Download Chapter 8 - earthjay science

THE EARTH THROUGH TIME
TENTH EDITION
H A R O L D L. L E V I N
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
1
CHAPTER 8
Earth's Formative
Stages and the Archean
Eon
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HADEAN
Hadean—An informal time units prior to the Archean
which takes into account the time before the
preservation of the rock record (4.56 billion years
ago to around 4.0 billion years ago)
Earth’s accretion was complete 4.56 billion years ago. The
crust began to form about 4.0 billion years ago.
Geologic processes recycled, destroyed or altered the
original rocks of the Earth during the Hadean.
Much of our knowledge of the Earth's earliest history
comes from indirect evidence: meteorites.
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ARCHEAN EON
Archean (Archean Eon) is the oldest unit on the
geologic time scale.
It began about 4.0 billion years ago and ended 2.5
billion years ago.
Archean lasted for 2.1 billion years (2,100,000,000
years).
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EARTH'S OLDEST ROCKS

Earth's oldest rocks are found in Canada.
They are about 4.04 billion years old.

But there are even older mineral grains.
Sand-sized zircon grains in metamorphosed
sedimentary rocks from Australia are 4.4
billion years old.
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PRECAMBRIAN
Archean and
Proterozoic Eons
comprise in interval of
time informally called
Precambrian, which
spans 87% of the
geologic time scale.
FIGURE 8-1 The Precamrian
eons span 87% of Earth history.
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THE BIG BANG
Calculations indicate that the Big Bang occurred 18–
15 billion years ago.
The Big Bang marked the instantaneous creation of
all matter in the Universe followed by an expansion.
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THE SOLAR SYSTEM
The Sun and the planets, moons, asteroids, comets and
other objects that orbit it, comprise the Solar System.
FIGURE 8-4 Schematic
view of the Solar System,
showing orbits of the eight
planets, the planetoid
Pluto, the asteroid belt,
and a comet.
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OBSERVATIONS THE MUST BE
CONSIDERED FOR ANY HYPOTHESIS ON
THE ORIGIN OF THE SOLAR SYSTEM
1.
2.
3.
Planets revolve around sun in same
direction—counterclockwise (CCW)
Planets lie roughly within sun's equatorial
plane (plane of sun's rotation)
Solar System is disk-like in shape
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OBSERVATIONS THE MUST BE
CONSIDERED FOR ANY HYPOTHESIS ON
THE ORIGIN OF THE SOLAR SYSTEM
4.
Planets rotate CCW on their axes,
except for:
a.
b.
5.
Venus: slowly clockwise
Uranus: on its side
Moons go CCW around planets (with a
few exceptions)
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OBSERVATIONS THE MUST BE
CONSIDERED FOR ANY HYPOTHESIS ON
THE ORIGIN OF THE SOLAR SYSTEM
6.
Distribution of planet densities and
compositions is related to their distance
from sun
a.
b.
7.
Inner, terrestrial planets have high density
Outer, Jovian planets have low density
Age—Meteorites are as old as 4.56 billion
years
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SOLAR NEBULA HYPOTHESIS
OR NEBULAR HYPOTHESIS
1.
2.
3.
4.
Cold cloud of gas and dust contracts,
rotates, and flattens into a disk-like
shape.
Roughly 90% of mass becomes
concentrated in the center, due to
gravitational attraction.
Turbulence in cloud caused matter to
collect in certain locations.
Clumps of matter begin to form in the
disk.
FIGURE 8-6 The solar nebula hypothesis
for the origin of the Solar System.
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SOLAR NEBULA HYPOTHESIS
OR NEBULAR HYPOTHESIS
5.
6.
7.
Accretion of matter (gas and dust)
around clumps by gravitational
attraction. Clumps develop into
protoplanets.
Solar nebula cloud condenses, shrinks,
and becomes heated by gravitational
compression to form Sun.
Ultimately hydrogen (H) atoms begin to
fuse to form helium (He) atoms,
releasing energy (heat and light). The
Sun "ignites."
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FIGURE 8-6 The solar nebula hypothesis
for the origin of the Solar System.
SOLAR NEBULA HYPOTHESIS
OR NEBULAR HYPOTHESIS
8.
9.
10.
The Sun's solar wind drives lighter
elements outward, causing observed
distribution of masses and densities in the
Solar System.
Planets nearest Sun lose large amounts of
lighter elements (H, He), leaving them with
smaller sizes and masses, but greater
densities than the outer planets. Inner
planets are dominated by rock and metal.
Outer planets retain light elements such as
H and He around inner cores of rock and
metal. Outer planets have large sizes and
masses, but low densities.
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FIGURE 8-6 The solar nebula hypothesis
for the origin of the Solar System.
HOW OLD IS THE SOLAR SYSTEM?
Based on radiometric dates of moon rocks
and meteorites, the Solar System is about
4.56 billion years old.
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METEORITES: SAMPLES OF
THE SOLAR SYSTEM


Meteors = "shooting stars." The glow comes from
small particles of rock from space being heated as
they enter Earth's atmosphere.
Meteorites = chunks of rock from the Solar System
that reach Earth's surface. They include fragments
of:



Asteroids
Moon rock
Planets, such as Mars (i.e., "Martian meteorites")
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TYPES OF METEORITES
1.
2.
3.
4.
Ordinary chondrites
Carbonaceous chondrites
Iron meteorites
Stony-iron meteorites
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METEORITES: ORDINARY CHONDRITES



