GOL 106 LAB 8
TECTONIC SETTINGS
Group ________
Member Names
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Question(s)
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Chapter 7
Evolution (Part I)
The Theory and Its Supporting
Evidence
1
Darwin and the Galápagos
• During Charles Darwin’s five-year voyage
– (1831-1836) on the HMS Beagle, he visited the
Galápagos Islands where he made important
observations that changed his ideas about
the then popular concept called the fixity of species
• an idea holding that all present-day species had been
created in their present form and had changed little or
not at all
• Darwin fully accepted
– the Biblical account of creation before the voyage
2
Route of HMS Beagle
• Map showing the route (red line) followed
– by Charles Darwin when he was aboard HMS
Beagle from 1831 to 1836
• The Galápagos Islands
– are in the Pacific Ocean west of Ecuador
3
The Galápagos Islands
• The Galápagos
Islands
– are specks of
land
– composed of
basalt
– in the eastern
Pacific
4
Darwin Developed the Theory
• During the voyage Darwin observed
– that fossil mammals in South America are similar
yet different from present-day llamas, sloths, and
armadillos
– that the finches and giant tortoises living on the
Galápagos Islands vary from island to island
– and still resemble ones from South America, even
though they differ in subtle ways
• These observations convinced Darwin
– that organisms descended with modification from
ancestors that lived during the past
– the central idea of the theory of evolution
5
Galápagos Finches
• Darwin’s finches from the Galápagos Islands
– arranged to show evolutionary relationships
Insect eaters
Insect eaters
Berry
eater
Seed Cactus
eaters eaters
– Notice
that beak
shape
– varies
depending
on diet
6
Why Study Evolution?
• Evolution
– involving inheritable changes in organisms through
time
• is fundamental to biology and paleontology
– Paleontology is the study of life history as revealed
by fossils
• Evolution is a unifying theory
• like plate tectonic theory
– that explains an otherwise encyclopedia collection
of facts
• Evolution provides a framework for discussion
of life history
7
Evolution: Historical Background
• Evolution, the idea that today’s organisms have
descended with modification from ancestors that lived
during the past is usually attributed solely to Charles
Darwin
– but it was seriously considered long before he was
born, even by some ancient Greeks and by
philosophers and theologians
• during the Middle Ages
• Nevertheless, the prevailing belief in the 1700s was
that Genesis and the works of Aristotle explained the
origin of life and contrary views were heresy
8
Evolution: Historical Background
• During the 18th century, naturalists were
discovering evidence that could not be
reconciled with literal reading of the Bible
• In this changing intellectual atmosphere,
scientists gradually accepted a number of ideas:
•
•
•
•
the principle of uniformitarianism,
Earth’s great age,
that many types of plants and animals had become extinct,
and that change from one species to another occurred
• What was lacking, though,
– was a theoretical framework to explain evolution
9
Lamarck
• Jean-Baptiste de Lamarck (1744-1829) is best
remembered for his theory of inheritance of acquired
characteristics, though he greatly contributed to our
understanding of the natural world
• According to this
theory, new traits
arise in organisms
because of their
needs and are
somehow passed on
to their descendants
• Lamarck’s theory seemed logical at the time
10
Lamarck’s Theory
• Lamark’s theory was
not totally refuted
– until decades later
– with the discovery that
genes
• units of heredity
– cannot be altered by
any effort by an
organism during its
lifetime
11
Darwin
• In 1859, Charles Robert
Darwin (1809-1882)
– published On the Origin
of Species
• in which he detailed
– his Natural Selection
ideas
– formulated 20 years
earlier
– and proposed a
mechanism for evolution
12
Natural Selection
• Plant and animal breeders practice artificial
selection by selecting those traits they deem
desirable and then breed plants and animals with
those traitsthereby bringing about a great amount
of change
• Observing artificial selection gave Darwin the idea
that a process of selection among variant types in
nature could also bring about change
• Therefore, a natural process was selecting only a
few individuals for survival
13
Artificial Selection
• Through artificial selection, humans have given rise to
dozens of varieties of
– domestic dogs, pigeons, sheep, cereal crops, and vegetables
• Wild mustard was selectively bred
– to yield broccoli, cauliflower, kale, and cabbage
14
Natural Selection—Main Points
• Organisms in all populations possess heritable
variations such as
– size, speed, agility, visual acuity, digestive
enzymes, color, and so forth
• Some variations are more favorable than others
– some have a competitive edge in acquiring
resources and/or avoiding predators
• Those with favorable variations
– are more likely to survive
– and pass on their favorable variations
15
“Survival of the Fittest”
• In colloquial usage, natural selection is sometimes
expressed as “survival of the fittest”
• This is misleading
because
– natural selection is
not simply a
matter of survival
but involves
inheritable
variations leading
to reproductive
success
16
Not only Biggest, Strongest, Fastest
• One misconception about natural selection
– is that among animals, only the biggest, strongest,
and fastest are likely to survive
– These characteristics might provide an advantage
• but natural selection may favor
–
–
–
–
–
the smallest if resources are limited
the most easily concealed
those that adapt most readily to a new food source
those having the ability to detoxify some substance
and so on…
17
Limits of Natural Selection
• Natural selection works on existing variation in
a population
• It could not account for the origin of variations
• Critics reasoned that should a variant trait arise,
– it would blend with other traits and would be lost
• The answer to these criticisms
– existed even then in the work of Gregor Mendel,
but remained obscure until 1900
18
Mendel and the Birth of Genetics
• During the 1860s, Gregor Mendel, an Austrian
monk, performed a series of controlled
experiments with true-breeding strains of garden
peas strains that when self-fertilized always
display the same trait, such as flower color
• Traits are controlled by a pair of factors, now
called genes
• Genes occur in alternate forms, called alleles
– One allele may be dominant over another
– Offspring receive one allele of each pair from each
parent
19
Mendel’s Experiments
• The parental generation consisted of
– true-breeding strains,
– RR = red flowers,
rr = white flowers
• Cross-fertilization
yielded a second
generation
– all with the Rr
combination of
alleles,
– in which the R
(red) is dominant
over r (white)
20
Mendel’s Experiments
• The second generation, when self-fertilized produced
a third generation with a ratio of three red-flowered plants
to one white-flowered plant
21
Importance of Mendel’s Work
• The factors (genes) controlling traits
– do not blend during inheritance
• Traits not expressed in each generation
– may not be lost (it’s simply recessive)
• Therefore, some variation in populations
– results from alternate expressions of genes (alleles)
• Variation can be maintained
22
Genes and Chromosomes
• Complex, double-stranded
helical molecules of
deoxyribonucleic acid (DNA)
• called chromosomes
– are found in cells of organisms
• Specific segments of DNA are
the basic units of heredity (genes)
• The number of chromosomes
– varies from one species to another
– fruit flies 8; humans 46; horses 64
23
Sexually Reproducing Organisms
• In sexually reproducing organisms,
– the production of sex cells
• pollen and ovules in plants
• sperm and eggs in animals
– results when cells undergo a type of cell division
known as meiosis
• This process yields cells with only one
chromosome of each pair
– so all sex cells (gametes) have only 1/2 the
chromosome number of the parent cell
24
Meiosis
• During meiosis,
– sex cells that contain
one member of each
chromosome pair
form
• Formation of sperm
is shown here
• Eggs form the same
way,
– but only one of the
four final eggs is
functional
25
Fertilization
• The full number of chromosomes is restored
when a sperm fertilizes an egg
– or when pollen fertilizes an
ovule
• The zygote then
– has a full set of
chromosomes typical for
that species
• As Mendel deduced,
– 1/2 the genetic makeup of
fertilized egg comes from
each parent
• The fertilized egg
– grows by mitosis
26
Mitosis
• Mitosis is cell division
– that results in the
complete duplication of a
cell
• In this example,
– a cell with four
chromosomes (two pairs)
produces two cells each
with four chromosomes
• Mitosis takes place in all
cells except sex cells
• Once an egg has been
fertilized, the developing
embryo grows by mitosis
27
Modern View of Evolution
• During the 1930s and 1940s, paleontologists,
population biologists, geneticists, and others
developed ideas that merged to form a modern
synthesis or neo-Darwinian view of evolution
• They incorporated chromosome theory of
inheritance into evolutionary thinking
• They saw changes in genes (mutations) as one
source of variation
• They completely rejected Lamarck’s idea of
28
inheritance of acquired characteristics
Modern View of Evolution
• They reaffirmed the importance of natural
selection
• According to modern evolutionary theory,
populations rather than individuals evolve.
