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12.1 The Fossil Record
KEY CONCEPT Specific environmental conditions are necessary in order for fossils to form.
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12.1 The Fossil Record
Fossils can form in several ways.
• Permineralization occurs when minerals carried by water are deposited around a hard structure.
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12.1 The Fossil Record
• A natural cast forms when flowing water removes all of the original tissue, leaving an impression.
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12.1 The Fossil Record
• Trace fossils record the activity of an organism.
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12.1 The Fossil Record
• Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.
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12.1 The Fossil Record
• Preserved remains form when an entire organism becomes encased in material such as ice.
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12.1 The Fossil Record
• Specific conditions are needed for fossilization.• Only a tiny percentage of living things became fossils.
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12.1 The Fossil Record
Radiometric dating provides an accurate way to estimate the age of fossils.
• Relative dating estimates the time during which an organism lived.– It compares the placement
of fossils in layers of rock.– Scientists infer the order in
which species existed.
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12.1 The Fossil Record
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their number of neutrons.
neutrons protrons
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12.1 The Fossil Record
– A half-life is the amount of time it takes for half of the isotope to decay.
• Radiometric dating uses decay of unstable isotopes.
– Isotopes are atoms of an element that differ in their number of neutrons.
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12.1 The Fossil Record
KEY CONCEPT 12.3The origin of life on Earth remains a puzzle.
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12.1 The Fossil Record
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.
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12.1 The Fossil Record
Several sets of hypotheses propose how life began on Earth.
• There are two organic molecule hypotheses.– Miller-Urey experiment
– meteorite hypothesis
electrodes
heat source amino acids
water
“atmosphere”
“ocean”
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12.1 The Fossil Record
• There are different hypotheses of early cell structure.
– iron-sulfide bubbles hypothesis
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12.1 The Fossil Record
• There are different hypotheses of early cell structure.– lipid membrane hypothesis
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12.1 The Fossil Record
• 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.