Download Fossils and the diversity of life

Precambrian Time
• “Precambian” is the informal term for the
interval of time prior to the evolutionary
radiation of skeletonized animals at 543 Ma
• “Precambrian” is subdivided into:
– Archean Eon, from the origin of the Earth (4.6
Ga) to the stabilization of Earth’s basic
structure (core/mantle/crust) (2.5 Ga)
– Proterozoic Eon, from 2.5 Ga to the beginning
of Cambrian time (543 Ga)
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Geologic time
Archean
43%
Phanerozoic
12%
Proterozoic
45%
Precambrian = 88%
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Precambrian rocks
• Although Precambrian time accounts for
88% of Earth’s history, Precambrian rock
exposures make up only about 20% of
Earth’s land surface
• Most Precambrian rocks have been
destroyed in the course of plate tectonic
cycles (and most remaining ones are buried
beneath the veneer of Phanerozoic rocks)
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Precambrian rocks
• Cratons are the stable, interior regions of continents
that have not undergone major deformation since
Precambrian or early Phanerozoic time
• Most Precambrian rocks are confined to cratons,
where they may be exposed in a “Precambrian
shield”
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Precambrian
shield area in
NW Canada
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Archean Time:
From the very beginning….
• Age of universe is estimated at ~13 billion
years (redshift evidence)
• Oldest radiometrically dated rocks on Earth
are ~4.1 billion years old
• But, meteorites and lunar rocks have been
dated at 4.6 billion years, suggesting that
our solar system is about that old
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Origin of our
solar system
Solar nebula forms
(remains of supernova)
Rotation and
contraction to disk
Central concentration
of matter
Formation of discrete
rings of matter
Condensation of matter
into planets
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Solar system
• Outer planets are composed largely of volatile
compounds
• Denser, less volatile compounds make up the inner
planets
• Asteroid belt is a ring of debris that has not coalesced into
a planet
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Origin of Earth
• Primordial Earth accreted from successive
impacts of hot, giant asteroids (some the size of
Mars)
• Early Earth was molten because of heat from
energy of impacts and radioactive decay
• Dense materials sank to center of planet, with less
dense materials rising toward surface
• “Magma ocean” at surface eventually cooled to
form oceanic crust
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Early Earth
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Origin of Earth (cont.)
Homogeneous
molten Earth
Segregation of
materials by density
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Final differentiation
of core/mantle/crust
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Earth’s early heat flow
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Earth’s internal heat
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Earth’s early atmosphere
• Earth did not inherit its atmosphere from
the initial asteroids that coalesced to form it
• Earliest atmosphere was generated by
emission of internal gases :
– Nitrogen, water vapor, carbon dioxide,
• Note absence of oxygen, which was rare
prior to the advent of photosynthetic
organisms!
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Earth’s early oceans
• Ocean water originated mostly from emitted
water vapor and partly from icy comets as they
melted upon entry into Earth’s atmosphere
– 15 million small comets (~12 meters in diameter) enter
Earth’s atmosphere every year!
• Salts were added to the oceans from rivers
carrying by-products of chemically weathered
rocks
– Salinity stabilized very early in Archean time because
salt is removed from the oceans by precipitation of salt
minerals
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Origin of continents
• Earth’s early crust was entirely oceanic
crust of mafic composition
• Earliest continental (felsic) crust must have
originated from a mafic parent, but how?
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Origin of continents:
Iceland example
• Iceland is a volcanic island situated over a hot spot along
the mid-Atlantic ridge
• Here, lower oceanic crust contains isolated “pods” of felsic
material that have segregated from igneous material in the
mantle
• Mafic magma flows to the surface along faults; in doing so
it melts felsic bodies along the way  felsic volcanics
• As volcanics pile up, isostatic sinking of Iceland causes
partial melting and further segregation of felsics  more
felsic volcanics
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Origin of continents:
Iceland example
About 10% of Iceland’s crust is felsic in composition
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Origin of continents:
Iceland example
• Iceland’s crust is 8–10 km thick, about
twice the average thickness of oceanic crust
• Iceland is only about 16 million years old
and still growing—it’s a protocontinent!
