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History of Life on Earth
Chapter 19
Section 1: How Did Life Begin?
Section 1
History of Life on Earth
Section 1
Key Ideas
• What did the Miller-Urey experiment show about the
formation of the basic molecules of life?
• What are two theories that propose where the building
blocks of life originated in early Earth?
• How could molecules have become packaged into cells
that contain heritable cellular instructions?
History of Life on Earth
Section 1
The Basic Chemicals of Life
• Oparin and Haldane
– Suggested early oceans contained organic molecules
– 1920’s
• Molecules formed spontaneously
– Energy from sun, volcanoes, lightning
• Hypothesized early atmosphere contained H2, H2O, NH3,
and CH4.
• The hypothesis was tested in the 1950s by American
scientists Harold Urey and Stanley Miller.
History of Life on Earth
Section 1
The Basic Chemicals of Life, continued
The Miller-Urey Experiment
• In this experiment, Miller placed hydrogencontaining gases into a device.
• He used electrical sparks to simulate lightning.
• Miller found organic molecules in his device,
including some of life’s basic building blocks:
amino acids, fatty acids, and other hydrocarbons
History of Life on Earth
Miller-Urey Experiment
Section 1
History of Life on Earth
Section 1
The Basic Chemicals of Life, continued
• The Miller-Urey experiment
– organic molecules could form from inorganic molecules.
• Some molecules used in Miller’s experiment could not
have existed in abundance on early Earth.
• Early Earth did not have a protective ozone layer.
History of Life on Earth
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The Basic Chemicals of Life, continued
• UV radiation from the sun would have destroyed
any methane and ammonia in the atmosphere.
• Lack of CH4 and NH3
– Key organic compounds were not made
• However, the experiment shows that complex
biological compounds can form from simple
building blocks.
History of Life on Earth
Section 1
Life’s Building Blocks
• One hypothesis
– early biological molecules formed close to
hydrothermal vents.
– protected from harmful solar radiation.
• Another hypothesis
– arrived on Earth in meteorites or comets.
– Some meteorites contain amino acids.
– could have arrived on early Earth, when frequent
meteorite impacts were common.
History of Life on Earth
Section 1
The First Cells
• lipids tend to combine in water.
• Certain lipids can form a tiny droplet
– resembles a cell membrane.
• short chains of amino acids can form tiny
spherical structures called microspheres.
History of Life on Earth
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The First Cells, continued
• Microspheres
– may have been the first step toward cellular organization.
– could not be considered cells unless they had the characteristic of
heredity.
• In the laboratory, scientists have not been able to make proteins or
DNA form spontaneously in water.
• They have been able to form short chains of RNA, the nucleic acid
that helps to carry out the instructions of DNA.
• RNA molecules may have been the first self-replicating molecule.
History of Life on Earth
Section 1
The First Cells, continued
• Cech and Altman found a type of RNA molecule, called a
ribozyme, that can act like an enzyme.
• They also showed that RNA can form spontaneously in
water, without DNA.
History of Life on Earth
Section 1
The First Cells, continued
• Cech and Altman further hypothesized that RNA could
have changed—evolved—from one generation to the
next.
• Scientists hypothesize that DNA and proteins eventually
took over these roles in the cell.
History of Life on Earth
Section 1
Summary
• The Miller-Urey experiment showed that, under certain
circumstances, organic compounds could form from
inorganic molecules.
• Among the scientific theories that address the origin of
life, one suggests that life began close to hydrothermal
vents, and another proposes that organic molecules
arrived on early Earth from a meteorite.
• The formation of microspheres might have been the first
step toward cellular organization.