Most abundant type of meteorite
About 4.6 billion years old
May contain chondrules: spherical bodies
that solidified from molten droplets thrown
into space during Solar System impacts
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METEORITES: CARBONACEOUS
CHONDRITES



Contain about 5% organic
compounds, including
amino acids—the building
blocks of proteins, DNA,
and RNA
May have supplied basic
building blocks of life to
Earth
Contain chondrules
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Harold Levin
METEORITES: IRON METEORITES



Iron-nickel alloy
Coarse-grained inter-grown
crystal structure
About 5% of all meteorites
(c) AP/Wide World Photos
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METEORITES: STONY-IRON METEORITES


Composed partly of Fe, Ni and partly of silicate
minerals, including olivine (like Earth's mantle).
About 1% of all meteorites. Least abundant type.
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THE SUN
The Sun is a star
 Composition:

 70%
hydrogen
 27% helium
 3% heavier elements
Size: About 1.5 million km in diameter
 Contains about 98.8% of the matter in the
Solar System

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THE SUN
Temperature: may exceed 20 millionoC in the
interior
 Sun's energy comes from fusion, a
thermonuclear reaction in which hydrogen
atoms are fused together to form helium,
releasing energy
 The Sun's gravity holds the planets in their
orbits

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SUN'S ENERGY IS THE FORCE BEHIND
MANY GEOLOGIC PROCESSES ON EARTH




Evaporation of water to produce clouds, which
cause precipitation, which causes erosion.
Uneven heating of the Earth's atmosphere causes
winds and ocean currents.
Variations in heat from Sun may trigger continental
glaciations or change forests to deserts.
Sun and moon influence tides which affect the
shoreline.
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THE PLANETS
1.
2.
3.
4.
Mercury
Venus
Earth
Mars
5.
6.
7.
8.
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Jupiter
Saturn
Uranus
Neptune
THE PLANETS
Terrestrial planets:
Jovian planets:
 Small
 Large
 Dense (4 - 5.5 g/cm3)
 Low density (0.7 - 1.5
 Rocky + Metals
g/cm3)
 Mercury, Venus, Earth,
 Gaseous
Mars
 Jupiter, Saturn, Uranus,
Neptune
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MERCURY






Smallest of the terrestrial planets
Revolves rapidly around the sun; its year is 88 Earth
days
Densely cratered
Thin atmosphere of sodium and lesser amounts of
helium, oxygen, potassium and hydrogen
Weak magnetic field and high density suggest an
iron core
No moons
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VENUS






Similar to Earth in size, mass, volume, density and
gravity
No oceans or liquid water
Very high atmospheric pressure
Atmosphere is 98% carbon dioxide
Dense clouds of sulfuric acid droplets in
atmosphere
Greenhouse effect causes temperature on planet's
surface to reach 470°C, hot enough to melt lead
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VENUS






Rotates once on its axis (one day on Venus) in 243
Earth days
Rotates on axis in opposite direction to other
planets, possibly due to collision with other object
Has volcanoes
Has craters
Surface rocks resemble basalt
No moons
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EARTH










Diameter = nearly 13,000 km (8000 mi)
Oceans cover 71% of surface
Atmosphere = 78% nitrogen and 21% oxygen
Surface temperature approx. –50 and +50oC
Average density = 5.5 g/cm3
Surface rock density = 2.5-3.0 g/cm3
Core about 7000 km in diameter
Mantle surrounds core. Extends from base of crust
to depth of 2900 km
Geologically active. Plate tectonics
Only body in the Universe known to support life
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EARTH'S INTERNAL LAYERED STRUCTURE

The Earth is internally layered, with a basic
structure consisting of:




Crust
Mantle
Inner and outer core
The Earth's internal structure may be
primary (formed initially as the Earth
formed), or secondary due to later heating.
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FACTORS THAT MAKE EARTH
HOSPITABLE FOR LIFE





Distance from Sun maintains temperatures in the
range where water is liquid.
Temperature relatively constant for billions of
years.
Rotation allows all sides of Earth to have light and
heat.
Atmosphere absorbs some heat from the Sun and
reflects some solar radiation back to space.
Magnetic field protects life from dangerous high
energy particles and radiation in the solar wind.
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EARTH'S MOON






Diameter = about 1/4 that of Earth.
Density = about 3.3 g/cm3 (similar to Earth's
mantle).
Rotates on its axis at same rate as it revolves
around Earth (29.5 days). Results in same side of
Moon always facing Earth.
Far side of moon is more densely cratered
No atmosphere.
Ice is present at the poles.
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GEOLOGY OF THE MOON
Dominant rock type is anorthosite (related to
gabbro; rich in Ca plagioclase feldspar).
 Basalt is also present.
 Two types of terranes

 Lunar
highlands
 Maria (singular = mare)
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LUNAR HIGHLANDS
Lunar Highlands
 Light-colored
 Rough topography
 Highly cratered
 Rocks more than
4.2 billion years old
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LUNAR MARIA
Lunar Maria
 Large, dark areas
 Immense basins covered
with basaltic lava flows
 Age of basalt is 3.8 to 3.2
billion years
 Maria have few craters.
This indicates a decrease
in meteorite
bombardment after about
3.8 billion years ago.
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ORIGIN OF THE MOON



Moon may have formed as a result of an impact of
a large body with Earth about 4.4 billion years ago.
Debris from the impact was thrown into orbit
around Earth and collected to form the Moon.
Heat from impacts led to melting and
differentiation (or segregation of materials of
different density; low density materials rose and
high density materials sank).
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MARS




Has white polar caps made of frozen carbon
dioxide ice
Has seasonal changes. Polar ice caps expand and
contract
Rusty orange color due to iron oxides on surface
Heavily cratered due to early bombardment by
meteorites and asteroids
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MARS
Diameter is about half that of Earth
 Mass is only about 10% of Earth's mass, so
gravity is much less
 Thin atmosphere (less than 1% as dense as
Earth's).
 Dominant gas is carbon dioxide; small
amounts of nitrogen, oxygen and carbon
monoxide. No greenhouse effect.