• Archean continents probably were small,
too
• Modern continents formed via the fusion of
multiple protocontinents
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Greenstone belts
• Elongate belts of weakly metamorphosed rock
separating larger masses of felsic protocontinents
• Make up large portions of Archean terranes
• Age of most greenstone belts is ~2.5–3.0 billion
years
• Include metamorphosed mixtures of mafic and
felsic volcanics, volcanic sediments, turbidites
• Probably formed along subduction zones where
protocontinents were sutured together
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Formation of greenstone belts
Time 1
Time 2
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Greenstone belts
Satellite view of
Archean greenstone belts
and felsic protocontinents
in western Australia
25 mi
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Archean sedimentary rocks
• Archean sedimentary rocks are mostly of
deep-water origin
– Mostly sandstones, cherts, shales
– Very few, if any, limestones or evaporites
– No well developed continental shelves for
accumulation of shallow-water deposits
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Archean sedimentary rocks
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Life on Earth
• Earth is the right size
– Gravitational pull of larger planets creates an
atmosphere too dense for penetration of
sunlight
– Gravitational pull of smaller planets is too weak
to retain an atmosphere
• Earth’s temperature is just right
– Most H2O is in the form of liquid water, not ice
or water vapor
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The Archean fossil record
• All Archean fossils are prokaryotes
– Archeobacteria and Eubacteria
• The oldest known forms are bacterial
filaments like modern cyanobacteria
– 3.2 to 3.5 billion years old, from Western
Australia
• Stromatolites known in rocks 3.4 billion
years old and younger
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The Archean fossil record (cont.)
3.5 billion year old bacteria
preserved in chert from
Western Australia
Modern cyanobacterial
filaments
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Fossilized bacterial filaments:
3.2 billion years old, NW Australia
diameter of
filaments = 2 µm
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Oldest known stromatolites:
3.45 Ga, Western Australia
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The Archean fossil record (cont.)
3.2 billion year old stromatolite
from South Africa
Growth of cyanobacterial mats
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Origin of life
• Basic attributes of life:
– Ability to reproduce
– Self-regulation (ability to sustain orderly
internal chemical reactions)
• Amino acids are the building blocks of
proteins
• Proteins are among the compounds
required for reproduction and regulation
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Stanley Miller
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Laboratory synthesis of amino acids
• Stanley Miller, 1953
–
–
–
–
–
–
Hydrogen (H)
Ammonia (NH3)
Methane (CH4)
Water vapor (H2O)
Electrical spark
No O2
Amino acids and
simple protein-like
molecules
collected here
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Origin of life
• Miller’s assumption was that no O2 existed in
Earth’s early atmosphere
– Incorrect: at least some was there (but not much)
• Experiment did produce many types of amino
acids that combined to form simple protein-like
compounds
• Amino acids later discovered in Murchison
meteorite (1969) in the same relative proportions
as in Miller’s soup
– Thus, amino acids could have been delivered to the
Archean Earth from space
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Further experiments
• Joan Oro i Florensa
(1961):
– Hydrogen cyanide (HCN)
+ ammonia (NH3)
produced the nucleotide
base adenine
– Later experiments
produced other nucleotide
bases found in DNA and
RNA
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Origin of life
• Nucleic acids
DNA and RNA
carry genetic code
and have ability to
replicate
nucleotide
bases
sugar
phosphate group
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Origin of life
• Messenger RNA carries information from
DNA to sites where proteins are formed
• Transfer RNA ferries amino acids to sites
where proteins are formed, and serves as a
catalyst in protein growth
• RNA probably was the nucleic acid in the
earliest true form of life, with DNA
evolving later
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Origin of life
• Once RNA and DNA had
originated, semipermeable
cell membranes evolved
– protect the chemical system of
the primitive organism while
allowing certain compounds to
pass in and out
– bilayered lipids spontaneously
form vesicles in water
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Origin of life
• Where did life form?
– Probably not at the Earth’s surface in shallow
pools, as once believed
• Presence of oxygen would have inhibited the
prebiotic synthesis of “Stanley Miller soup”
– Most likely in the deep sea, away from O2, and
probably near a “vent” of hot water
• Modern chemosynthetic bacteria are abundant near
vents on mid-ocean ridges
• They derive energy by consuming chemical
compounds and allowing reactions to occur within
their cell membranes
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Mid-ocean ridge “vents”
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Origin of life
• Mid-ocean ridges are the most likely sites
for origin of life and early bacterial
evolution
– Enormous size  many opportunities for key
events to take place
– Anoxic (no O2) water with necessary amino
acid building blocks present
– Other key materials present
• Phosphorus, nickel, zinc, clays
– Modern “vent” bacteria are genetically the
most primitive archeobacteria known
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