The Origin of Life
Spontaneous Generation:
Prior to the 17th century, it was generally believed that living organisms arose from nonliving matter. People thought that maggots were produced from raw meat that was
allowed to rot and that rats were produced from various kinds of refuse. A series of
carefully controlled experiments gradually disproved these theories. For example, the
Italian scientist Francesco Redi showed that maggots were not produced when decaying
meat was kept under a screen. This was because flies could not lay their eggs on the
meat .
The theory of spontaneous generation is now a thing of the past. Or is it? Certainly no
reasonable scientist believes that it is a common occurrence. But if spontaneous
generation, the creation of life from non-living matter, is impossible then the following
question has to be asked:
“If life can only arise from other living things, how did the very first life form on
earth come to be?”
Possible Theories
1) Creation by a Supreme Being
2) Life Came to into Contact with the Earth from an Outside Source
3) Spontaneous Generation
Theory #1 is widely believed by many different religions and constitutes the type of
theory that, while difficult to disprove, is also difficult, if not impossible to prove using
science. Theory two is also a possibility and while this may explain how life came to
exist on earth, it does not explain the bigger question. Namely, how did life originate?
Thus, 20th century scientists wishing to discover how life first came to be were forced to
re-examine the theory of spontaneous generation.
The Simplest Life Form
If the theory of evolution is to be believed, the first life forms were extremely simple
compared to the type of life that exists today. As you have already learned, in order for
something to be considered living, it must have several characteristics. These include the
ability to:
 Produce similar offspring
 Grow and repair itself
 Respond to its environment
 Convert fuel into a useful form of energy that it can use
So the simplest life form must have some type of structural component (amino acids),
some way of passing on information about itself to its offspring (nucleic acids) and must
be able to use some type of fuel to satisfy its energy needs (glucose).
Simply put, the very first organisms were probably little more than a protein shell with a
genetic code inside.
How Were the First Proteins Formed?
It is believed that the earth’s early atmosphere consisted mainly of methane (CH4),
ammonia (NH3), water (H2O) and hydrogen. In 1953, Stanley Miller and Harold Urey
conducted an experiment where they recreated the conditions that existed in the earth’s
early atmosphere. They placed these gases in a closed system (see below) and ran a
continuous electric current through it. This was done to simulate the frequent and
powerful electrical storms that were believed to have occurred on the early earth.
After one week, Miller and Urey analysed the results of their experiment and found that a
large number of simple amino acids were formed. This provided strong evidence that the
atmosphere of the early earth was conducive to the formation of the basic building blocks
of life.
How Did Amino Acids Aggregate to Form Proteins?
An amino acid alone is not a living thing. If a living organism were a house, an amino
acid would be a brick. You would need many of them to come together in the right way
to make a living thing. And so the only way for this house to built would be for these
amino acids to randomly collide together and combine in just the right way. This seems
like a highly unlikely scenario. Fortunately, there were two things that worked in life’s
favour on the early earth. These were:
1. Time and
2. Lack of oxygen
First off, molecules floating around in the sea that was the earth almost 4 billlion years
ago had “all the time in the world”. It may have taken a million years for two amino acids
to collide together in just the right way, and then another million years for another amino
acid to collide with the first two. But there was no hurry because of factor 2, lack of
oxygen. Matter breaks down in the atmosphere today because of oxygen. Oxygen makes
metal rust, plants decompose and food go rotten. Since there was no oxygen, once amino
acids formed, they would not break down. And so they remained in the earth’s waters
while more and more were formed. Eventually the waters became so full of amino acids
that the chances of them colliding increased significantly.
In 1961, Juan Oro found that the nucleotide base, adenine could be produced from
hydrogen cyanide and ammonia in an aqueous solution. 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 by simulating the early earth’s atmosphere.
And so biologists believe that amino acids and nucleic acids slowly but inevitably formed
over hundreds of millions of years until one moment, approximately 3.5 billion years ago,
a combination of amino acids and nucleic acids formed that enabled this thing to survive
and make a copy of itself.
THE FIRST LIVING THING!
This may have happened only one time in the history of the world. And so all life from
amoebas to pine trees to sharks to humans arose from this one life form.
It should be noted that this theory is far from universally accepted. Many biologists
disagree with Miller and Urey’s methods and believe that the early earth’s atmosphere
was much different than what they used in their experiment.
So while it is controversial, it is still a solid theory that is backed up by some
experimental evidence.
Questions:
1. List the three major theories of the origin of life on earth.
i)
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ii)
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iii)
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2. Explain why scientists prefer to study the theory of spontaneous generation over
the others.
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3. Describe the characteristics of the first life form on earth.
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4. Explain why DNA or RNA are crucial components of any life form.
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5. What compounds did Miller and Urey use to simulate the early earth’s
atmosphere?
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6. Why did they administer electrical shocks to their experimental set up?
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7. Comment on the following statement:
“The true origin of life on earth with never be known for certain”
(i.e. do you agree with the statement? Why or why not?)
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8. It is generally felt that a virus is the simplest life form yet evolutionary biologists
believe that the first life forms on earth resembled bacteria. Why do you think
they have this belief?
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