– Individuals with favorable traits
• are more likely to survive and reproduce
• if their variations are favorable
– As a result, descendant populations possess
variations in greater frequency
29
What Brings about Variation?
• Evolution by natural selection
– works on variation in populations most of which is
accounted for by the reshuffling of genes from
generation to generation during sexual reproduction
• The potential for variation is
enormous
– with thousands of genes,
each with several alleles,
– and with offspring
receiving 1/2 of their
genes from each parent
• New variations arise by
mutations
– change in the
30
chromosomes or genes
Mutations
• Mutations result in a change
– in hereditary information
• Mutations that take place in sex cells
– are inheritable,
– whether they are chromosomal mutations
• affecting a large segment of a chromosome
– or point mutations
• individual changes in particular genes
• Mutations are random with respect to fitness
– they may be beneficial, neutral, or harmful
31
Mutations
• If a species is well adapted to its environment,
– most mutations would not be particularly useful
and perhaps would be harmful
• But what was a harmful mutation
– can become a useful one if the environment
changes
32
Neutral Mutations
• Information in cells is carried on chromosomes
– which direct the formation of proteins
– by selecting the appropriate amino acids
– and arranging them into a specific sequence
• Neutral mutations may occur
– if the information carried on the chromosome
– does not change the amino acid or protein that is
produced
33
What Causes Mutations?
• Some mutations are induced by mutagens
– agents that bring about higher mutations rates such as
•
•
•
•
some chemicals
ultraviolet radiation
X-rays
extreme temperature changes
• Some mutations are spontaneous
– occurring without any known mutagen
34
Species
• Species is a biological term for a population
– of similar individuals that naturally interbreed and
produce fertile offspring
• Species are reproductively isolated from one another
• Goats and sheep do not interbreed in nature,
– so they are separate species
• Yet in captivity
– they can produce fertile offspring
35
Speciation
• Speciation is the phenomenon of a new species
arising from an ancestral species
• It involves change in the genetic makeup
– of a population,
– which also may bring about changes in form and
structure
• During allopatric speciation,
– species arise when a small part of a population
– becomes isolated from its parent population
36
Allopatric Speciation
• A few individuals of a species on the mainland
– reach isolated island 1
– Speciation follows genetic divergence in a new
habitat.
37
Allopatric Speciation
• Later in time, a few individuals of the new
species colonize island 2
– In this new habitat, speciation follows genetic
divergence.
38
Allopatric Speciation
• Speciation may also follow colonization of
islands 3 and 4
• Invasion of island 1 by genetically different
descendants of the ancestral species!
39
Rate of Speciation
• Although widespread agreement exists on allopatric
speciation scientists disagree on how rapidly a new
species might evolve
– Phyletic gradualism
• the gradual accumulation of minor changes eventually brings
about the origin of new species
– Punctuated equilibrium
• holds that little or no change
takes place in a species during
most of its existence
• Then evolution occurs rapidly
giving rise to a new species
– in as little as a few thousands of
years
40
Styles of Evolution
• Divergent evolution occurs
when an ancestral species
gives rise to diverse
descendants that differ
markedly from their ancestors
• Convergent evolution
involves the development
of similar characteristics in
distantly related
organisms
• Parallel evolution involves
the development of similar
characteristics in closely
related organisms
41
Microevolution and Macroevolution
• Microevolution is any change in the genetic make-up
of a species, and involves changes within a species
• Macroevolution involves changes such as the origin of
a new species or changes at even higher levels
– For example, the origin of birds from reptiles
• The cumulative effects of microevolution are
responsible for macroevolution
42
Cladistics and Cladograms
• Traditionally, scientists have depicted evolutionary
relationships with phylogenetic trees
• in which the horizontal axis represents anatomical
differences
• and the vertical axis denotes time
• In contrast, a cladogram shows the relationships
among members of a clade
• a group of organisms including its most recent common
ancestor
• Cladistics focus on derived characteristics
sometimes called evolutionary novelties
– as opposed to primitive characteristics
43
Phylogenetic Tree
• A phylogenetic
tree showing
the
relationships
among various
organisms
44
Cladogram
• A cladogram showing inferred relationships
• Some of the characteristics used
– to construct this cladogram are indicated
45
QUESTIONS?
46
Chapter 7
Evolution (Part II)
The Theory and Its Supporting
Evidence
47
Evolutionary Novelties
• All land-dwelling vertebrate animals possess
bone and paired limbs so these characteristics
are primitive and of little use in establishing
relationships among land vertebrates
• However, hair and three middle ear bones are
derived characteristics because only one
subclade, the mammals, has them
48
Evolutionary Novelties
• If considering only mammals, hair and middle
ear bones are primitive characteristics
– but live birth is a derived characteristic that serves to
distinguish most mammals from the egg-laying
mammals
49
Evolutionary Trends
• Evolutionary changes do not involve
– all aspects of an organism simultaneously
• A key feature we associate with a descendant
group might appear before other features typical
of that group
• For example, the oldest known bird
– had feathers and the typical fused clavicles of birds,
– but it also retained many reptile characteristics
• Mosaic evolution is the concept that
– organisms possess recently evolved characteristics
as well as some features of their ancestral group
50
Phylogeny
• Phylogeny is the evolutionary history of a
group of organisms
• If sufficient fossil material is available,
– paleontologists determine the phylogeny and
evolutionary trends for groups of organisms
• For example, one trend in ammonoids
• extinct relatives of squid and octopus
– was the evolution of an increasingly complex shell
51
Evolutionary Trends
• Abundant fossils show the evolutionary trends of
– the Eocene mammals, Titanotheres
• These extinct relative of
horses and rhinoceroses
– evolved from small ancestors
– to giants standing 2.4 m at
the shoulder
– developed large horns
– and the shape of their skull
changed
– Only 4 of the 16 known
genera are shown
52
Evolutionary Trends
• Size increase is one of the most common
evolutionary trends
• However, trends are complex
– they might reverse
– more than one can take place at the same time at
different rates
• Trends in horses included
– generally larger size
• but size decreased in some now-extinct horses
– changes in teeth and skull
– lengthening legs
– reduction in number of toes
• These trends occurred at different rates
53
Adaptations
• Evolutionary trends are a series of
adaptations to changing
environment or in response to
exploitation of new habitats
• Some organisms
– show little evolutionary change for
long periods
• Lingula is a brachiopod
– whose shell has not changed
significantly since the Ordovician
54
“Living Fossils”
• Several organisms have shown
– little or no change for long periods
• If these still exist as living organisms today
– they are sometimes called living fossils
• For example:
– horseshoe crabs
– Latrimaria (fish)
– Ginkgo trees
• Some of these are generalized and can live
under a wide variety of environments
55
A Living Fossil
• Latrimaria
– belongs to a group of fish
– once thought
to have gone
extinct at the
end of the
Mesozoic
Era
A specimen was caught off the
coast of East Africa in 1938
56
A Second Living Fossil
• Ginkgos
– have changed very
little for millions of
years
57
Randomness in Natural Selection?
• But isn’t evolution by natural selection a
random process?
• If so, how is it possible
– for a trend to continue long enough to account just
by chance for such complex structures as
– eyes, wings, and hands?
58
Two Steps in Natural Selection
• Evolution by natural selection
– is a two-step process
– Only the first step involves chance
• Variation must be present
– or arise in a population
• Whether a mutation is favorable
– is a matter of chance
• The natural selection of favorable variations
– is not by chance
59
Extinctions
• Perhaps as many as 99% of all species
– that ever existed are now extinct
• Organisms do not always evolve toward some kind of
higher order of perfection or greater complexity
• Vertebrates are more complex
– but not necessarily superior in some survival sense.
– Bacteria have persisted for at least 3.5 billion
years!