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MARS
Previously had a denser atmosphere
 Evidence of abundant liquid water in the
past.
 An ocean once existed, at least 0.5 km deep
and larger than all 5 U.S. Great Lakes.
 Temperatures range from –85oC to 21oC
(21oC is about room temperature)

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MARS
Lower density than other terrestrial planets
 Little to no magnetic field, suggesting only a
small iron-rich core.
 Lack of magnetic field exposed planet to
solar winds which swept away atmosphere
and liquid water.
 Two small moons, Phobos and Deimos

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ASTEROID BELT
Thousands of asteroids, primarily between
Mars and Jupiter.
 Asteroids are composed of rocks and metal
(Fe & Ni).
 Size of asteroids ranges from a few km in
diameter to about 1/10 the size of Earth.

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JUPITER
Largest planet in the Solar System.
(Diameter 11 times greater than Earth)
 Low density. (Density is about 1/4 that of
Earth)
 Most of planet's interior is probably liquid
metallic hydrogen.
 Rotates on axis rapidly. One day on Jupiter is
10 hours on Earth.
 Rotation causes bands in atmosphere

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JUPITER



Note Great Red Spot, a
cyclonic storm
Atmosphere composed of H,
He, with lesser amounts of
methane and ammonia
Has a faint ring of debris
which encircles the planet
Courtesy NASA
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JUPITER


Has more than 60 moons
Four largest moons are:




Io: covered by sulfur volcanoes
Europa: has sea of liquid water
beneath an icy surface
Ganymede: planet-sized body
larger than Mercury. Cratered with
sinuous ridges; has sea of liquid
water beneath an icy surface
Callisto: highly cratered; has sea of
liquid water beneath an icy surface
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Courtesy NASA
SATURN


Second largest planet
Has prominent rings of debris (mostly ice with minor silicates)
encircling planet in equatorial plane. Debris ranges from
microns to meters.
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SATURN
Density is less than that of water; it could
float. (Density = 0.7g/cm3)
 Mostly H and He; also contains methane,
ammonia, and water; may have iron core
 Has magnetic field, radiation belts, and
internal heat source
 Has more than 30 moons.

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URANUS






About 4 x larger than Earth
Low density (density = 1.3 g/cm3)
Axis of rotation is tipped on its side,
possibly due to collision with
another Solar System object
Has more than two dozen moons
Atmosphere of hydrogen, helium,
and methane
Has planetary ring system
E. Karkoschka, University of
Arizona/Space Telescope
Science Institute
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NEPTUNE






Similar in size and color to Uranus
Low density (density = 1.6 g/cm3)
Atmosphere = H, He, and methane
Has more than a dozen moons
Has Great Dark Spot, a cyclonic
storm
Has planetary ring system
Courtesy NASA
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SOLAR NEBULA HYPOTHESIS OR
COLD ACCRETION MODEL
(SECONDARY DIFFERENTIATION)




Earth formed by accretion of dust and larger particles of
metals and silicates.
Earth was originally homogeneous throughout - a
random mixture of space debris.
Origin of layering requires a process of differentiation.
Differentiation is the result of heating and at least partial
melting.
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POSSIBLE SOURCES OF
HEAT FOR MELTING:
1.
2.
3.
Accretionary heat from bombardment (meteorite impacts)
Heat from gravitational compression as material
accumulated
Radioactive decay
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DIFFERENTIATION AFTER ACCRETION




Iron and nickel sink to form core.
Less dense material (silicon and oxygen combined
with remaining iron and other metals) forms mantle
and lighter crust (dominated by silicon and oxygen).
Presence of volatile gases on Earth indicates that
complete melting did not occur.
Earth was repeatedly partly melted by great
impacts, such as the Moon-forming impact.
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AN ALTERNATIVE MODEL: HOT
ACCRETION (PRIMARY
DIFFERENTIATION)
Internal zonation of planets is a result of hot
heterogeneous accretion.
 Hot solar nebula (over 1000oC).
 Initial crystallization of iron-rich materials
forms planet's core.
 With continued cooling, lower density silicate
materials crystallized.

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WHICH MODEL?
Solar Nebula Hypothesis also known as the
Cold Accretion Model (secondary differentiation)
OR
Hot Accretion Model (primary differentiation)
???
Parts of both models may have been in
operation.
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ARCHEAN CRUST
Once differentiation occurred, Earth's crust was
dominated by Fe & Mg silicate minerals.
 If Earth experienced heating and partial
melting, it may have been covered by an
extensive magma ocean during Archean.
 Magma cooled to form rocks called komatiites.
 Archean rocks form South Africa indicate that
the Earth had a magnetic field.


The existence of a magnetic field is important to the
development of life on Earth
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KOMATIITES
Komatiites are ultramafic rocks composed
mainly of olivine and pyroxene.
 Komatiites form at temperatures greater
than those at which basalt forms (greater
than 1100oC).
 This rock formed Earth's Archean crust.

