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The History of Life
Chapter 17
17-1 The Fossil Record
• Fossils are the preserved remains
of ancient organisms.
• By observing fossils and making
comparisons, scientists can
determine relationships between
ancient organisms
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–
–
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What is the structure of the organism?
What did it eat?
What ate it?
What was the organisms environment
like?
• By comparing fossils to one
another, scientists ordered
organisms according to when they
lived (The Fossil Record)
Fossil Record Continued
• The fossil record reveals
amazing facts!
– Fossils occur in a
particular order
– Certain fossils appear in
older rocks but not newer
ones
– This shows that life has
changed over time
– Out of all the species that
have ever lived, 99% of
them are extinct!
17-2,17-3 The Evolution of
Variation
• Up until 1 Billion years ago, the earth consisted
of a primordial soup
• As oxygen levels in the atmosphere were
elevated, more single-cell living organisms
appeared.
• 1 Billion years ago, prokaryotic cells began
reproducing sexually. They also began to join
together, creating the first eukaryotic cell.
(Endosymbiotic theory)
– This led to an increase in variation and the birth of
multicellular organisms
17-4 Extinction
• Out of all the species that
ever lived, only 1% still
exist
• Extinction of species was
caused by many factors,
such as:
– Competition for resources
– Environmental changes
– Gradual extinction via
natural selection
– Mass extinction
17-4 Continued
• Mass Extinction
– Happens occasionally and
wipes out entire
ecosystems
– Volcanoes, Sea Level
changes, and continental
drift have all lead to mass
extinction
– Scientists have reason to
believe that an asteroid
lead to the extinction of the
dinosaurs
Is Evolution constant?
• Gradualism: Evolution • Punctuated Equilibrium:
Evolution happens in
happens gradually
and continually over a short bursts interrupted
by long periods of no
long period of time
change