• Natural selection yields organisms adapted
– to a specific set of circumstances
– at a particular time
60
Background and Mass Extinction
• The continual extinction of species is referred
to as background extinction
• It is clearly different from mass extinction
– during which accelerated extinction rates sharply
reduce Earth’s biotic diversity
• Extinction is a continual occurrence
– but so is the evolution of new species that usually
quickly exploit the opportunities another species’
extinction creates
• Mammals began a remarkable diversification
– when they began occupying niches the extinction
of dinosaurs and their relatives left vacant
61
Evidence in Support of Evolution
• Darwin cited supporting evidence
– for evolutionary theory such as
•
•
•
•
•
classification
embryology
comparative anatomy
geographic distribution
fossil record, to a limited extent
• He had little knowledge
– of the mechanism of inheritance,
– and biochemistry and molecular biology were
unknown at his time
62
Evidence in Support of Evolution
• Since Darwin’s time, studies from additional
fields
– in biochemistry
– molecular biology
– more complete and better understood fossil record
• have convinced scientists that the theory is as
well supported by evidence as any other major
theory
63
Is the Theory of Evolution
Scientific?
• An idea can only be a truly scientific theory if
testable predictive statements can be made from it
• No theory in science is ever proven in the final
sense, although substantial evidence may support
it
• All theories are always open
– to question, revision and occasionally
– to replacement by a more comprehensive theory
64
Theories Must Be Predictive
• Not all predictions are about future events, and
– evolutionary theory cannot make predictions about
the far distant future
• Nevertheless, we can make a number of
predictions
– about the present-day biological world and about
the fossil record that should be consistent with
evolutionary theory if it is correct
65
Some Predictions from Evolution
• If evolution has taken place,
– closely related
species such as
wolves and
coyotes should be
similar in
anatomy and
biochemistry,
genetics, and
embryonic
development
66
Testable
• Suppose that contrary to evolutionary prediction
– wolves and coyotes were not similar in terms of their
biochemistry, genetics and embryonic development,
then
– our prediction would fail and we would at least have to
modify the theory
• If other predictions also failed,
– for example, if mammals appeared in the fossil record
before fishes then we would have to abandon the
theory and find a better explanation for our
67
observations
Classification
• Classification uses a nested pattern of
similarities
• Carolus Linneaus (1707-1778) proposed
– a classification scheme in which organisms receive
a two-part name consisting of genus and species
– for example, the coyote is Canis latrans
• Linnaeus’s classification is an ordered list
– of categories that becomes more inclusive as one
proceeds up the list
68
Linnaean Classification
• the coyote, Canis latrans
• Animalia
– Chordata
Most inclusive
• Kingdom
– Phylum
• Subphylum
– Class
» Order
• Family
– Genus
• Species
• Vertebrata
– Mammalia
» Carnivora
• Canidae
– Canis
• latrans
Least inclusive
69
Classification —shared Characteristics
• Subphylum
vertebrata
– including
fishes,
amphibians,
reptiles, birds
and mammals,
– have a
segmented
vertebral
column
• Only warmblooded
animals with
hair/fur and
mammary
glands are
mammals
70
Coyote, Canis latrans
• 18 orders of
mammals exist
including order
Carnivora
• The Family
Canidae are
doglike
carnivores
• and the genus
Canis includes
only closely
related species
• Coyote, Canis
latrans, stands
alone as a
species
71
Coyote and Wolf
• Coyote (Canis latrans) and wolf (Canis lupus)
– share numerous characteristics as members of the
same genus
• They share some but fewer characteristics
– with the red fox (Volpes fulva) in the family
Canidae
• All canids share some characteristics with cats,
– Bears, and weasels in the order Carnivora
– which is one of 18 living orders of the class
Mammalia
• Shared characteristics are evidence for
evolutionary relationships
72
Biological Evidence
Supporting Evolution
• If all existing organisms descended with
modification from ancestors that lived during
the past,
• all life forms should have fundamental
similarities:
– all living things consist mainly of carbon, nitrogen
hydrogen and oxygen
– their chromosomes consist of DNA
– all cells synthesize proteins
• in essentially the same way
73
Evolutionary Relationships
• Biochemistry provides evidence for
evolutionary relationships
• Blood proteins are similar among all mammals
– Humans’ blood chemistry is related
•
•
•
•
most closely to the great apes
then to Old World monkeys
then New World monkeys
then lower primates such as lemurs
• Biochemical tests support the idea
– that birds descended from reptiles
• a conclusion supported by evidence in the fossil record
74
Structures with Similarities
• Homologous structures
– are basically similar structures that have been
modified for different functions
– They indicate derivation from a common ancestor.
75
Homologous Structures
• Forelimbs of humans, whales, dogs, and birds
– are superficially dissimilar,
– yet all are made up of the same bones,
– have similar
arrangement
– of muscles,
nerves and
blood
vessels,
– are similarly
arranged with respect to other structures,
– have similar pattern of embryonic development
76
Structures with Similarities
• Analogous structures are structures with
similarities unrelated to evolutionary relationships
that serve the same function but are quite
dissimilar in both structure and development
• Wings of insects and birds
– serve the same function but differ considerably
– in structure and development
77
Vestigial Structures
• Vestigial structures are remnants
– of structures in organisms that were functional
– in their ancestors
• Why do dogs have tiny,
– functionless toes on their
feet (dewclaws)?
• Ancestral dogs had five
toes
– on each foot,
– all of which contacted the
ground
• As they evolved
– they became toe-walkers with only four toes on the ground
– and the big toes and thumbs were lost or reduced
– to their present state
78
Remnants of Rear Limbs in Whales
• The Eocene-aged whale, Basilosaurus,
– had tiny vestigial back limbs
– but it did not
use limbs to
support its
body weight.
79
Evolution in Living Organisms
• Small-scale evolution can be observed today.
• For example
– adaptations of some plants to contaminated soils
– insects and rodents developing resistance to new
insecticides and pesticides
– development of antibiotic-resistant strains of
bacteria
• Variations in these populations
– allowed some variant types to live and reproduce,
– bringing about a genetic change
80
What do We Learn from Fossils?
• The fossil record consists of first appearances of
various organisms through time
– One-celled organisms appeared before multi-celled
ones
– plants appeared before animals
– invertebrates before vertebrates
• Fish appeared first followed
– in succession by amphibians, reptiles, mammals, and
birds
81
Advent of Various Vertebrates
• Times when major groups of vertebrates appeared
in the fossil record
• Thickness
of
spindles
shows
relative
abundance
82
Fossils Are Common
• Fossils are much more common than many
people realize
• However the origin and initial diversification
– of a group is generally the most poorly represented
• But fossils showing the diversification of
horses, rhinoceroses, and tapirs from a
common ancestor are known
• as are ones showing the origin
– of birds from reptiles
• and the evolution
– of whales from a land-dwelling ancestor
83
The Evidence: Summary
• Scientists agree that the theory of evolution is
as well-supported by evidence as any other
theory
• Transitional fossils provide compelling
evidence
– but fossils aren’t the only evidence
• Much evidence comes from
–
–
–
–
–
–
comparative anatomy
biogeography
molecular biology
genetics
embryology
and biochemistry
84
Summary
• The central claim of evolution is that all organisms
– have descended from ancestors that lived during the
past , with modification
• Jean Baptiste de Lamarck proposed
– the first mechanism to account for evolution
– Inheritance of Acquired Characteristics
• Darwin’s observation of variation in populations
and artificial selection
– and his reading of Malthus’ essay on population
– helped him formulate his idea of natural selection
85
Summary
• In 1859 Charles Darwin and Alfred Wallace
– published their ideas of natural selection
– which hold that in populations of organisms,
some have favorable traits that make it more
likely that they will survive and reproduce
86
Summary
• Gregor Mendel’s breeding experiments
– with garden peas provided some of the answers
regarding how variation is maintained in
populations
– Mendel’s work is the basis for present-day
genetics
• Genes are the hereditary units in all organisms
– Only the genes in sex cells are inheritable
87
Summary
• Sexual reproduction and mutations
– account for most variation in populations
• Evolution by natural selection has two steps
– First, variation must exist or arise and be
maintained in interbreeding populations
– Second, favorable variants must be selected for
survival
88
Summary
• Most species evolve by allopatric speciation
– which involves isolation of a small population from its
parent population, that is then subjected to different
selection pressures
• Divergent evolution involves
– A common ancestor stock giving rise to diverse species
• The development of similar adaptive types in
different groups of organisms results from parallel
and convergent evolution
89
Summary
• Microevolution involves changes within a
species,
– while macroevolution encompasses all
changes above the species level.