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ORIGIN OF MAFIC CRUST
The first mafic, oceanic crust formed about
4.5 billion years ago by partial melting of rocks
in the upper mantle.
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EARTH'S CRUST TODAY
Earth has two types of crust today:
1.
2.
Dense, mafic (Mg- and Fe-rich) oceanic
crust dominated by basalt.
Less dense, silicic (Si- and Al-rich)
continental crust dominated by granite.
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ORIGIN OF CONTINENTAL CRUST
Continental crust developed after the initial
mafic to ultramafic crust.
 Continental crust is silicic or felsic (such as
granite). Dominated by light-colored
minerals such as quartz and feldspar.
 Felsic crust began forming around 4.4
billion years ago.

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ORIGIN OF CONTINENTAL CRUST
Felsic crust formed in subduction zones
where descending slabs of crust partially
melted.
 The early-melting, less dense components of
the melt rose to the surface where they
cooled to form continental crust.

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EARTH'S OLDEST ROCKS

One of the oldest dated felsic Earth rocks is the 4.04
billion year old Acasta Gneiss from northwestern
Canada. Dates are from zircon grains in tonalite
gneisses.
(Tonalite gneiss is metamorphosed tonalite, a rock similar
to diorite, with at least 10% quartz).

The Amitsoq Gneiss from Greenland is another old
tonalite gneiss (3.8 b.y. old).

Patches of old felsic crust have also been found in
Antarctica (3.9 b.y. old).
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EARTH'S OLDEST LAND SURFACE


A 3.46 b.y. old fossil soil zone (or paleosol)
associated with an unconformity in the Pilbara
region of Australia indicates that Archean continents
stood above sea level.
This paleosol represents the oldest land surface
known, and provides evidence that subaerial
weathering, erosion, and soil formation processes
were at work during Archean.
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THE OLDEST MINERAL GRAINS





The oldest zircon grains are 4.4 b.y. old.
Found in quartzite in western Australia.
Sedimentary structures in the quartzite resemble those in
modern stream deposits.
Interpreted as fluvial (river) deposits.
Derived from weathering of granitic rocks (some of the
earliest continental crust), and deposited above sea level,
indicating the presence of both liquid water and continental
crust by 4.4 b.y. ago.
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EARTH'S CRUST
Oceanic Crust
Continental Crust
First appearance
About 4.5 b.y. ago
About 4.4 b.y. ago
Where formed
Mid-ocean ridges
Subduction zones
Composition
Komatiite & basalt
Tonalite & granodiorite,
and later, granites
Lateral extent
Widespread
Local
(few 100 km or mi)
How formed
Partial melting of
ultramafic rocks in upper
mantle
Partial melting of wet,
sediment-covered mafic
rocks in subduction
zones
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EVOLUTION OF EARTH'S ATMOSPHERE
AND HYDROSPHERE
Earth's first, primitive atmosphere lacked
free oxygen.
 The primitive atmosphere was derived from
gases associated with the comets and
meteorites which formed the Earth during
accretion.
 The gases reached the Earth's surface
through a process called outgassing.

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GASES ASSOCIATED WITH COMETS


Comets are made of frozen gases, ice and dust.
Halley's comet is composed of:
 80% water ice
 Frozen carbon dioxide (dry ice)
 Hydrogen cloud surrounds comet
 Dust near the nucleus contains iron, oxygen, silicon,
magnesium, sodium, sulfur, and carbon
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GASES ASSOCIATED WITH METEORITES

Carbonaceous chondrites are mainly
composed of silicate minerals, but also
contain:




Nitrogen
Hydrogen
Water
Carbon in the form of complex organic molecules
(proteins and amino acids)
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GASES ASSOCIATED WITH EARTH’S
MATERIAL
Water and gaseous elements would have
been released from the newly accreted Earth
by the heat associated with bombardment and
accretion, or by melting and volcanism
accompanying later differentiation.
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VOLCANIC OUTGASSING


