The Beginning of life on Earth
The age of the Earth
The age of the Earth has been estimated at approximately 4 700 million (4.7 billion)
years old. At this time the cooling gases around our Sun began to accrete (gather and
build upon each other) and the planet Earth was formed.
Conditions on early Earth
For the first billion years of its life, the Earth existed as a relatively cool and
homogenous mass of silicon compounds; iron and magnesium oxides. These materials
were distributed relatively evenly throughout the Earth’s interior. It has been
estimated that the average temperature reached as high as 1000°C during the early
stages of the Earth’s development. The Earth was molten at this time and it took 600
to 800 million years for the crust to solidify.
Between 3500 and 4000 million years ago the crust cooled to less than the boiling
point of water. At this point rain formed and the oceans and an atmosphere formed
from volcanic emissions. The volcanic emissions filled the atmosphere with water
vapour (H2O), methane (an organic molecule CH4), carbon monoxide (CO), carbon
dioxide (CO2), ammonia (NH3), hydrogen (H2) and hydrogen sulfide (H2S). This
was a reducing atmosphere. Importantly there was no free oxygen. Without free
oxygen there could be no ozone layer and ultraviolet radiation hit the Earth’s
surface at a greater rate than today. At this time there were also violent electrical
storms and acidic rain fell forming the warm mineral-rich oceans. Erosion of the
surface began and the temperature of the surface dropped below 100°C.
Comparison with present day conditions
In contrast, today’s atmosphere consists principally of nitrogen (N2); oxygen (O2)
which is an oxidising agent; small amounts of argon; variable amounts of water
vapour; and carbon dioxide.
Organic molecules
The conditions on the early Earth were right for the build up of the available materials
into more complex molecules. The combination of the right conditions and the
presence of the chemical elements necessary for life was the first step in the evolution
of life. On the Earth today life would use up these organic molecules as soon as they
were formed so once life is around it starts to prevent the build up of organic
molecules.
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Simple organic molecules
Complex organic molecules, such as those that make up your body, are made from
simpler organic molecules. These simple organic molecules contain the elements
carbon, hydrogen, oxygen, nitrogen with smaller amounts of phosphorous and sulfur.
They are known as the building blocks of life and consist of:
• amino acids
• nucleic acid bases
• sugars and
• phosphoric acid.
All of these molecules are organic molecules (the chemicals in living systems.)
Amino acids consist of a basic structure containing nitrogen, carbon, oxygen and
hydrogen. They are commonly referred to as the building blocks of life because they
combine in different arrangements to form different types of proteins. Proteins in turn
are responsible for the formation, repair and function of all cells in living organisms.
There are only 20 different amino acids and these are assembled into thousands of
different proteins. The human body, for example contains about 150 000 different
proteins.
Sugars are very important in the formation of nucleic acids such as RNA and DNA.
These are formed when sugars link with phosphates to form a nucleotide with a
nucleic acid base.
How does the knowledge of the chemistry of living organisms relate to the
conditions of early Earth?
The molecules produced in a reducing atmosphere, such as the early atmosphere, form
the basic elements of living cells: membranes, proteins, RNA and DNA. So for life to
arise you needed an atmosphere that existed on Earth millions of years ago.
The next step in the origin life is for the simple organic molecules to build up to more
complex organic molecules. That is, amino acids into proteins and nucleic bases,
sugars and phosphoric acid into DNA and RNA. This is sometimes called chemical
evolution. But for this to occur where did the organic molecules come from?
Organic molecules in the cosmos
There are two possible sources for organic molecules. They can be:
• made on the Earth from simpler molecules
• arrive from space on meteorites.
Earth is not the only place in the cosmos (Universe) where organic molecules have
been found. Astronomers and biologists agree that the key elements or molecules
required to initiate life may have arrived in meteorites. This is the theory of
Panspermia (meaning seeds everywhere.)