• Scientists traditionally used phylogenetic
trees to depict evolutionary relationships
– but now they more commonly use cladistic
analyses and cladograms to show these
relationships
90
Summary
• Background extinctions occur continually,
– but several mass extinctions have also taken
place, during which Earth’s biologic diversity
has decreased markedly
• The theory of evolution is truly scientific
91
Summary
• Much of the evidence supporting the theory of
evolution comes from
– classification, comparative anatomy, embryology, genetics,
biochemistry, molecular biology, and present-day examples
of microevolution
• The fossil record also provides evidence for evolution
– in that it shows a sequence of different groups appearing
through time,
– and some fossils show features we would expect in the
ancestors of birds or mammals, horses, whales, and so on
92
QUESTIONS?
93
Precambrian Earth
and
Life History
(The Proterozoic Eon – Part I)
1
The Length of the Proterozoic
• The Proterozoic
Eon alone,
– at 1.958 billion
years long,
– accounts for
42.5% of all
geologic time
– yet we review
this long
episode of Earth
and life history
in a single
chapter
2
The Phanerozoic
• The Phanerozoic,
– consisting of
• Paleozoic,
• Mesozoic,
• Cenozoic eras,
– lasted a
comparatively
brief 542 million
years
3
Proterozoic Rocks
• The Vishnu schist in the Grand Canyon was originally
lava flows and sedimentary rocks, but was intruded by
the Zoraster Granite 1.7 billion years ago
4
Proterozoic Rocks
• The outcrop of sandstone and mudstone 1.0 billion years
old has only been slightly altered by metamorphism
5
Archean-Proterozoic Boundary
• Geologists have rather arbitrarily placed the ArcheanProterozoic boundary at 2.5 billion years ago because
it marks the approximate time of changes in the style
of crustal evolution
• However, we must emphasize “approximate”,
because Archean-type crustal evolution was
not completed at the same time in all areas
6
Style of Crustal Evolution
• Archean crust-forming processes generated
– granite-gneiss complexes and greenstone belts
that were shaped into cratons
• Although these same rock associations
continued to form during the Proterozoic,
they did so at a considerably reduced rate
7
Archean vs. Proterozoic
• Many Archean rocks have been metamorphosed,
• However, vast exposures of Proterozoic rocks are
unaltered or nearly so
• In many areas, Archean rocks are separated from
Proterozoic rocks by an unconformity
• Widespread associations of sedimentary rocks of
passive continental margins were deposited during the
Proterozoic by a plate tectonic style essentially the
same as it is now
8
Other Differences
• The Proterozoic was also a time in evolution of
the atmosphere and biosphere as well as the
origin of some important natural resources
• Oxygen-dependent organisms evolved during
this time
• The first multicelled organisms and animals
made their appearance.
• The fossil record is still poor compared to the
9
Phanerozoic
Evolution of
Proterozoic Continents
• Archean cratons assembled during collisions of
island arcs and mini-continents, providing the
nuclei around which Proterozoic crust accreted,
thereby forming much larger landmasses
• Proterozoic accretion
– probably took place more rapidly than today
because Earth possessed more radiogenic heat,
– and the plates moved faster
10
Focus on Laurentia
• Our focus here is on the geologic evolution of
Laurentia, a large landmass that consisted of
what is now
• North America,
• Greenland,
• parts of northwestern Scotland,
• and perhaps some of the Baltic shield of
Scandinavia
11
Early Proterozoic History of
Laurentia
• Laurentia underwent important changes between
2.0 and 1.8 billion years ago
• During this time, collisions among various plates
formed several orogens, which are linear or
arcuate deformation belts in which many of the
rocks have been
• metamorphosed and intruded by magma, thus
forming plutons, especially batholiths
12
Proterozoic Evolution of Laurentia
• Archean cratons were sutured along these
orogens, thereby forming a larger landmass which
makes up much of Greenland, central Canada,
and the north-central United States
13
Wilson Cycle
• Rocks of the Wopmay orogen in northwestern
Canada are important because they record the
opening and closing of an ocean basin or what
is called a Wilson cycle
• A complete Wilson cycle, named after the
Canadian geologist J. Tuzo Wilson, involves
• rifting of a continent,
• opening and closing of an ocean basin,
• and finally reassembly of the continent
14
Wilson Cycle
• Some geologists
think that the
Wopmay orogen
– represents a
complete Wilson
cycle
15
Accretion along Laurentia’s
Southern Margin
• Following the initial episode of amalgamation of
Archean cratons, accretion took place along Laurentia’s
southern margin as it collided with volcanic island arcs
and oceanic terranes
• From 1.65 to 1.76 billion years ago, the Yavapai and
Mazatzal orogens were added to the evolving continent
• The rocks have been deformed, altered by
metamorphism, intruded by granitic batholiths, and
incorporated into Laurentia.
16
Southern Margin Accretion
• Laurentia grew along its southern margin
– by accretion of the Central Plains, Yavapai, and
Mazatzal orogens
17
BIF, Red Beds, Glaciers
• This was also the time during which most of Earth’s
banded iron formations (BIF) were deposited
• The first continental red beds, sandstone and shale
with oxidized iron were deposited
• A significant Paleoproterozoic event was a huge
meteorite impact that took place in northern
Ontario, Canada
• In addition, some Early Proterozoic rocks and
associated features provide excellent evidence for
widespread glaciation
18
Mesoproterozoic Accretion &
Igneous Activity
• During the interval from 1.35 to 1.55 billion years ago,
extensive igneous activity took place
– that seems to be unrelated to orogenic activity and
accounted for the addition of the Granite-Rhyolite
province
• Some of the igneous activity resulted in plutons being
emplaced in existing continental crust.
• The resulting igneous rocks are exposed in eastern Canada
extend across Greenland, and are also found in the Baltic
Shield, Scandinavia
19
Grenville Orogeny
• Rocks of the Grenville Orogen
– These metamorphic rocks are uncomformably
overlain by the Upper Cambrian Potsdam Formation.
20
Sedimentary
Basins in the West
• Meso- to
Neoproterozoic basin
– in the western United
States and Canada
• Belt Basin
• Uinta Basin
• Apache Basin
21
Sedimentary Rocks
• Meso- and Neoproterozoic sedimentary rocks are
exceptionally well exposed in the northern Rocky
Mountains of Montana and Alberta, Canada
• Indeed, their colors, deformation features, and
erosion by Pleistocene and recent glaciers have
yielded some fantastic scenery
• Like the Paleo-proterozoic rocks in the Great
Lakes region, they are mostly sandstones, shales,
and stromatolite-bearing carbonates
22
Belt Basin, Glacier National Park
• Meso- and Neoproterozoic rocks in the Belt basin
23
Proterozoic Sandstone
• Proterozoic rocks of the Grand Canyon Supergroup lie unconformably upon Archean rocks
– and in turn are overlain unconformably by
Phanerozoic-age rocks
• The rocks, consisting mostly of sandstone,
shale, and dolostone, were deposited in shallowwater marine and fluvial environments
• The presence of stromatolites and carbonaceous
impressions of algae in some of these rocks
indicate probable marine deposition
24
Grand Canyon Super-group
• Neoproterozoic sandstone in the Grand Canyon
25
Proterozoic Supercontinents
• A continent is a landmass made up of granitic
crust with much of its surface above sea level
• A supercontinent consists of at least two
continents merged into one, but usually
includes all or most of all Earth’s landmasses
• The supercontinent Pangaea, which existed at
the end of the Paleozoic Era, is familiar,
– but few people are aware of earlier supercontinents
26
Early Supercontinents
• Supercontinents may have existed as early as
the Neoarchean,
– but if so we have little evidence of them
• The first that geologists recognize with some
certainty, known as Rodinia, assembled
between 1.3 and 1.0 billion years ago
– and then began fragmenting 750 million years ago
27
Early Supercontinent (Rodinia)
• Possible
configuration of
the Neoproterozoic
supercontinent
Rodinia
– before it began
fragmenting about
750 million years
ago
28
Early Supercontinent (Pannotia)
• Judging by the Pan-African orogeny and the large-scale
deformation that took place in what are now the
Southern Hemisphere continents, Rodinia’s separate
pieces reassembled and formed another supercontinent
– Pannotia, about 650 million years ago
• Fragmentation was underway again, by the latest
Proterozoic, about 550 million years ago, giving rise to
the continental configuration
– that existed at the onset of the Phanerozoic Eon
29
Ancient Glaciers
• Very few instances of widespread glacial
activity have occurred during Earth history
• The most recent one during the Pleistocene 1.8
million to 10,000 years ago is the best known,
– but we also have evidence for Pennsylvanian
glaciers and two major episodes of Proterozoic
glaciation
30
Recognizing Glaciation
• How can we be sure that there were
Proterozoic glaciers?