Outgassing = release of water vapor and other
gases from Earth through volcanism.
Gases from Hawaiian eruptions consist of:
 70% water vapor (H2O)
 15% carbon dioxide (CO2)
 5% nitrogen (N2)
 5% sulfur (in H2S)
 chlorine (in HCl)
 hydrogen
 argon
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VOLCANIC OUTGASSING
Most of the water on the surface of the Earth
and in the atmosphere was outgassed
during the first billion years of Earth history.
We know this because there are 3.8 b.y.-old
marine sedimentary rocks, indicating the
presence of an ocean by 3.8 billion years
ago.
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FORMATION OF THE HYDROSPHERE
Once at the Earth's surface, gases and other
volatile elements underwent a variety of
changes.
1. Water vapor condensed and fell as rain.
2. Liquid water probably began to fall on the
Earth's surface as early as 4.4 billion years
ago.
3. Rain water accumulated in low places to form
seas. The seas were originally freshwater
(rain).
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FORMATION OF THE HYDROSPHERE
4.
5.
6.
Carbon dioxide and other gases dissolved in the
rain made the water more acidic than today.
Carbon dioxide and water combine to form
carbonic acid.
Acid waters caused rapid chemical weathering of
the exposed rocks, adding Na, Ca, K, and other
ions to seawater.
A change to more alkaline water may have
occurred rapidly as large amounts of Ca, Na, and
Fe were introduced by submarine volcanism,
neutralizing the acid.
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FORMATION OF THE HYDROSPHERE
7.
8.
9.
Ions accumulated in the seas, increasing the
salinity. Sea salinity is relatively constant today
because salts are precipitated at about the same
rate they are supplied to the sea. Sodium
remains in sea water due to its high solubility.
Later, when the seas became less acidic, Ca ions
bonded with CO2 to form shells of marine
organisms and limestones (CaCO3).
The presence of marine fossils suggests that
sodium has not varied appreciably in sea water
for at least the past 600 million years.
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HYDROLOGIC CYCLE
Today Earth's water is continuously
recirculated through the hydrologic cycle
(evaporation and precipitation, powered by
the sun and by gravity).
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EVOLUTION OF THE ATMOSPHERE
FIGURE 8-21 The relative amounts of gases in the primordial atmosphere were
different from the abundances vented to the exterior of Earth during differentiation.
Note: Gases released by volcanoes, condensation of water
vapor, precipitation, and accumulation of liquid water,
photochemical reactions in the atmosphere, and formation of
carbonate rocks (limestones) later, after the seas became less
acidic.
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THE EARLY ANOXIC ATMOSPHERE
Earth's early atmosphere was strongly reducing and
anoxic (lacked free oxygen or O2 gas), and probably
consisted primarily of:
 Water vapor (H2O)
 Carbon dioxide (CO2)
 Nitrogen (N2)
 Carbon monoxide (CO)
 Hydrogen sulfide (H2S)
 Hydrogen chloride (HCl)
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THE EARLY ANOXIC ATMOSPHERE
The atmosphere composition would have
been similar to that of modern volcanoes,
but probably with more hydrogen, and
possibly traces of methane (CH4) and
ammonia.
If any free oxygen had been present, it would
have immediately been involved in chemical
reactions with easily oxidized metals such
as iron.
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EVIDENCE FOR A LACK OF FREE OXYGEN
IN EARTH'S EARLY ATMOSPHERE
1.
2.
3.
Lack of oxidized iron in the oldest sedimentary
rocks. (Instead, iron combined with sulfur to form
sulfide minerals like pyrite. This happens only in
anoxic environments.)
Urananite and pyrite are readily oxidized today,
but are found unoxidized in Precambrian
sedimentary rocks.
Archean sedimentary rocks are commonly dark
due to the presence of carbon, which would have
been oxidized if oxygen had been present.
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EVIDENCE FOR A LACK OF FREE OXYGEN
IN EARTH'S EARLY ATMOSPHERE
4.
Archean sedimentary sequences lack
carbonate rocks but contain abundant
chert, presumably due to the presence of
an acidic, carbon dioxide-rich atmosphere.
In an acidic environment, alkaline rocks
such as limestone do not form.
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BANDED IRON FORMATION
5.
Banded iron formations (BIF) appear during
Precambrian (3.8 bys to 1.8 b.y.).
Cherts with alternating laminations of red oxidized
iron and gray unoxidized iron.
Formed as precipitates on shallow sea floor. Some iron
probably came from weathering of iron-bearing rocks
on continents. Most iron was probably from submarine
volcanoes and hydrothermal vents (hot springs).
Great economic importance; major source of iron mined
in the world.
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BANDED IRON FORMATIONS
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ADDITIONAL EVIDENCE FOR
AN ANOXIC ATMOSPHERE
6.
7.
The simplest living organisms have an
anaerobic metabolism. They are killed by
oxygen.
Includes some bacteria (such as botulism),
and some or all Archaea, which inhabit
unusual conditions.
Chemical building blocks of life (such as
amino acids, DNA) could not have formed in
the presence of O2.
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FORMATION OF AN
OXYGEN-RICH ATMOSPHERE
The change from an oxygen-poor to an oxygenrich atmosphere occurred by Proterozoic, which
began 2.5 billion years ago, at the end of
Archean.
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FORMATION OF AN
OXYGEN-RICH ATMOSPHERE
The development of an oxygen-rich atmosphere is
the result of:
1.
2.
Photochemical dissociation: Breaking up of water
molecules into H and O in the upper atmosphere,
caused by ultraviolet radiation from the Sun (a
minor process today)
Photosynthesis: The process by which
photosynthetic bacteria and plants produce
oxygen (major process).
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EVIDENCE FOR FREE OXYGEN IN THE
PROTEROZOIC ATMOSPHERE
1.
Red beds: Sedimentary rocks with iron
oxide cements (including shales, siltstones,
and sandstones), appear in rocks younger
than 1.8 billion years old. This occurred
during Proterozoic, after the disappearance
of the banded iron formations (BIFs).
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EVIDENCE FOR FREE OXYGEN IN THE
PROTEROZOIC ATMOSPHERE
2.
Carbonate rocks (limestones and
dolostones) appear in the stratigraphic
record at about the same time that red
beds appear.
This indicates that CO2 was less abundant in
the atmosphere and oceans so that the
water was no longer acidic.
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PRECAMBRIAN
FIGURE 8-31 Generalized cross-section through two greenstone belts.
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
PRECAMBRIAN
Precambrian covers about 4 billion years (and
87%) of Earth history.
Precambrian is divided into 2 eons:
 Proterozoic
Eon 2.5–0.542 billion years ago
(or 2500–542 million years ago)
 Archean Eon 4.6–2.5 billion years ago (lower
limit not defined)
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PRECAMBRIAN IS NOT WELL KNOWN
OR COMPLETELY UNDERSTOOD. WHY?