Certain types of meteorites called carbonaceous chondrites contain a variety of
carbon-rich compounds. It has been suggested that it may be possible for these
compounds to form shapes similar to simple organisms. A chondritic meteorite that
fell to Earth in South Australia in 1969, has been found to contain amino acids. Some
of the rock samples from Mars arguably contain fossils of bacteria. The presence of
these elements is not enough to sustain life or form organic molecules. This is evident
from the fact that most of these elements have been found on the Moon, yet no life
has been detected.
It was the particular conditions found on the Earth especially the presence of liquid
water that allowed for chemical evolution to occur. It does however lead to the
fascinating prospect of life originating in other parts of the cosmos and evolution
producing organisms vastly different from the life forms on this planet.
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Theories about the evolution of life
The chemicals for life were available and organic molecules could build up on the
early Earth but more than this is required for life to form. How did life start? Here are
some theories.
Spontaneous generation
During the Middle Ages it was believed that living organisms could arise
spontaneously and survive to replicate or even transform from other living things.
This appearance of life was called spontaneous generation. This theory was well
supported in the Middle ages using the transformation of caterpillars into moths or
butterflies, tadpoles into frogs and maggots emerging from decaying matter as
examples to justify this belief.
Scientists began to question this theory. However, it was not until Louis Pasteur
(1822-1895), a French scientist, provided evidence to show conclusively that this was
not the case. Louis Pasteur performed experiments to demonstrate that life did not
spontaneously generate from non-living materials.
However just because life doesn’t arise spontaneously now does not mean that it did
not happen during the time of chemical evolution on the early Earth.
Synthesis of organic molecules
It is very difficult to recreate the conditions that would have been present on the Earth
at the time of its formation. However scientists have tried, through experiments, to
create the conditions that prevailed at the time. There are three views on where life
developed on the early Earth. These are:
• tidal pools producing froth
• Panspermia
• deep-sea thermal vents (black smokers).
Some of the experiments that were carried out to illustrate the possible mechanism for
the formation of simple organic molecules are outlined below.
Charles Darwin had speculated about the possibility of life originating in warm,
phosphate-rich water, however this was not seriously pursued until earlier in the 20th
century.
Haldane and Oparin
J B S Haldane, a Scottish biochemist, and A P Oparin, a Russian scientist began
working on the possible theories for the formation of organic molecules. Haldane and
Oparin proposed that soon after the formation of the Earth, the conditions were very
different from the present, as mentioned earlier. These conditions were thought to be
suitable for the precursors of living molecules (amino acids, sugars and nucleotide
bases), to form from the hydrogen based molecules of ammonia, methane and water.
The formation of these precursor molecules was possible due to the absence of
oxygen and an abundance of energy from electrical discharges (lightning), ultraviolet
light, heat and radiation. There were no organisms present to destroy the
spontaneously formed molecules, so they were able to accumulate in the water
(‘seas’) present. Haldane and Oparin suggested that continued synthesis and
increasing concentrations led to the formation of polypeptides from amino acids and
eventually to a diverse range of molecules including enzymes and other proteins.
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Oparin continued to work on what was called the heterotrophic hypothesis, in 1924.
This hypothesis was based on the premise that the first organisms were heterotrophic,
meaning they obtained their organic material from the environment, rather than
producing it like an autotrophic organism, such as cyanobacteria (previously known
as blue-green algae.)
Oparin went on to describe the idea of a reducing atmosphere in which it is possible to
produce organic compounds. A reducing atmosphere is one that contains methane,
ammonia, hydrogen and water. Such an atmosphere was thought to have been present
on early Earth. This theory was not tested until the middle of the 20th century.
Urey and Miller
In 1953, S L Miller and H C Urey, who were both chemists, carried out an
experimental simulation of the primitive conditions proposed by Haldane and Oparin.
The device used (pictured below) circulated a mixture of steam (water vapour),
ammonia, methane and hydrogen through liquid water forming a solution. This
mimicked the conditions on the early Earth. A high-energy electrical discharge (to
represent the frequent lightning) was sparked through the solution forming a number
of amino and hydroxy acids, which are vital components or building blocks of living
things. After a few days organic compounds such as amino acids could be collected.