– Their most common deposit, called tillite, is simply
a type of conglomerate/breccia that may look much
like conglomerates originating from other
processes
• Tillite or tillite-like deposits are known from at
least 300 Precambrian localities, and some of
these are undoubtedly not glacial deposits
31
Proterozoic Glacial Evidence
• Tillite in Norway
– overlies striated bedrock surface of sandstone
32
Geologists Convinced
• Geologists are now convinced, based on this kind
of evidence, that widespread glaciation took
place during the Paleoproterozoic
• The occurrence of tillites
of about the same age in
Michigan, Wyoming,
and Quebec indicates
that North America may
have had an ice sheet
centered southwest of
Hudson Bay
33
Snowball Earth?
• Some geologists think that glaciers covered all land and
all seas were frozen
– a snowball Earth
• This hypothesis is controversial but proponents claim
that onset of this glacial episode may have been triggered
by the near-equatorial location of the continents
– Accelerated weathering would absorb huge quantities
of CO2
– With little CO2 glaciers would form and reflect solar
radiation back into space, forming more glacier 34
Snowball Earth?
• Volcanoes would continue spewing gases, which
would warm the atmosphere and end the glacial
episode
• One criticism of the snowball Earth hypothesis:
How would life survive?
– Suggestions include
•
•
•
•
Life persisted at hydrothermal vents on the seafloor
Photosynthesis can take place beneath thin glacial ice
Life may have persisted in sub-glacial lakes
There may have been pools of water near volcanoes
35
The Evolving Atmosphere
• Geologists agree that the Archean atmosphere
– contained little or no free oxygen so the atmosphere
was not strongly oxidizing as it is now
• Even though processes were underway that added
free oxygen to the atmosphere, the amount
present at the beginning of the Proterozoic was
probably no more than 1% of that present now
• In fact, it might not have exceeded 10% of
present levels even at the end of the Proterozoic
36
Cyanobacteria and Stromatolites
• Remember that cyanobacteria, were present
during the Archean, but stromatolites
• the structures they formed,
– did not become common until about 2.3 billion
years ago,
• that is, during the Paleoproterozoic
• These photosynthesizing organisms
– and to a lesser degree, photochemical dissociation
• added free oxygen to the evolving atmosphere
37
Oxygen Versus Carbon Dioxide
• Earth’s early atmosphere had abundant carbon
dioxide
• More oxygen became available whereas the
amount of carbon dioxide decreased
• Only a small amount of CO2 still exists in the
atmosphere today
• It is one of the greenhouse gases
– partly responsible for global warming
38
Life of the Proterozoic
• Archean fossils are not very common, and consists
of archea and bacteria, although there were many
types of these organisms
• Likewise, the Paleoproterozoic fossil record is
characterized by the same organisms although
stromatolites became common
• The lack of biotic diversity is not surprising
because prokaryotic cells reproduce asexually and
do not share their genetic material
– so evolution is a comparatively slow process.
39
Gunflint Microfossils
• Proterozoic microfossils from the Gunflint Iron
Formation of Canada, resemble bacteria living today
40
Precambrian Earth
and
Life History
(The Proterozoic Eon – Part II)
41
Stromatolites
• Two Proterozoic stromatolites had grown together
42
Sexual Reproduction Increased
the Pace of Evolution
• Organisms that reproduced sexually probably
evolved by the Mesoproterozoic, and the tempo
of evolution increased markedly
– though from our perspective it was still
exceedingly slow.
43
Eukaryotic Cells
• Eukaryotic cells are much larger than prokaryotic
cells
– have an internal membrane-bound nucleus and other
internal structures not found in prokaryotes
• Many eukaryotes are multi-celled and aerobic
– Most of them reproduce sexually
• Although 6 kingdoms are recognized,
– only 3 domains of living things exits
– Two domains are prokaryotic, and the other is
eukaryotic
44
Prokaryotic Cell
• Prokaryotic cells
– do not have a cell nucleus
– do not have organelles
– are smaller and not nearly as complex as eukaryotic
cells
45
Eukaryotic Cell
• Eukaryotic cells have
– a cell nucleus
containing
– the genetic material
– and organelles
– such as mitochondria
– and plastids,
– as well as chloroplasts
in plant cells
46
Domains of Life
• The inferred relationship among these organisms
are based on analyses of a type of ribosomal RNA
– Eukarya is more closely related to the archaea than to
bacteria
47
Eukaryotic Fossil Cells
• The oldest known eukaryotes are found in 1.2 billion year
Mesoproterozoic rocks in Canada
– These tiny organisms, Bangiomorpha,
• were single celled
• probably reproduced sexually
• and resemble red algae.
• The 2.1 billion year Negaunee Iron Formation has fossils
– but these megafossils were likely single-celled bacteria
– or some kind of algae.
48
Oldest Eukaryotes
• At 1.2 billion
years,
Bangiomorpha
is the oldest
known
eukaryote.
49
Oldest Eukaryotes
• Grypania, at 2.1 billion years, is the oldest
known megafossil.
– It was probably a bacterium or
– some kind
of algae.
50
Acritarchs
• Cells larger than 60 microns appear at least 1.4 billion
years ago
– and many of them show an increase in organizational
complexity
– An internal membrane-bounded nucleus is present in
some
• Hollow fossils known as acritarchs
– were probably cysts of planktonic algae
– and became common during the Meso- and
Neoproterozoic
51
Acritarchs
• These Proterozoic fossils are probably from
eukaryotic organisms
• Acritarchs are very likely the cysts of algae
52
Endosymbiosis and the
Origin of Eukaryotic Cells
• Eukaryotic cells probably formed from
prokaryotic cells that entered into a symbiotic
relationship
– Symbiosis,
• involving a prolonged association of two or more
dissimilar organisms, is common today
• In many cases both symbionts benefit from the
association
– as occurs in lichens,
• once thought to be plants
• but actually symbiotic fungi and algae
53
Endosymbiosis
• An aerobic bacterium and a larger host bacterium
united to form a mitochondria-containing amoeboid.
54
Endosymbiosis
• In a symbiotic relationship, each symbiont is
usually capable of metabolism and
reproduction,
– but the degree of dependence in some relationships
• is such that one or both symbionts cannot live
independently
• This may have been the case with Proterozoic
symbiotic prokaryotes
– that became increasingly interdependent until the
unit could exist only as a whole
• In this relationship
– one symbiont lived within the other, which is a
special type of symbiosis called endosymbiosis
55
Multicelled Organisms
• Multicelled organisms are made up of many
cells, with cells specialized to perform specific
functions
– such as reproduction and respiration
• Multicelled organisms were present by the
Neoproterozoic
– but the fossil record does not show the transition
56
Multicelled Organisms
• Some living organisms,
– while multicelled
– possess as few as four identical cells
– all of which are capable of living on their own.
57
The Multicelled Advantage?
• Is there any particular advantage to
being multicelled?
• For something on the order of 1.5
billion years
– all organisms were single-celled
– and life seems to have thrived
• In fact, single-celled organisms
– are quite good at what they do
– but what they do is very limited
58
The Multicelled Advantage?
• For example, single celled organisms
– can not grow very large, because as size increases,
proportionately less of a cell is exposed to the
external environment in relation to its volume
– and the proportion of surface area decreases
• Transferring materials from the exterior to the
interior becomes less efficient
59
The Multicelled Advantage?
