagine that by random strokes on the
typewriter you type the words to be. You might then be reminded of the
famous line: "To be, or not to be: that is the question." A further jump
of imagination might lead you to the idea that the remainder of Hamlet
could then emerge from the random strokes. But any sober calculation of
the odds reveals that the chances of producing a play or even a sonnet
in this way are hopeless, even if every atom of material on Earth were a
typewriter that had been turning out text without interruption for the
past four and a half billion years.
Other thinkers, including a number of religious people, have argued that
the formation of life from nonliving matter by natural means is
impossible. They cite the Second Law of Thermodynamics, and claim that
the formation of organized matter from disorganized matter is forbidden
by it. But the Second Law applies only to sealed-in (closed) systems. It
does not forbid nonliving chemicals on Earth to absorb energy from an
outside source such as the sun and becoming more organized. The gain in
organization here would be balanced by a greater loss of organization in
the sun, and the Second Law would thus be satisfied.
When chemical systems absorb energy, however, they usually use it to
heat themselves up, or to form new bonds in ways that do not lead to any
gain in organization. We don't know the key recipe-the set of special
ingredients and forms of energy that could lead chemical systems up the
ladder of organization on the first steps to life. These circumstances
may be quite rare and difficult or, once you have grasped the trick, as
simple as brewing beer or fermenting wine.
How can we find out more? One way would be to run more prebiotic
experiments. Many have been run, of course, but they usually have
searched for the chemicals present in life today, rather than seeking to
identify the process of self-organization. It isn't likely that the
highly evolved biochemicals of today-proteins, nucleic acids, and other
complexities-were present during the first faltering steps to life. What
we need is more understanding of how simple chemical substances such as
minerals, soaps, and components of the air will behave when exposed to
an abundant and continuing supply of energy, such as ultraviolet light.
Would the material simply turn into a tar or dissipate the energy as
heat? Most of the time this would happen, and little of importance would
have been learned. But perhaps, if the right mixture were chosen,
complex chemical cycles would establish themselves and continue to
evolve. If so, we would have gained an important clue about the
beginning of life. Some experiments of this type could be carried out in
undergraduate and even high-school laboratories, as they would not
require complex and costly equipment. This area remains one where
amateurs could make a significant contribution to fundamental science.
Another scientific approach to seeking our origins is very expensive,
but it would provide great excitement and inspiration along the way. In
the past generation, we have developed the ability to explore our solar
system, but not good enough reasons to ensure that this is done. By that
I mean objectives that would capture the attention of the public and
motivate them to support the costs, rather than ones of interest only to
scientists deeply immersed in their specialities. The solar system
offers a dazzling array of worlds, each containing different chemical
systems that have been exposed to energy for billions of years. Some of
them may have developed in the direction of organization.
By discovering a system that has started on such a path, even a
different path from the one followed on our own planet, we may get vital
clues about the principles involved in self-organization and the nature
of our own first steps. A treasure hunt of this type among the worlds
surrounding our sun might or might not discover early forms of life, but
would certainly put some life back into our space program.
"Where do we come from?" In my title, I put the origin of life in terms
of a question of location, as a child might do. Some scientists have
argued that life started elsewhere, then migrated to planet Earth. Even
if this were so, that fact would still not solve the central question,
which is one of mechanism: "How did we come to be?" Location may be
critical, though, in another way: To learn how we started, even if it
happened here, we may have to venture out into the greater universe that
awaits us.
ROBERT SHAPIRO is professor of chemistry at New York University. He is
author or coauthor of over ninety articles, primarily in the area of DNA
chemistry. In particular, he and his coworkers have studied the ways in
which environmental chemicals can damage our hereditary material,
causing changes that can lead to mutations and cancer. His research has
been supported by numerous grants from the National Institutes of
Health; the Department of Energy; the National Science Foundation; and
other organizations.
In addition to his research, Professor Shapiro has written three books
for the general public. The topics have included the extent of life in
the universe (Life Beyond Earth, with Gerald Feinberg); the origin of
life on earth (Origins: A Skeptic's Guide to the Creation of Life on
Earth); and the current effort to read the human genetic message (The
Human Blueprint).
____________________________________
http://www.wbateman.demon.co.uk/gcse2003/
gcsesums/chemsums/earth/earth.htm
The Earth and its atmosphere
Return to GCSE science
C6.08 The composition of the atmosphere
The atmosphere is made up of the following gases, Nitrogen 78%, Oxygen
21%, Argon 1%, Carbon dioxide 0.03%, water - variable
C6.09 Earth's early atmosphere and volcanoes
The primary atmosphere of the Earth was hydrogen and helium. These light
gases were slowly lost. They were replaced by a secondary atmosphere
produced by the action of volcanoes.
C6.10 Composition of the Earth's early atmosphere
The Earth's secondary atmosphere was made up of some left over hydrogen,
carbon dioxide, water vapour, nitrogen, carbon monoxide, sulphur dioxide
ammonia and methane. (apparently primarily carbon dioxide)
C6.11 Origin of the oceans
As the Earth cooled to below 100oC oceans were formed when water vapour
condensed and formed liquid water. Oceans are reservoirs for carbon
dioxide because they can store the gas when it dissolves in them.
The new oceans dissolved a great deal of the carbon dioxide in the
atmosphere. The oceans still play a part in keeping carbon dioxide
levels constant. If there is a lot of carbon dioxide in the air then
more can dissolve. If there is less carbon dioxide in the air then
some comes out of solution back into the air.
C6 12 The release of oxygen into the atmosphere
As the temperature of the Earth cooled simple green plants evolved in
the oceans to use the carbon dioxide in the environment. These green
plants steadily removed carbon dioxide and produced oxygen by
photosynthesis. Oxygen levels in the atmosphere slowly increased.