Precambrian rocks are often poorly exposed.
Many Precambrian rocks have been eroded or
metamorphosed.
Most Precambrian rocks are deeply buried beneath
younger rocks.
Many Precambrian rocks are exposed in fairly
inaccessible or nearly uninhabited areas.
Fossils are seldom found in Precambrian rocks;
only way to correlate is by radiometric dating.
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SHIELDS AND CRATONS
Most of what we know about Precambrian is
based on studies of rocks from cratons —arge
portions of continents which have not been
deformed since Precambrian or Early
Paleozoic.
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SHIELDS AND CRATONS
FIGURE 8-25 Exposed Precambrian rocks.
Areas where Precambrian rocks are exposed are shown in
yellow, as well as in the red areas in orogenic belts.
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SHIELDS AND CRATONS




The most extensive exposures of Precambrian rocks are in
geologically stable regions of continents called shields.
Example = Canadian shield in North America. Mostly igneous
and metamorphic rocks; few sedimentary rocks. Overlying
sedimentary rocks were scraped off by glaciers during last
Ice Age.
Stable regions of the craton where shields are covered by
sedimentary rocks are called platforms.
Precambrian rocks are often called basement rocks because
they lie beneath a covering of fossil-bearing sedimentary
strata.
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North American
craton, shield,
platform, and
orogenic belts.
FIGURE 8-26 North American
craton, shield, and platform.
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PRECAMBRIAN PROVINCES
Various Precambrian provinces can be
delineated within the North American continent,
based on radiometric ages of rocks, style of
folding, and differences in trends of faults and
folds.
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PRECAMBRIAN PROVINCES IN NORTH
AMERICA, WITH DATES
 Oldest
(Archean)
rocks are shown
in orange.
 Younger
(Proterozoic)
rocks are shown
in green.
FIGURE 8-27 Precambrian
Provinces of North America.
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ORIGIN OF PLATE TECTONICS
By about 4 b.y. ago, the Earth had probably
cooled sufficiently for plate formation.
 Once plate tectonics was in progress, it
generated crustal rock that could be partially
melted in subduction zones and added to
the continental crust.

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ORIGIN OF PLATE TECTONICS
Continents also increased in size by addition
of microcontinents along subduction zones.
 Greater heat in Archean would have caused
faster convection in mantle, more extensive
volcanism, more midoceanic ridges, more
hot spots, etc.
 Growth of volcanic arcs next to subduction
zones led to formation of greenstone belts.

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GRANULITES AND GREENSTONES
The major types of Archean rocks on the
cratons are:
 Granulites
 Greenstones
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GRANULITES

Granulites: Highly metamorphosed gneisses
(metamorphosed tonalites, granodiorites,
and granites) and anorthosites (layered
intrusive gabbroic rocks).
Granulites formed from partially melted crust
and sediments in subduction zones.
Metamorphism altered the rocks to form
granulites.
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GREENSTONES

Greenstones: Metamorphosed volcanic rocks and
sediments derived from the weathering and
erosion of the volcanic rocks.
Greenstone volcanic rocks commonly have pillow
structures, (called pillow basalts), indicating
extrusion under water.
The green color is the result of low-grade
metamorphism, producing green minerals such as
chlorite and hornblende.
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GREENSTONES


Mostly found in trough-like or synclinal belts.
Sequence of rock types:
 Ultramafic volcanic rocks near the bottom
(komatiites)
 Mafic volcanic rocks (basalts)
 Felsic volcanic rocks (andesites and rhyolites)
 Sedimentary rocks at the top (shales,
graywackes, conglomerates, and sometimes BIF),
deposited in deep water environments adjacent
to mountainous coastlines.
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GRANULITES AND GREENSTONES
FIGURE 8-31 Generalized cross-section through two greenstone belts.
Generalized cross-section through two greenstone
belts. Note sequence of rock types and relationships
between granulites and the greenstones. Granulites
are present between greenstone belts.
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EARTH'S EARLIEST GLACIATION
By 2.8 billion years ago, Earth had cooled
sufficiently for glaciation to occur. Earth's
earliest glaciation is recorded in 2.8 billion
year-old sedimentary rocks in South Africa.
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EARLIEST EVIDENCE OF LIFE
The earliest evidence of life occurs in
Archean sedimentary rocks.
 Stromatolites
 Microscopic
cells of prokaryotes
 Algal filaments
 Molecular fossils
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STROMATOLITES


An organo-sedimentary structure built by photosynthetic
cyanobacteria or blue-green algae.
Stromatolites form through the activity of cyanobacteria in
the tidal zone. The sticky, mucilage-like algal filaments of the
cyanobacteria trap carbonate sediment during high tides.
Modern stromatolites, Shark Bay,
western Australia
Jane Gould/Alamy
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STROMATOLITES

More abundant in Proterozoic rocks than in
Archean rocks.
Examples:
 Oldest
are 3.5 b.y. old, Warrawoona Group,
Australia's Pilbara Shield
 3 b.y. old Pongola Group of southern Africa
 2.8 b.y. old Bulawayan Group of Australia
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STROMATOLITES



Stromatolites are scarce today because
microorganisms that build them are eaten by
marine snails and other grazing invertebrates.
Stromatolites survive only in environments that are
too saline or otherwise unsuitable for most grazing
invertebrates.
The decline of stromatolites is associated with the
evolutionary appearance of new groups of marine
invertebrates during Early Paleozoic.
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OLDEST DIRECT EVIDENCE OF LIFE






Microscopic cells and filaments of prokaryotes.
Associated with stromatolites
Similar to cyanobacteria living today, which produce
oxygen.
Fossiliferous chert bed associated with the Apex
Basalt
Found in Warrawoona Group, Pilbara Supergroup,
western Australia
3.460–3.465 billion years old
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OTHER EVIDENCE OF ARCHEAN LIFE

Indirect evidence of life in older rocks
Found in banded iron deposits in Greenland.
Carbon-13 to carbon-14 ratios are similar to
those in present-day organisms.
3.8 b.y.
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OTHER EVIDENCE OF ARCHEAN LIFE
Algal filament fossils
Filamentous prokaryotes preserved in
stromatolites.
Found at North Pole, western Australia;
3.4–3.5 b.y. old.
 Spheroidal bacterial structures
Found in rocks of the Fig Tree Group, South
Africa (cherts, slates, ironstones, and
sandstones).
Prokaryotic cells, showing possible cell
division; 3.0–3.1 b.y. old.