• Also, multicelled organisms live longer,
– because cells can be replaced and more offspring
can be produced
• Cells have increased functional efficiency when
they are specialized into organs with specific
functions
60
Neoproterozoic Animals
• Biologists set forth criteria such as
– method of reproduction and type of metabolism
– to allow us to easily distinguish between animals
and plants
• Or so it would seem,
– but some present-day organisms blur this
distinction—and the same is true for some
Proterozoic fossils
• Nevertheless, the first relatively controversyfree fossils of animals come from the Ediacaran
fauna of Australia
– and similar faunas of similar age elsewhere
61
Ediacaran Fauna
• The Ediacaran fauna of Australia
Tribrachidium heraldicum, a possible primitive
echinoderm or cnidarian
Spriggina floundersi, a possible
ancestor of trilobites
62
Ediacaran Fauna
Parvanconrina is
perhaps related to
arthropods
• Restoration of the
Ediacaran Environment
63
Represented Phyla
• Three present-day phyla may be represented
– in the Ediacaran fauna:
• jellyfish and sea pens (phylum Cnidaria),
• segmented worms (phylum Annelida),
• and primitive members of the phylum Arthropoda (the
phylum with insects, spiders crabs, and others)
• One Ediacaran fossil, Spriggina,
– has been cited as a possible ancestor of trilobites
• Another might be a primitive member
– of the phylum Echinodermata
64
Other Proterozoic Animal Fossils
• Although scarce, a few animal fossils older
than those of the Ediacaran fauna are known
• A jellyfish-like impression is present in rocks
2000 m below the Pound Quartzite
• Burrows, in many areas, presumably made by
worms, are found in rocks at least 700 million
years old
• Some possible fossil worms are found
– from 700- to 900 million-year-old rocks in China65
Wormlike Fossils from China
• Wormlike
fossils from
Late
Proterozoic
rocks in China
66
Proterozoic Mineral Resources
• Most of the world’s iron ore comes from
– Paleoproterozoic banded iron formations
• Canada and the United States have large
deposits of these rocks
– in the Lake Superior region and in eastern Canada
• Thus, both countries rank among
– the ten leading nations in iron ore production
67
Iron Mine
• The Empire Mine at Palmer, Michigan
– where iron ore from the Paleoproterozoic
Negaunee Iron Formation is mined
68
Nickel
• In the Sudbury mining district in Ontario,
Canada,
– nickel and platinum are extracted from Proterozoic
rocks
• Nickel is essential for the production of nickel
alloys such as
• stainless steel
• and Monel metal (nickel plus copper),
– which are valued for their strength and resistance to
corrosion and heat
• The United States must import more than 50%
of all nickel used mostly from the Sudbury
mining district
69
Platinum and Chromium
• Some platinum for jewelry, surgical
instruments, and chemical and electrical
equipment is exported to the United States from
Canada,
– but the major exporter is South Africa
• The Bushveld Complex of South Africa is a
layered igneous complex containing both
• platinum
• and chromite
– the only ore of chromium,
– United States imports much of the chromium from
South Africa
– It is used mostly in stainless steel
70
Oil and Gas
• Economically recoverable oil and gas have
been discovered in Proterozoic rocks in China
and Siberia, arousing some interest in the
Midcontinent rift as a potential source of
hydrocarbons
• So far, land has been leased for exploration, and
numerous geophysical studies have been done
• However, even though some rocks within the
rift are known to contain petroleum,
– no producing oil or gas wells are operating
71
Proterozoic Pegmatites
• A number of Proterozoic pegmatites are
important economically
• The Dunton pegmatite in Maine, whose age is
generally considered to be Neoproterozoic,
– has yielded magnificent gem-quality specimens of
tourmaline and other minerals
• Other pegmatites are mined for gemstones, tin,
industrial minerals, such as feldspars, micas,
and quartz
– and minerals containing such elements as cesium,
rubidium, lithium, and beryllium
72
Proterozoic Pegmatites
• Geologists have identified more than 20,000
pegmatites in the country rocks adjacent to the
Harney Peak Granite
– in the Black Hills of South Dakota
• These pegmatites formed ~ 1.7 billion years ago
– when the granite was emplaced as a complex of
dikes and sills
• A few have been mined for gemstones, tin,
lithium, micas,
– and some of the world’s largest known mineral
crystals were discovered in these pegmatites
73
QUESTIONS?
74
Precambrian Earth
and
Life History
Part I
(The Archean Eon)
1
Archean Rocks
• The Beartooth Mountains on the Wyoming and Montana
border consists of Archaean-age gneisses
– some of the oldest rocks in the US.
2
Precambrian
• The Precambrian lasted for more than 4 billion
years!
– This large time span is difficult for humans to comprehend
• Suppose that a 24-hour
clock represented all 4.6
billion years of geologic
time then the Precambrian
would be slightly more
than 21 hours long,
constituting about 88% of
all geologic time
3
Precambrian
4
Precambrian
• The term Precambrian is informal but widely
used when referring to both time and rocks
• The Precambrian includes
– time from Earth’s origin 4.6 billion years ago to the
beginning of the Phanerozoic Eon, 542 million
years ago
• It encompasses all rocks below the Cambrian
system
• No rocks are known for the first 600 million
years of geologic time
– The oldest known rocks on Earth are 4.0 billion
years old
5
Rocks Difficult to Interpret
• The earliest record of geologic time preserved in rocks is
difficult to interpret because many Precambrian rocks
have been
•
•
•
•
•
altered by metamorphism
complexly deformed
buried deep beneath younger rocks
fossils are rare, and
the few fossils present are not useful in biostratigraphy
• Subdivisions of the Precambrian have been difficult to
establish
• Two eons for the Precambrian are the Archean and
Proterozoic which are based on absolute ages
6
Eons of the Precambrian
• Eoarchean refers to all time from Earth’s origin to
the Paleoarchean, 3.6 billion years ago
• Earth’s oldest body of rocks, the Acasta Gneiss in
Canada is about 4.0 billion years old
• We have no geologic record for much of the
Archaen
• Precambrian eons have no stratotypes
– unlike the Cambrian Period, for example
7
What Happened
During the Eoarchean?
• Although no rocks of Eoarchean age are present
on Earth,
– except for meteorites,
• We do know some events that took place then
– Earth accreted from planetesimals and differentiated
into core and mantle
• and at least some crust was present
–
–
–
–
Earth was bombarded by meteorites
Volcanic activity was widespread
An atmosphere formed, quite different from today’s
Oceans began to accumulate
8
Hot, Barren, Waterless Early Earth
• about 4.6 billion years ago
• Shortly after accretion, Earth was
–
–
–
–
a rapidly rotating, hot, barren, waterless planet
bombarded by meteorites and comets
with no continents, intense cosmic radiation
and widespread volcanism
9
Oldest Rocks
• Continental crust was present by 4.0 billion
years ago
– Sedimentary rocks in Australia contain detrital
zircons (ZrSiO4) dated at 4.4 billion years old
– so source rocks at least that old existed
• The Eoarchean Earth probably rotated in as
little as 10 hours
– and the Earth was closer to the Moon
• By 4.4 billion years ago, the Earth cooled
sufficiently for surface waters to accumulate
10
Eoarchean Crust
• Early crust formed as upwelling mantle currents
of mafic magma, and numerous subduction zones
developed to form the first island arcs
• Eoarchean continental crust may have formed
– by collisions between island arcs
– as silica-rich materials were metamorphosed.
– Larger groups of merged island arcs
• protocontinents
– grew faster by accretion along their margins
11
Origin of Continental Crust
• Andesitic
island arcs
– form by
subduction
– and partial
melting of
oceanic crust
• The island arc
collides with
another
12
Continental Foundations
• Continents consist of rocks with composition
similar to that of granite
• Continental crust is thicker and less dense than
oceanic crust which is made up of basalt and
gabbro
• Precambrian shields consist of vast areas of
exposed ancient rocks and are found on all
continents
• Outward from the shields are broad platforms of
buried Precambrian rocks that underlie much of
each continent
13
Cratons
• A shield and its platform
make up a craton,
– a continent’s ancient
nucleus
• Along the margins of
cratons, more continental
crust was added as the
continents took their present
sizes and shapes
• Both Archean and Proterozoic rocks are present in
cratons and show evidence of episodes of deformation
accompanied by igneous activity, metamorphism, and
mountain building
• Cratons have experienced little deformation since the
Precambrian
14
Distribution of Precambrian Rocks
• Areas of
exposed
– Precambrian rocks
– constitute
the shields
• Platforms
consist of
– buried Precambrian
rocks
– Shields and adjoining platforms make up cratons
15
Canadian Shield
• The exposed part of the craton in North
America is the Canadian shield
– which occupies most of northeastern Canada
– a large part of Greenland
– parts of the Lake Superior region
• in Minnesota, Wisconsin, and Michigan
– and the Adirondack Mountains of New York
• Its topography is subdued,
– with numerous lakes and exposed Archean and
Proterozoic rocks thinly covered in places by
Pleistocene glacial deposits
16
Evolution of North America
• North America
evolved by the
amalgamation of
Archean cratons
that served as a
nucleus around
which younger
continental crust
was added.