C6.13 The carbon cycle
The carbon cycle helps to keep the atmospheric composition constant by
adding carbon dioxide to the atmosphere and also taking carbon dioxide
away from the atmosphere. Carbon dioxide is taken away from the
atmosphere or out of the cycle by photosynthesis, dissolving in water
and by chemical reactions, for example with rock. It is brought into the
atmosphere or into the cycle by respiration, combustion, volcanic
activity and decay.
C6.14 Formation of igneous rocks
Igneous rocks are formed when magma pushes up into the crust and
cools. It is made up of crystals It does not contain any fossils.
Any living thing falling into the molten rock, from which it is made
would be burnt and leave no trace. Igneous rock forms as magma cools
slowly under the surface e.g. Granite. Magma reaching the surface
through a volcano cools quickly e.g. basalt. Rock cycle presentation
(needs Powerpoint)
C6.15 Crystal size and igneous rock
Igneous rocks which cool slowly have large crystals e.g. granite but
rock forming quickly has smaller crystals e.g. balsalt.
C6.16 The formation of sedimentary rock
This rock is formed in shallow seas. After long periods of time
sediment layers pile up and the lower ones come under great pressure.
This pressure pushes the water out of the layers or sediments and salt
crystallizes and sticks the particles together to form sedimentary
rocks.
This process is called lithification. Living things falling into the
sediments leave an impression as the rock forms (a fossil). Fossils
show that a rock was made from sediments.
C6.17 Dating sedimentary rock
Sedimentary rock forms in layers. New sediments are layered on top of
old ones so the age of sedimentary rock increases with depth. Species
become extinct and new species take their place so fossils of different
species are of known age can be used to date the rock.
C6.18 Formation of metamorphic rock
Metamorphic rocks are formed by the action of heat and pressure over
long periods of time on rocks that are already formed. Earth movements
push all types of rock underground, here they are compressed and heated
and the mineral structure and texture may change. For example marble is
formed from limestone.
C6.19 The composition of metamorphic rock
Metamorphic rock has the same chemicals in it as the rock that it was
made from. Limestone is made of calcium carbonate. Marble is a
metamorphic rock made from limestone. It is also made of calcium
carbonate. Mudstone in a sedimentary rock which turns into the
metamorphic rock called slate. Both mudstone and slate contain the
same clay like minerals.
_____________________________________
http://www.ldeo.columbia.edu/edu/ dees/ees/climate/lectures/earth.html
Early Earth probably had an atmosphere dominated by carbon dioxide
similar to the atmosphere of Venus today.
There are a group of one-celled organisms that can live in an oxygen
free environment. These are the bacteria or prokaryotes. They do not
have a nucleus and reproduce only by cell division. These creatures are
the earliest evidence of life on earth. They were the first organisms to
develop photosynthesis. Photosynthesis today is balanced by oxygen using
respiration.
Hypothesis: Oxygen was nearly absent in the atmosphere of early Earth so
photosynthesis would have created a net gain of oxygen first in the
ocean and later in the atmosphere.
Eventually with sufficient oxygen in the atmosphere respiration would
have balanced photosynthesis except when burial removed the organic
material from the oxygenated water or air. Before oxygen could build up
in the atmosphere it must have oxidized reduced ions in seawater.
Evidence to support the above hypothesis:
Iron (Fe) is a very abundant element in the earth's crust so much is
released by the chemical disintegration of minerals contained in rocks.
Fe++ is slightly soluble in seawater while Fe+++ is insoluble (Figure
6). During the time when the earth had a reducing atmosphere Fe++ should
have accumulated as dissolved ions in seawater. However at some point
the oxygen build-up in the ocean from prokaryote photosynthesis should
have oxidized the Fe++ to Fe+++ resulting in the precipitation of
insoluble iron compounds. Are such ancient iron rich compounds
preserved? Yes there are, in fact the bulk of the iron ore mined to
produce steel comes from iron deposits that are about two billion years
old (Figure 7). Such deposits are found on all continents and all look
much the same (Figure 8). They are reddish and have clearly visible
bands hence they are called Banded Iron Formations. The Messabi range of
Minnesota is an example of such a deposit. It was for much of US history
the primary source of iron ore for the steel mills of Pittsburg,
Pennsylvania and Gary, Indiana. If we know the mass of these banded iron
formations and the rate at which we mine them we can calculate their
residence time and determine how long they will last, or when we will
run out of this kind of iron ore (Figure 9).
A second line of evidence, to suggest that the early earth had a
reducing atmosphere like Venus and Mars, is the presence of detrital
(formed from the products of erosion of pre-existing rocks) pyrite in
sedimentary deposits older than two billion years old. Iron pyrite forms
in reducing environment and is quickly chemically decomposed in the
presence of oxygen. Today such minerals are only preserved in rocks that
formed in reducing environments such as swamps etc. However, in rocks
older than two billion years old this mineral (iron pyrite) is found in
rocks that were probably formed in streambeds.
The possible changing composition of the Earth's atmosphere during its
early history is shown in Figure 10. All nucleated cells (Eucaryote
cells) require oxygen for metabolism. We and all other plants and
animals are built of eukaryotic cells so we all require oxygen. Hence
early primitive life (procaryote cells) modified our planet by
converting CO2 and H2O to organic matter and releasing oxygen to the
environment. As a consequence these organisms moved carbon from the
atmosphere to the rocks (Figure 11) and broke down water molecules
releasing oxygen to the ocean and eventually to the atmosphere. Life
therefore is a powerful force controlling the composition of the Earth's
atmosphere which in turn exerts a powerful control on our planet's
climate.
Text by James D. Hays, Spring 2004.
"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well without it."
Stephen Hawking
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