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12.3 Origin of Life
KEY CONCEPT
The origin of life on Earth remains a puzzle.
What is Evolution
12.3 Origin of Life
Earth was very different billions of years ago.
• There have been many hypotheses of Earth’s origins.
• The most widely accepted hypothesis of Earth’s origins is
the nebula hypothesis.
12.3 Origin of Life
Several sets of hypotheses propose how life began on
Earth.
• There are two organic molecule hypotheses.
– Miller-Urey experiment
electrodes
“atmosphere”
water
“ocean”
heat source
– meteorite hypothesis
amino acids
12.3 Origin of Life
• There are different hypotheses of early cell structure.
– iron-sulfide bubbles hypothesis
12.3 Origin of Life
• There are different hypotheses of early cell structure.
– lipid membrane hypothesis
12.3 Origin of Life
• A hypothesis proposes that RNA was the first genetic
material.
– Ribozymes are RNA
molecules that catalyze
their own replication.
– DNA needs enzymes to
replicate itself.
12.3 Origin of Life
KEY CONCEPT
Single-celled organisms existed 3.8 billion years ago.
12.3 Origin of Life
Microbes have changed the physical and chemical
composition of Earth.
• The oldest known fossils are a group of marine
cyanobacteria.
– prokaryotic cells
– added oxygen to
atmosphere
– deposited minerals
12.3 Origin of Life
• Fossil stromatolites provide evidence of early colonies of
life.
12.3 Origin of Life
Eukaryotic cells may have evolved through
endosymbiosis.
• Endosymbiosis is a relationship in which one organism
lives within the body of another.
• Mitochondria and chloroplasts may have developed through
endosymbiosis.
12.3 Origin of Life
The evolution of sexual reproduction led to increased
diversity.
• Genetic variation is an advantage of sexual reproduction.
• Sexual reproduction may have led to the evolution of
multicellular life.