17
Archean Rocks
• Only 22% of Earth’s exposed Precambrian crust is Archean
• The most common Archean rock associations are granitegneiss complexes
• Other rocks range from peridotite to various sedimentary rocks
– all of which have been metamorphosed
• Greenstone belts are subordinate in quantity,
– account for only 10% of Archean rocks
– but are important in unraveling Archean tectonic events
• Outcrop of Archean
gneiss cut by a granite
dike from a granitegneiss complex in
Ontario, Canada
18
Archean Rocks
• Shell Creek in the Bighorn Mountains of Wyoming
has cut a gorge into this 2.9 billion year old granite
19
Archean Plate Tectonics
• Plate tectonic activity has operated since the
Paleoproterozoic or earlier
• Most geologists are convinced that some kind
of plate tectonic activity took place during the
Archean as well
– but it differed in detail from today
• Plates must have moved faster
– with more residual heat from Earth’s origin
– and more radiogenic heat,
– and magma was generated more rapidly
20
Archean Plate Tectonics
• As a result of the rapid movement of plates,
– continents grew more rapidly along their margins, a
process called continental accretion, as plates
collided with island arcs and other plates
• Also, ultramafic extrusive igneous rocks,
– komatites, were more common
21
The Origin of Cratons
• Certainly several small cratons existed during
the Archean and grew by accretion along their
margins
• They amalgamated into a larger unit
– during the Proterozoic
• By the end of the Archean,
– 30-40% of the present volume of continental crust
existed
• Archean crust probably evolved similarly to the
evolution of the southern Superior craton of
Canada
22
Atmosphere and Hydrosphere
• Earth’s early atmosphere and hydrosphere were
quite different than they are now
• They also played an important role in the
development of the biosphere
• Today’s atmosphere is mostly
– nitrogen (N2)
– abundant free oxygen (O2),
• or oxygen not combined with other elements such as in
carbon dioxide (CO2)
– water vapor (H2O)
– small amounts of other gases, like ozone (O3)
• which is common enough in the upper atmosphere to
block most of the Sun’s ultraviolet radiation
23
Present-day
Atmosphere Composition
• Nonvariable gases
Nitrogen N2 78.08%
Oxygen O2 20.95
Argon
Ar
0.93
Neon
Ne
0.002
Others
0.001
• Variable gases
Water vapor H2O
Carbon dioxide CO2
Ozone
O3
Other gases
0.1 to 4.0
0.038
0.000006
Trace
• Particulates
normally trace
in percentage by volume
24
Earth’s Very Early Atmosphere
• Earth’s very early atmosphere was probably
composed of
– hydrogen and helium,
• the most abundant gases in the universe
• If so, it would have quickly been lost into space
– because Earth’s gravity is insufficient to retain them
– because Earth had no magnetic field until its core
formed (magnetosphere)
• Without a magnetic field,
– the solar wind would have swept away any
atmospheric gases
25
Outgassing
• Once a magnetosphere
was present
– Atmosphere began
accumulating as a result of
outgassing, released during
volcanism
• Water vapor is the most
common volcanic gas today
– but volcanoes also emit
carbon dioxide, sulfur
dioxide, carbon monoxide,
sulfur, hydrogen, chlorine,
and nitrogen
26
Archean Atmosphere
• Archean volcanoes probably emitted the same
gases, and thus an atmosphere developed
– but one lacking free oxygen and an ozone layer
• It was rich in carbon dioxide,
– and gases reacting in this early atmosphere
probably formed
• ammonia (NH3)
• methane (CH4)
• This early atmosphere persisted throughout the
27
Archean
Evidence for an
Oxygen-Free Atmosphere
• The atmosphere was chemically reducing
– rather than an oxidizing one
• Some of the evidence for this conclusion comes
from detrital deposits containing minerals that
oxidize rapidly in the presence of oxygen
• pyrite (FeS2)
• uraninite (UO2)
• But oxidized iron becomes increasingly
common in Proterozoic rocks
– indicating that at least some free oxygen was present
then
28
Introduction of Free Oxygen
• Two processes account for introducing free
oxygen into the atmosphere,
• one or both of which began during the Eoarchean.
1. Photochemical dissociation involving ultraviolet
radiation in the upper atmosphere
• The radiation disrupts water molecules and releases their
oxygen and hydrogen
• This could account for 2% of present-day oxygen
• but with 2% oxygen, ozone forms, creating a barrier
against ultraviolet radiation
2. More important were the activities of organisms
that practiced photosynthesis
29
Photosynthesis
• Photosynthesis is a metabolic process
– in which carbon dioxide and water are used in
making organic molecules
– and oxygen is released as a waste product
6CO2 + 6H2O ==> C6H12O6 + 6O2
• Even with photochemical dissociation and
photosynthesis,
– probably no more than 1% of the free oxygen level
of today was present by the end of the Archean
30
Oxygen Forming Processes
• Photochemical dissociation and photosynthesis
added free oxygen to the atmosphere
– Once free
oxygen was
present, an
ozone layer
formed
– and blocked
incoming
ultraviolet
radiation
31
Earth’s Surface Waters
• Outgassing was responsible for the early
atmosphere and also for some of Earth’s surface
water
• the hydrosphere, most of which is in the oceans >
97%
• Another source of our surface water was meteorites and
icy comets
• Numerous erupting volcanoes, and an early episode of
intense meteorite and comet bombardment accounted for
rapid rate of surface water accumulation
32
Ocean Water
• Volcanoes still erupt and release water vapor
– Is the volume of ocean water still increasing?
– Perhaps it is, but if so, the rate has decreased
considerably because the amount of heat needed to
generate magma has diminished
33
Decreasing Heat
• Ratio of radiogenic heat production in the past to
the present
– The width of
the colored
band indicates
variations in
ratios from
different
models
• Heat production
4 billion years
ago was 3 to
6 times as great
as it is now
• With less heat
outgassing
decreased
34
First Organisms
• Today, Earth’s biosphere consists of millions of species
of Archea, Bacteria, Fungi, Protists, Plants, and Animals,
– whereas only bacteria and archea are found in
Archean rocks
• We have fossils from Archean rocks
– 3.5 billion years old
• Chemical evidence in rocks in Greenland that are 3.8
billion years old convince some investigators that
organisms were present then
35
What Is Life?
• Minimally, a living organism must reproduce
– and practice some kind of metabolism
• Reproduction ensures the long-term survival of
a group of organisms
• whereas metabolism maintains the organism
• The distinction between living and nonliving
things is not always easy
• Are viruses living?
– When in a host cell they behave like living
organisms
– but outside, they neither reproduce nor metabolize
36
What Is Life?
• Comparatively simple organic (carbon based)
molecules known as microspheres
– form spontaneously
– can even grow and
divide in a somewhat
organism-like
fashion
– but their processes
are more like random
chemical reactions,
so they are not living
37
How Did Life First Originate?
• To originate by natural processes, from non-living
matter (abiogenesis), life must have passed
through a prebiotic stages
– in which it showed signs of living
– but was not truly living
• The origin of life has 2 requirements
– a source of appropriate elements for organic molecules
– energy sources to promote chemical reactions
38
Elements of Life
• All organisms are composed mostly of
–
–
–
–
carbon (C)
hydrogen (H)
nitrogen (N)
oxygen (O)
• all of which were present in Earth’s early
atmosphere as
–
–
–
–
–
carbon dioxide (CO2)
water vapor (H2O)
nitrogen (N2)
and possibly methane (CH4)
and ammonia (NH3)
39
Basic Building Blocks of Life
• Energy from
• Lightning, volcanism,
• and ultraviolet radiation
– probably promoted chemical reactions during
which C, H, N, and O combined
– to form monomers
• such as amino acids
• Monomers are the basic building blocks of
more complex organic molecules
40
Experiment on the Origin of Life
• Is it plausible that monomers originated in the
manner postulated?
– Experimental evidence indicates that it is
• During the late 1950s
– Stanley Miller
synthesized several
amino acids
– by circulating gases
approximating the early
atmosphere
– in a closed glass vessel
41
Experiment on the Origin of Life
• This mixture was subjected to an electric spark
– to simulate lightning
• In a few days
– it became cloudy
• Analysis showed that
– several amino acids
typical of organisms
had formed
• Since then,
– scientists have
synthesized all 20
amino acids found in
organisms
42
Polymerization
• The molecules of organisms are polymers
– such as proteins
– and nucleic acids
• RNA (ribonucleic acid) and DNA (deoxyribonucleic acid)
consisting of monomers linked together in a specific
sequence
• How did polymerization take place?
