The History of Life
Spontaneous Generation
The belief that life arose from non-living
things.
 As scientific thinking progressed through
the ages, free-thinkers set out to prove
that spontaneous generation was not
true.
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Examples
1. Redi performed a test that showed meat
didn’t become flies, but rather flies laid their
eggs in meat and maggots in the meat
became flies.
2. Louis Pasteur showed that life did not come
from “gravy.”
Curved neck is
removed.
Broth is boiled.
Broth remains free of
microorganisms.
Microorganisms
grow in broth
The fossil record
The fossil record provides evidence
about the history of life on earth.
 It also shows how different organisms,
including species, have changed over
time.
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Earth’s early history
4.6 billion years old
 Earth’s early atmosphere probably
contained hydrogen cyanide, CO2, CO,
N, H, S, and H2O.
 There was no free oxygen; free oxygen
was the result of organisms doing
photosynthesis and releasing oxygen as
a by-product into the atmosphere.

Free oxygen
The ancient atmosphere had no free
oxygen.
 Prokaryotes (later eukaryotes) began
photosynthesis as a means to obtain
energy.
 The by-product (oxygen) accumulated in
the atmosphere => ozone
 The rise of oxygen in the atmosphere
drove some life forms to extinction, while
other life forms evolved new, more efficient
metabolic pathways that used oxygen for
respiration.
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The first organic molecules:
Miller and Urey pg. 424
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Miller and Urey’s experiments
suggested how mixtures of the
organic compounds necessary
for life could have risen from
simpler compounds present on a
primitive earth.
They filled a flask with H, CH4,
Ammonia, and H2O, made
certain no microorganisms could
get in, and passed electric sparks
through the mixture.
Over a few days, amino acids
(building blocks of protein)
formed.
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The puzzle of life’s origins
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Formation of microspheres: under
certain conditions, large, organic
molecules can form tiny bubbles called
proteinoid microspheres.
They are not cells, but they do have:
1. selectively permeable
membranes
2. a simple means of
releasing and storing
energy
3. over time, they grow and
divide.
Evolution of RNA and DNA
Under the right conditions, RNA can
help DNA replicate.
 Some RNA can grow and duplicate
itself.
 Some RNA can catalyze chemical
reactions.
 Scientists believe RNA came before
DNA.

Origin of eukaryotic cells
the endosymbiotic theory (Lynn
Margulis, Boston University)
Eukaryotic cells formed from a symbiosis
among several different prokaryotic
organisms.
 Early prokaryotes (primitive mitochondria
and chloroplasts) that could process
energy began living together, formed
communities, then eventually complex
eukaryotic organisms.

Sexual Reproduction and
Multicellularity
Some time after eukaryotes arose, they
began to reproduce sexually.
 Prior to that, organisms reproduced
through simple division.
 Sexual reproduction shuffled genes, and
added diversity, making more variations.
 A few hundred million years after the
onset of sexual reproduction, organisms
became multi-cellular.

Read pgs. 429-434 about the
timeline of the earth.
Patterns of Evolution
Macroevolution: large-scale evolutionary
patterns and processes that occur over
long periods of time.
 6 important topics in macroevolution:
1. extinction
2. adaptive radiation
3. convergent evolution
4. co-evolution
5. punctuated equilibrium
6. changes in developmental genes

Extinction
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More than 99% of all species that have ever
lived are now extinct.
 Reasons include: competition/loss of resources,
etc.
 Mass extinctions wipe out entire ecosystems,
collapsing food webs and disrupting energy flow
in the ecosystem.
 Whatever the cause (volcanoes, asteroids, etc.),
the result is that the disappearance of certain
organisms will open up habitats and niches that
allow organisms to flourish and new species to
appear.

Adaptive Radiation
A single species or a small group of
species has evolved, through natural
selection, into diverse forms that live in
different ways (like Darwin’s finches or
dinosaurs).
 The loss of the dinosaurs opened up the
opportunity for mammals to grow new
species and spread out.

Convergent Evolution
The process by which unrelated organisms
come to resemble one another.
 How does this happen?
 Sometimes groups of organisms undergo
adaptive radiation in different places or at
different times but in ecologically different
environments.
 Organisms may start out with different raw
material (DNA) to work with, but as time
progresses and they face similar environmental
demands, they come to resemble each other
over time.
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Co-evolution
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Organisms that are closely connected to one
another by ecological interactions evolve
together.
Example: some flowers have very specific
flower, shape, and odor that attract very specific
pollinators.
If the flower’s pollinator changes, a change in
the flower may change as well.
An evolutionary change in one organism may
be followed by a corresponding change in
another organism.
Co-evolution: when two species evolve in
response to changes in other over time.
Punctuated Equilibrium
Long, stable periods (millions of years)
interrupted by brief periods (hundreds of
thousands of years) of more rapid change.
ex: horseshoe crab has remain
unchanged when compared to its
ancient fossils; no selection pressures =
no change. They are still in equilibrium.
 Controversial amongst scientists who study
evolution.
 Evolution has proceeded at different rates
for different organisms at different times
during the long history of the earth.

Developmental Genes and
Body Genes

The timing of cell differentiation and
gene expression can have an impact on
organisms and the generations that
come from them.