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OTHER EVIDENCE OF ARCHEAN LIFE

Molecular fossils
Preserved organic molecules that only
eukaryotic cells produce.

Indirect evidence for eukaryotes.
In black shales from northwestern Australia;
2.7 b.y.

Origin of eukaryotic life is pushed back to
2.7 b.y.
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THE ORIGIN OF LIFE
The basic materials from which microbial organisms
(i.e., life) could have developed initially. May have
arrived on Earth during Archean in meteorites called
carbonaceous chondrites, which contain organic
compounds.
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LIFE REQUIRES THESE ELEMENTS:
Carbon
 Hydrogen
 Oxygen
 Nitrogen
 Phosphorus
 Sulfur
Each of these is abundant in the Solar System.

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FOUR ESSENTIAL COMPONENTS OF LIFE:
1.
2.
Proteins: Chains of amino acids. Proteins are used to
build living materials, and as catalysts in chemical
reactions in organisms.
Nucleic acids: Large complex molecules in cell
nucleus.


3.
4.
DNA (carries the genetic code and can replicate itself)
RNA
Organic phosphorus compounds: Used to transform
light or chemical fuel into energy required for cell
activities.
Cell membrane to enclose the components within
the cell.
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ORIGINAL LIFE WAS ANAEROBIC



The earliest organisms developed in the presence
of an atmosphere which lacked oxygen. The
organisms must have been anaerobic (i.e., they did
not require oxygen for respiration).
Organic molecules could not assemble into larger
structures in an oxygenated environment. Oxidation
and microbial predators would break down the
molecules.
Because the atmosphere lacked oxygen, there was
no ozone shield to protect the surface of the Earth
from harmful ultraviolet (UV) radiation.
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ORIGIN OF AMINO ACIDS
UV radiation can recombine atoms in mixtures of
water, ammonia and hydrocarbons, to form amino
acids.
(The energy in lightning can do the same thing.)
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MILLER EXPERIMENT
Lab simulation experiments by S. Miller in the
1950's formed amino acids from gases present in
Earth's early atmosphere:
 H2,
 CH4 (methane),
 NH3 (ammonia), and
 H2O (water vapor or steam),
along with electrical sparks (to simulate lightning).
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MILLER EXPERIMENT
This was the first laboratory synthesis of
amino acids. A liquid was produced that
contained a number of amino acids and other
complex organic compounds that comprise
living organisms. A main requirement was the
lack of free oxygen.
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JOINING AMINO ACIDS TO FORM
PROTEINS
Amino acids are monomers and have to be joined
together to form proteins, which are polymers (or
chains).
This requires:
 Input of energy
 Removal of water
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JOINING AMINO ACIDS TO FORM
PROTEINS
How could this occur?
1.
Heating (volcanic activity)
2.
At lower temperatures in the presence of
phosphoric acid
3.
Evaporation
4.
Freezing
5.
Involve water in a dehydration chemical reaction
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JOINING AMINO ACIDS TO FORM
PROTEINS
6.
7.
On surface of clay particles, which have
charged surfaces, and to which polar
molecules could attach. Metallic ions on
clays could concentrate organic molecules in
an orderly array, causing them to align and
link into protein-like chains.
On pyrite, which has a positively charged
surface to which simple organic compounds
can become bonded. Formation of pyrite
yields energy which could be used to link
amino acids into proteins.
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PROTEINOIDS


Proteinoids are protein-like chains produced in the
lab by Fox from a mixture of amino acids.
Considered to be possibly like the transitional
structures leading to proteins billions of years ago.
Similar proteinoids are also found in nature around
Hawaiian volcanoes.
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PROTEINOIDS
Hot aqueous solutions of proteinoids will cool
to form microspheres, tiny spheres that have
many characteristics of living cells:
 Film-like
outer wall
 Capable of osmotic shrinking and swelling
 Budding similar to yeast
 Divide into daughter microspheres
 Aggregate into lines to form filaments, as in
some bacteria
 Streaming movement of internal particles, as in
living cells
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WHERE DID LIFE ORIGINATE?
Early life may have avoided UV radiation by living:
Deep beneath the water
 Beneath the surface of rocks (or below sediment,
such as stromatolites)

Life probably began in the sea, perhaps in areas
associated with submarine hydrothermal vents
or black smokers.
Other possibilities:
Deep underground
Comets
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EVIDENCE FOR LIFE BEGINNING IN THE
SEA NEAR HYDROTHERMAL VENTS
1.
2.
3.
Sea contains salts needed for health and growth.
Water is universal solvent, capable of dissolving
organic compounds, producing a "rich organic
broth" or primordial soup.
Ocean currents mix these compounds, leading to
collisions between molecules, leading to
combination into larger organic molecules.
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EVIDENCE FOR LIFE BEGINNING IN THE
SEA NEAR HYDROTHERMAL VENTS
4.
5.
6.
Microbes at vents are hyperthermophiles
that thrive in seawater hotter than boiling
point (100oC).
These microbes derive energy by
chemosynthesis, without light, rather than by
photosynthesis (suggests origin in deep
water in absence of light).
Hyperthermophiles are Archaea, with DNA
different from bacteria.
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FEEDING LIFE ON EARTH—
OBTAINING NUTRIENTS
Examples of types of feeding modes:
1.
Fermenters: digest chemicals, such as sugar, in
the absence of oxygen, to obtain energy. Produce
CO2 and alcohol. Example: Yeast
2.
Autotrophs: manufacture their own food.
Examples: sulfur bacteria, nitrifying bacteria, and
photoautotrophs (such as plants and
photosynthetic bacteria) that use photosynthesis
3.
Heterotrophs : can't make their own food, so they
must find nutrients in the environment to eat.
Example: Animals.
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EVOLUTION OF EARLY LIFE