These compounds were obtained only when oxygen gas was not present in the
reacting vessel.
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The results of Urey and Miller’s experiments have lead to further research and
experimentation by other scientists. It is an important experiment because it shows
that amino acids can be produced by more simple raw materials.
The relevance of Urey and Miller’s experiments have been questioned, as it is thought
that the experimental atmospheric conditions used may not be a true reflection of
those that existed on the early Earth. This had lead to other scientists trying different
combinations of raw materials and using ultraviolet radiation as a source of energy.
This has resulted in building different amino acids.
The nature and practice of Science
The experiment of Urey and Miller illustrates the nature and practice of Science.
Science is self-correcting. The knowledge available is used to form a hypothesis.
When errors are found through new knowledge a new hypothesis is formed that
accounts for the new information. What is believed to be true at the moment may be
different in two years time. Over time Science corrects its mistakes and builds
stronger understandings of the world. Religions differ form this because they are
not self-correcting and rely on faith. During the Middle Ages when it was thought that
rats where made by rubbish and maggots came from rotten meat (spontaneous
generation) there was no experimentation to test this idea it was accepted by
everyone. In contrast to this Urey and Miller set out to find an answer by observation
and experimentation. Since Urey and Millers’s experiment there have been further
experiments and different theories for the origin of life have occurred.
Technological advances
As new technology becomes available there is a development in the understanding of
the origin of life and evolution of living things. As it became possible to identify
chemicals and molecular structure this gave evidence to support the action of organic
compounds. Other changes in technology have made it possible to accurate date
fossils and piece together a more complete picture. These include:
• electron microscopes
• spacecraft
• deep sea exploration vessels
• carbon dating
• molecular clock.
Electron microscopes
The invention of the electron microscope has meant that microfossils within rocks can
now be seen. This has extended the evidence of live on Earth and also may identify
fossils from other planets.
Spacecraft
Sending spacecraft to other planets makes it possible to examine conditions on these
planets to compare to the conditions on early Earth.
Deep sea exploration vessels
The presence of deep-sea vents was unknown until vessels were developed that could
travel to the depths of the oceans. Once this occurred it was discovered that there were
many unknown organisms living on the vents and using the energy from the vents as a
basis for an ecosystem.
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Radioactive carbon dating
When rocks are dated, their ages are referred to as either numerical or relative.
Numerical age refers to a specific number of years, such as 200 million years ago
(mya). Numerical age is sometimes called absolute age. Relative age provides a
comparison between rocks, in other words it states whether one rock formation is
older than another rock formation.
The numerical age of a rock comes from the decay of radioactive isotopes, resulting
in new (daughter) isotopes that are more stable. This process of change is called
radioactive decay. The unstable radioactive element is also called the parent material,
and the stable non-radioactive element produced from the decay of the parent material
is known as the daughter product. Therefore uranium will decay to lead and potassium
will decay to argon.
Using the known behavior of many isotopes, the age of rock can be calculated using
two methods:
• counting the number of new (daughter) isotopes
• using the known decay rate to calculate the length of time required to
produce a specific number of new isotopes.
Another way of determining the numerical age of rocks is by using carbon-14 (an
isotope of the element carbon). It is a radioactive isotope that is continually produced
in the atmosphere by the action of cosmic rays breaking down atmospheric nitrogen.
The carbon combines with oxygen to form carbon monoxide or carbon dioxide. It is
then absorbed by living things, so that all living things containing some carbon-14.
While an organism is alive, the decaying radiocarbon is continually replaced. This
means that the ratio of carbon-12 to carbon-14 remains constant. When the organism
dies, the amount of carbon-14 decreases as it decays into nitrogen-14. The age of a
sample is derived by comparing the proportions of carbon-12 to carbon-14.
