Transcript ch 17 the history of life 2

17-2 Earth's Early History
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17-2 Earth's Early History
Formation of Earth
Formation of Earth
• Evidence shows that Earth
was not “born” in a single event.
• The whole universe was concentrated into one
superdense mass that exploded = “big bang”
• Pieces of cosmic debris were probably attracted
to one another over the course of 100 million
years.
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17-2 Earth's Early History
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17-2 Earth's Early History
Formation of Earth
Once Earth formed (~4.6 billion years ago = bya), its
elements rearranged themselves according to
density.
The most dense elements formed the planet’s core.
Moderately dense elements floated to the surface,
cooled, and formed a solid crust.
The least dense elements formed the first
atmosphere.
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17-2 Earth's Early History
Formation of Earth
•4.6 billion years ago: Earth cooled and solid rocks
formed on its surface.
•Several million years afterwards: volcanic activity
shook Earth’s crust.
•~ 3.8 billion years ago: Earth’s surface cooled
enough for water to remain a liquid, and oceans
covered much of the surface.
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17-2 Earth's Early History
Conditions of Early Earth
Early earth was hostile to modern life:
1. organic compounds don’t form easily in the
presence of N & CO2
2. UV radiation from the sun would harm DNA – no
ozone (O3) layer for protection
3. no free oxygen
4. frequent volcanic eruptions & violent lightening
storms
5. extreme temperature
variations (no
greenhouse effect)
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17-2 Earth's Early History
Formation of Earth
What substances made up Earth's early
atmosphere?
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17-2 Earth's Early History
Formation of Earth
Earth's early atmosphere probably
contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen,
hydrogen sulfide, and water.
notice the 4 elements present:
C, O, H, N
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17-2 Earth's Early History
The First Organic Molecules
The First Organic Molecules
Could organic molecules have evolved under
conditions on early Earth?
•evidence from meteorites shows that organic
compounds form in space
•organic compounds form at
volcanic vents deep in the ocean
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17-2 Earth's Early History
The First Organic Molecules
Miller and Urey (1950’s) recreated the
conditions of the early earth including
volcanic gases, energy from heat &
electricity, & water vapor as rain
• formed simple amino acids in only 1 week
• more complex compounds formed with
more time.
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17-2 Earth's Early History
The First Organic Molecules
Miller and Urey’s Experiment
Mixture of gases
simulating
atmosphere of
early Earth
Spark simulating
lightning storms
Condensation
chamber
Water
vapor
Cold water cools
chamber, causing
droplets to form.
Liquid containing amino
acids and other organic
compounds
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17-2 Earth's Early History
Chemical Evolution
The Puzzle of Life's Origin
- Evidence suggests that 200–300 million years
after Earth had liquid water, cells similar to
modern bacteria were common.
- Organic compounds accumulated in the ocean =
primordial soup
- Life evolved from further chemical reactions &
transformations
- The 1st life forms were heterotrophs that fed on
organic compounds in the primordial soup
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17-2 Earth's Early History
Free Oxygen
Free Oxygen
Microscopic fossils, or microfossils, of unicellular
prokaryotic organisms resembling modern bacteria
have been found in rocks over 3.5 billion years old.
These first life-forms evolved without oxygen.
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17-2 Earth's Early History
Free Oxygen
About 2.2 billion years ago, photosynthetic bacteria
began to pump oxygen into the oceans.
Next, oxygen gas accumulated in the atmosphere,
but it was not abundant until 600-800 mya.
What occurred when oxygen
was added to Earth's atmosphere?
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17-2 Earth's Early History
Free Oxygen
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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17-2 Earth's Early History
Origin of Eukaryotic Cells
What hypothesis explains the origin of
eukaryotic cells?
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17-2 Earth's Early History
Origin of Eukaryotic Cells
The Endosymbiotic Theory
The endosymbiotic theory proposes that
eukaryotic cells arose from living
communities formed by prokaryotic
organisms.
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17-2 Earth's Early History
Endosymbiotic Theory
The Endosymbiotic Theory was not recognized as a
viable theory until the 1960’s by Lynn Margulis.
(from Boston!)
Eukaryotic cells developed from a symbiotic
relationship between several kinds of prokaryotes
(bacteria) – each had its own “specialty” and
together – formed a great “unit”
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17-2 Earth's Early History
Origin of Eukaryotic Cells
Endosymbiotic Theory
Ancient Prokaryotes
Chloroplast
Aerobic
bacteria
Nuclear
envelope
evolving
Ancient Anaerobic
Prokaryote
Photosynthetic
bacteria
Plants and
plantlike
protists
Mitochondrion
Primitive Aerobic
Eukaryote
Primitive Photosynthetic
Eukaryote
Animals,
fungi, and
non-plantlike
protists
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17-2 Earth's Early History
Concept Map
Evolution of Life Time Line
Section 17-2
4 billion
3.8 billion
Early Earth was hot; atmosphere contained poisonous gases.
Earth cooled and oceans condensed.
Simple organic molecules may have formed in the oceans..
Small sequences of RNA may have formed and replicated.
3.5 billion
First prokaryotes may have formed when RNA or DNA was enclosed in microspheres.
2 billion
Later prokaryotes were photosynthetic and produced oxygen.
An oxygenated atmosphere capped by the ozone layer protected Earth.
First eukaryotes may have been communities of prokaryotes.
500 mil
Multicellular eukaryotes evolved.
Sexual reproduction increased genetic variability, hastening evolution.
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17-2 Earth's Early History
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17-2
Which of the following gases was probably NOT
present in the early Earth’s atmosphere?
a. hydrogen cyanide
b. oxygen
c. nitrogen
d. carbon monoxide
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17-2
Miller and Urey's experiment was a simulation of
Earth's early
a. volcanic activity.
b. formation.
c. atmosphere.
d. life.
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17-2
As concentrations of oxygen rose in the ancient
atmosphere of Earth, organisms began to evolve
a. anaerobic pathways.
b. plasma membranes.
c. metabolic pathways that used oxygen.
d. photosynthesis.
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