• Water usually causes depolymerization,
– however, researchers synthesized molecules known as
proteinoids or thermal proteins
– some of which consist of more than 200 linked amino
acids
– when heating dehydrated concentrated amino acids
43
Proteinoids
• These concentrated amino acids
– spontaneously polymerized to form proteinoids
• Perhaps similar conditions for polymerization
existed on early Earth,
– but the proteinoids needed to be protected by an
outer membrane or they would break down
• Experiments show that proteinoids
spontaneously aggregate into microspheres
– which are bounded by cell-like membranes and
grow and divide much as bacteria do
44
Proteinoid Microspheres
• Proteinoid
microspheres
produced in
experiments
• Proteinoids grow
and divide much as
45
bacteria do
Protobionts
• These proteinoid molecules can be referred to
as protobionts
– These are intermediates between inorganic chemical
compounds and living organisms
46
Monomer and Proteinoid Soup
• The origin-of-life experiments are interesting,
– but what is their relationship to early Earth?
• Monomers likely formed continuously and in
billions
– They accumulated in the early oceans into a “hot,
dilute soup”
– The amino acids in the “soup” might have washed
up onto a beach or perhaps cinder cones
– where they were concentrated by evaporation and
polymerized by heat
• The polymers then washed back into the ocean
– where they reacted further
47
QUESTIONS?
48
Precambrian Earth
and
Life History
Part II
(The Archean Eon)
49
Next Critical Step
• Not much is known about the next critical step in
the origin of life
• the development of a reproductive mechanism
• The microspheres divide and may represent a
protoliving system
– but in today’s cells, nucleic acids,
• either RNA or DNA
– are necessary for reproduction
• The problem is that nucleic acids cannot replicate
without protein enzymes,
– and the appropriate enzymes cannot be made without
nucleic acids,
50
– or so it seemed until fairly recently
RNA World?
• Now we know that small RNA molecules
– can replicate without the aid of protein enzymes
• Thus, the first replicating systems
– may have been RNA molecules
• Some researchers propose
– an early “RNA world” in which these molecules
were intermediate between
• inorganic chemical compounds
• and the DNA-based molecules of organisms
• How RNA was naturally synthesized
– remains an unsolved problem
51
Much Remains to Be Learned
• Scientists agree on some basic requirements for
the origin of life,
– but the exact steps involved and significance of
results are debated
• Many researchers believe that
– the earliest organic molecules were synthesized
from atmospheric gases
– but some scientist suggest that life arose instead near
hydrothermal vents on the seafloor
52
Submarine Hydrothermal Vents
• Seawater seeps into the crust near spreading
ridges, becomes heated, rises and discharges
• Black smokers
– Discharge water
saturated with
dissolved
minerals
– Life may have
formed near these
in the past
53
Submarine Hydrothermal Vents
• Several minerals containing zinc, copper, and iron
precipitate around them
• Communities of organisms
– previously unknown to
science, are supported here.
– Necessary elements, sulfur,
and phosphorus are present in
seawater
– Polymerization can take place
on surface of clay minerals
– Protocells were deposited on
the ocean floor
54
Oldest Known Organisms
• The first organisms were archaea and bacteria
– both of which consist of prokaryotic cells,
– cells that lack an internal, membrane-bounded
nucleus and other structures
• Prior to the 1950s, scientists assumed that life
– must have had a long early history
– but the fossil record offered little to support this idea
• The Precambrian, once called Azoic
– (“without life”), seemed devoid of life
55
Oldest Know Organisms
• Charles Walcott (early 1900s) described structures from
the Paleoproterozoic Gunflint Iron Formation of Ontario, Canada
– that he proposed represented reefs constructed by algae
• Now called
stromatolites,
– not until 1954
were they
shown to be
products of
organic activity
56
Present-day stromatolites (Shark Bay, Australia)
Stromatolites
• Different types of stromatolites include
– irregular mats, columns, and columns linked by mats
57
Stromatolites
• Present-day stromatolites form and grow as
sediment grains are trapped on sticky mats of
photosynthesizing cyanobacteria
– although now they are restricted to environments
where snails cannot live
• The oldest known undisputed stromatolites are
found in rocks in South Africa
– that are 3.0 billion years old
• But probable ones are also known from the Warrawoona
Group in Australia
– which is 3.3 to 3.5 billion years old
58
Other Evidence of Early Life
• Chemical evidence in rocks 3.85 billion years old in
Greenland indicate life was perhaps present then
• The oldest known cyanobacteria were
photosynthesizing organisms
– but photosynthesis is a complex metabolic process
• A simpler type of metabolism must have preceded it
• No fossils are known of these earliest organisms
59
Earliest Organisms
• The earliest organisms must have resembled
– tiny anaerobic bacteria
– meaning they required no oxygen
• They must have totally depended on an external
source of nutrients
– that is, they were heterotrophic, as opposed to
autotrophic organisms
• that make their own nutrients, as in photosynthesis
• They all had prokaryotic cells
60
Earliest Organisms
• The earliest organisms, then, were anaerobic,
heterotrophic prokaryotes
• Their nutrient source was most likely
– adenosine triphosphate (ATP) from their
environment which was used to drive the energyrequiring reactions in cells
• ATP can easily be
synthesized from simple
gases and phosphate
– so it was available in
the early Earth
environment
61
Fermentation
• Obtaining ATP from the surroundings could
not have persisted for long
– because more and more cells competed for the
same resources
• The first organisms to develop a more
sophisticated metabolism
– probably used fermentation to meet their energy
needs
• Fermentation is an anaerobic process in which
molecules such as sugars are split, releasing
carbon dioxide, alcohol, and energy
62
Photosynthesis
• A very important biological event occurring in
the Archean was the development of the
autotrophic process of photosynthesis
• This may have happened as much as 3.5 billion
years ago
• These prokaryotic cells were still anaerobic,
– but as autotrophs they were no longer dependent on
preformed organic molecules as a source of
nutrients
63
Fossil Prokaryotes
• Photomicrographs from western Australia’s
– 3.3- to 3.5-billion-year-old Warrawoona Group,
– with schematic restoration shown at the right of each
64
Archean Mineral Resources
• A variety of mineral deposits are of Archean-age
– but gold is the most commonly associated, although it is
also found in Proterozoic and Phanerozoic rocks
• This soft yellow metal is prized for jewelry,
– but it is or has been used as a monetary standard, in glass
making, electric circuitry, and chemical industry
• About half the world’s gold since 1886 has come from
Archean and Proterozoic rocks in South Africa
• Gold mines also exist in Archean rocks of the Superior
craton in Canada
65
Archean Sulfide Deposits
• Archean sulfide deposits of
• zinc,
• copper
• and nickel
– occur in Australia, Zimbabwe, and in the Abitibi
greenstone belt in Ontario, Canada
• Some, at least, formed as mineral deposits
– next to hydrothermal vents on the seafloor, much as
they do now around black smokers
66
Chrome
• About 1/4 of Earth’s chrome reserves are in
Archean rocks, especially in Zimbabwe
• These ore deposits are found in
– the volcanic units of greenstone belts
– where they appear to have formed when crystals
settled and became concentrated in the lower parts
of plutons
– such as mafic and ultramafic sills
• Chrome is needed in the steel industry
• The United States has very few chrome deposits
– so must import most of what it uses
67
Chrome and Platinum
• One chrome deposit in the United States is in
the Stillwater Complex in Montana
• Low-grade ores were mined there during war
times,
– but they were simply stockpiled and never refined
for chrome
• These rocks also contain platinum,
– a precious metal, that is used
• in the automotive industry in catalytic converters
• in the chemical industry
• for cancer chemotherapy
68
Iron
• Banded Iron formations are sedimentary rocks
– consisting of alternating layers of silica (chert) and
iron minerals
• About 6% of the world’s banded iron
formations were deposited during the Archean
Eon
• Although Archean iron ores are mined in some areas
– they are neither as thick nor as extensive as those of
the Proterozoic Eon, which constitute the world’s
major source of iron
69
Pegmatites
• Pegmatites are very coarsely crystalline igneous
rocks, commonly associated with granite
plutons
• Some Archean pegmatites,
– such in the Herb Lake district in Manitoba, Canada,
– and Rhodesian Province in Africa,
– contain valuable minerals
• In addition to minerals of gem quality,
– Archean pegmatites contain minerals mined for
lithium, beryllium, rubidium, and cesium
70
QUESTIONS?
71