The earliest cells had to form and exist in anoxic
conditions (in the absence of free oxygen).
Likely to have been anaerobic bacteria or Archaea.
Some of the early organisms became
photosynthetic, possibly due to a shortage of raw
materials for energy.
Produced their own raw materials. Autotrophs.
Photosynthesis was an adaptive advantage.
Oxygen was a WASTE PRODUCT of photosynthesis.
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CONSEQUENCES OF OXYGEN BUILDUP
IN THE ATMOSPHERE
1.
2.
3.
4.
5.
Ozone layer which absorbs harmful UV radiation,
and protected primitive and vulnerable life forms.
End of banded iron formations which only formed
in low, fluctuating O2 conditions
Oxidation of iron, leading to the first red beds.
Aerobic metabolism developed. Uses oxygen to
convert food into energy.
Development of eukaryotic cell, which could cope
with oxygen in the atmosphere.
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PROKARYOTES VS. EUKARYOTES


Prokaryotes reproduce asexually by simple cell
division. This restricts their genetic variability.
Prokaryotes have shown little evolutionary change
for more than 2 billion years.
Eukaryotes reproduce sexually through the union of
an egg and sperm. This combines chromosomes
from each parent and leads to genetic
recombination and increased variability. Many new
genetic combinations. Led to a dramatic increase
in the rate of evolution.
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PROKARYOTES VS. EUKARYOTES
FIGURE 8-37 (A)
Comparison of a
prokaryote cell
(left) and a
eukaryote cell.
Note that the
prokaryotic cell is
tiny (0.5 to
1.0 micrometers).
The eukaryotic
cell is larger (10 to
100 micrometers),
contains a true
nucleus, and
various
organelles.
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PROKARYOTES VS. EUKARYOTES
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THE EARLIEST EUKARYOTES
Earliest large cells that appear to be eukaryotes
appear in the fossil record about 1.6–1.4 b.y. ago
(during Proterozoic).
Eukaryotes diversified around the time that the
banded iron formations disappeared and the red
beds appeared, indicating the presence of oxygen
in the atmosphere.
Origin of eukaryotic life was probably around 2.7 b.y.,
based on molecular fossils.
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ENDOSYMBIOTIC THEORY FOR THE
ORIGIN OF EUKARYOTES




Billions of years ago, several prokaryotic cells came
together to live symbiotically within a host cell as
protection from (and adaptation to) an oxygenated
environment.
These prokaryotes became organelles.
Evidence for this includes the fact that
mitochondria contain their own DNA.
Example: a host cell (fermentative anaerobe) +
aerobic organelle (mitochondrion) + spirochaetelike organelle (flagellum for motility).
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ENDOSYMBIOTIC
ORIGIN OF
EUKARYOTES
FIGURE 8-38 A theory for the
origin of eukaryotes.
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EUKARYOTES
The appearance of eukaryotes led to a dramatic
increase in the rate of evolution, and was ultimately
responsible for the appearance of complex
multicellular organisms.
© 2013 JOHN WILEY & SONS, INC. ALL RIGHTS RESERVED.
IMAGE CREDITS
• FIGURE 8-1 The Precamrian eons span 87% of Earth history. Source: Harold Levin.
• FIGURE 8-4 Schematic view of the Solar System, showing orbits of the eight planets, the planetoid
Pluto, the asteroid belt, and a comet. Source: From Fletcher, C., 2011, Physical Geology, Fig. 2.1. This
material is reproduced with permission of John Wiley & Sons, Inc.
• FIGURE 8-6 The solar nebula hypothesis for the origin of the Solar System. Source: Harold Levin.
• FIGURE 8-21 The relative amounts of gases in the primordial atmosphere were different from the
abundances vented to the exterior of Earth during differentiation. Source: Harold Levin.
• FIGURE 8-31 Generalized cross-section through two greenstone belts. Source: Harold Levin.
• FIGURE 8-25 Exposed Precambrian rocks. Source: Harold Levin.
• FIGURE 8-26 North American craton, shield, and platform. Source: Harold Levin.
• FIGURE 8-27 Precambrian Provinces of North America. Source: Hoffman, P.F., 1989, Precambrian
geology and tectonic history of North America, in Bally, A.W., and Palmer, A.R., eds., Geology of North
America—An Overview:, Boulder, Colorado, Geological Society of America, Geology of North America,
vol. A, p. 447–512.
• FIGURE 8-31 Generalized cross-section through two greenstone belts. Source: Harold Levin.
• FIGURE 8-37 (A) Comparison of a prokaryote cell (left) and a eukaryote cell. Source: Harold Levin.
• FIGURE 8-38 A theory for the origin of eukaryotes. Source: Thomas Brucker on behalf of John Wiley
& Sons, Inc.
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