This method of dating has been adopted for determining the ages of samples from the
last small fraction of geologic time as the amount remaining eventually is too small to
determine.
The molecular clock
Another technological advance that has had a huge impact on scientific thinking is the
molecular clock.
Biologists use a number of molecules, such as DNA, to determine similarities and
differences between organisms. The differences in the molecules can be said to
measure how long the organisms have been evolving independently of each other.
This method of comparison assumes however, that the rate of molecular change is
constant and measurable.
These claims are based on the assumption that the molecular clock, the rate of genetic
change any given species undergoes, is relatively constant over time. Thus, a
measurement of the genetic difference between any pair of species would reveal how
long ago their ancestral lineages went their separate ways..
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Exercise 1.1: Organic molecules and life
a) Describe the conditions on the early Earth at the time that chemical evolution of
organic molecules was occurring.
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b) For life to begin there must be a supply of organic molecules. Name two possible
sources of organic molecules.
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c) Using scientific information present one argument to support and one argument to
refute the statement below:
‘Life on Earth came from outer space.’
The statement can be supported
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The statement can be refuted because
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d) Organic molecules have been found on meteorites arriving on Earth. What are the
implications of the existence of organic molecules in the cosmos for the origin of life
on Earth?
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Exercise 1.2: Synthesis of organic molecules
1 Two scientific theories relating to the evolution of the chemicals of life and the
synthesis of organic molecules come from Haldane and Oparin and Urey and Miller.
Discuss the significance of both of these theories to the understanding the origin of
life on Earth.
Haldane and Oparin
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Urey and Miller
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2 Using information in this part and additional material you have
gathered from secondary sources analyse the experiments of Urey
and Miller.
a) What was the reason for their experiments?
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b) Outline the result of their experiments.
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c) How do the Urey and Miller experiments differ in application of
the nature and practice of science, compared to theories such as
spontaneous generation proposed in the Middle Ages?
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d) How do Urey and Miller’s experiments aid understanding about
the possible origins of life?
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Exercise 1.3: Technological advances
How has technology assisted in understanding the origin of life and
evolution of living things?
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Below is the theory of one Australian scientist based on recent information.
Where life originated
The theories on where life originated come down to two. It either originated external
to the Earth or it originated on the Earth. The evidence for the external origin comes
mainly from the location of organic molecules in chunks of rock known as meteorites
that have been found on Earth’s surface. No one seriously suggests that life ever
existed on these chunks of rock, only that organic molecules somehow or other were
produced on these chunks of rock by chemical processes. These extraterrestrial
organic molecules are always small molecules. The type called building block sized
molecules. Strangely these same chunks of rock also suggest that the big chunk of
rock known as the Earth probably had organic molecules present as a result of its
formation and chemical processes occurring on its surface layer. Similar chemical
processes would therefore have operated to ensure the presence of small organic
molecules on a primitive and lifeless Earth. Nobody ever disputes the idea that the
surface of the early Earth was an unpleasant and hostile place. The chance of organic
molecules being given time and stability to join together into the larger structured
molecules and to approach a size required favour the creation of life on the nprotected
surface of the Earth was almost certainly not good. The protective feature of the
atmosphere was under constant assault, being blown away almost entirely on
numerous occasions by the constant bombardment of extraterrestrial objects during
the days of the Early Solar System. The stability of the exposed surface was low. The
atmosphere was reactive; rain was almost certainly constant near coasts and would
have been rare inland with no mitigating climate controls as provided by life today.
The atmosphere was not able to protect from the UV rays emitted by a young more
dense hotter Sun. Damage to organic molecules forming at the surface would have
been horrendous. Mutations were not a possibility at this time, they only occur after
the evolution of life. Few if any environments other than hot springs on the surface
would have been able to supply the constant chemical and physical raw materials
necessary to allow the production of life molecules or life. The physical barrier of
distance in a hostile environment would mean if life evolved in a hot spring it was
trapped, unable to make it easily to an adjacent hot spring. The problem was building
the precursor molecules able to be spontaneously assembled into an order called
life. Time was essential. The life of most hot springs is short in a geological sense.
For that reason the most probable location for the initial growth of the precursor life
molecules was deep in the protective cover of the ocean. Luckily deep-sea exploration
now confirms the existence of chemosynthesis as a life supporting mechanism
especially around sites of active volcanism. The deep sea food webs are well
documented, photographed and the subject of intense study. That’s sexy science. On
the frontier of knowledge, evasive and therefore desirable. A challenge. The similarity
of life across vast separation distances also clearly shows that the ocean enables the
movement of life from one hot spring to another under the protection of the ocean.
The question arises then was it possible for conditions to exist at the ocean depths that
would enable life building molecules to come into being at the bottom of an ancient
ocean. The answer not surprisingly is yes. Life is made up of larger molecules built
from smaller molecules. You are no different to any other life form. You are made up
of large molecules built from smaller molecules produced through digestion of
larger molecules to form those small molecules which are then used to make up
required larger molecules using a template. That template has the name DNA, a self-
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replicating molecule. Without the organic template nature would have had to develop
its own template from an inorganic source. The most likely template was a clay
mineral. Clay minerals and the precursor feldspar minerals that weather and are
hydrated to form them abound on the floor of the ocean. The ocean floor is
predominantly basalt rock rich in feldspar. The opportunity for the feldspar to weather
and form clays is constant. The eruption of small organic molecules dissolved in
water is constant from deep-sea volcanic vents. The presence of ammonia necessary
for the formation of amino acids is guaranteed in such environments. The solution of
this mix of dissolved chemicals is a guarantee that the molecules will come into
contact under high pressures due to ocean depth and high but not excessive,
temperatures. Put that mix of chemicals near a clay inorganic template where the
catalysis of larger building blocks of life could occur in the constant presence of
active catalysts such as copper, nickel and platinum group elements erupted from
volcanic vents in the solutions of hot water and you have an ideal recipe to build
organic molecules that are larger and new. These physical conditions are all
documented to enhance chemical reaction that can lead to the growth of new organic
molecules spontaneously because of the chemistry. The question of time arises. How
long did the evolution of organic molecules and life take? How many failed attempts.
The answer is probably an astronomical number of attempts in a huge number of
locations simultaneously. No one expects that life evolved rapidly from an organic
soup. Almost everyone could imagine that an extended period of time during which
protection from hostile conditions near the surface of the Earth would be required to
allow life to begin. Life also does not remain stuck on the bottom of the ocean. It
currently occupies a diverse range of environments. Once evolved it required a
mechanism to slowly rise toward the surface from its deep sea cradle. The obvious
choice of slowly bringing life to the surface while maintaining its contact with the
volcanic vents supplying its energy source is a hotspot volcano environment. In the
primitive Earth volcanism across the surface was much more common that today.
The Earth was venting excess heat produced from its formation. Much of that
volcanic activity was probably erupting from hotspots, volcanoes slowly over millions
of years reaching for the surface of an already formed ocean from the bottom of the
sea. Life simply originated on the sides of such a volcano where conditions were
optimal and was brought to the surface by geological processes. Plate tectonic
processes ensured life’s distribution was global and eventually covered the entirety of
the planet. The slow rise of a volcano from the depths and its equally slow retreat
back to ocean depths as the volcano moved off the hotspots ensured that any life
evolved on the sides of that volcano was given ample time to evolve to suit changing
conditions. Selection was inherent in life from the beginning. A slowly evolving
environment ensured life diversity and a variety of habitats. The longlife of hotspots
in today’s oceans ensures that similar volcanic chains in ancient oceans provided
many opportunities for life to jump from one mature volcano to the juvenile volcano
next to it in the sub sea chain before the repeating of that volcano’s struggle to reach
the surface. The repetitive cycle of sea mountain building and decline put life on the
entire surface of the Earth.
Dr Ric Morante 05/05/2003 ©LMP
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