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Chapter 18
The History of Life
1
18.1 Origin of Life
• The last universal common ancestor
(LUCA) is common to all organisms that
live, and have lived, on Earth since life
began.
 Today, we say that “life only comes from life.”
 However, the first cells had to arise from
nonliving chemicals.
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Origin of Life
• The Earth came into being about 4.6 BYA (BYA).
• The Earth’s mass provides a gravitational field strong
enough to hold an atmosphere
• Primitive Earth atmosphere:
 Most likely consisted of:
•
•
•
•
Water vapor
Nitrogen
Carbon dioxide
Small amounts of hydrogen, methane, ammonia, hydrogen sulfide,
and carbon monoxide
 Little free oxygen
 Originally too hot for liquid water to form
• As the Earth cooled, water vapor condensed to liquid
water
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Origin of Life
• Evolution of Monomers:
 Several hypotheses suggest how monomers evolved
• Monomers came from outer space
– Comets and meteorites, perhaps carrying organic chemicals, have
pelted the Earth throughout history.
– Organic molecules could have seeded the chemical origin of life on
Earth.
– Bacterium-like cells could have been carried to Earth on a meteorite or
comet.
• Monomers came from reactions in the atmosphere
– Oparin/Haldane Hypothesis (early 1900s)
» Suggested organic molecules could be formed in the
presence of outside energy sources using atmospheric
gases
• Monomers came from reactions at hydrothermal vents
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Origin of Life
• Stanley Miller (1953)
 Conducted an experiment to test the Operin/Haldane
hypothesis
• Showed that gases (methane, ammonia, hydrogen, and
water) can react with one another to produce small organic
molecules (amino acids, organic acids).
• Strong energy sources
 Rainfall would have washed organic compounds from
the atmosphere into the ocean
 They would have accumulated in the ocean, making it
an organic soup
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Stanley Miller’s Experiment
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electrode
electric
spark
stopcock for
adding gases
CH4
NH3
gases
H2
stopcock for
withdrawing liquid
boiler
heat
H2O
hot water out
condenser
cool water in
liquid droplets
small organic molecules
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Origin of Life
• In cells, monomers join to form polymers
in the presence of enzymes
 Iron-Sulfur World Hypothesis
• Suggests organic molecules reacted with amino acids to form
peptides in the presence of iron-nickel sulfides:
 Protein-First Hypothesis:
• Assumes that protein enzymes arose first
• DNA genes came afterwards
 RNA-First Hypothesis:
• Suggests only RNA was needed to progress toward
formation of the first cell or cells
• DNA genes came afterwards
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Origin of Life
• Before the first true cell arose, there would have been a
protocell or protobiont, the hypothesized precursor to the
first true cells.
• A protocell would have an outer membrane and carry on
energy metabolism.
• Proteinoids are small polypeptides with catalytic properties.
• When proteinoids are placed in water, they form
microspheres, structures made of proteins with many
properties of a cell
 If lipids are made available to microspheres, lipids become
associated with microspheres, producing a lipid-protein
membrane.
• Lipids placed into water form cell-sized double-layered
bubbles called liposomes.
 May have provided the first membranous boundary
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Comparison of Protocell and
Modern Cell Plasma Membranes
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Protocell Membrane
Cell Membrane
a.
b.
hydrophilic “head”
2 fatty acids
hydrophobic “tail”
phospholipid
individual fatty acid
c.
outside protocell
inside protocell
fatty acid bilayer
d.
outside cell
inside cell
phospholipid bilayer
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Origin of Life
• Evolution of the protocell
 Nutrition
• The process would have had to carry on nutrition in
order to grow.
• If organic molecules formed in the atmosphere and
were carried into the ocean by rain, simple organic
molecules could have served as food.
– According to this hypothesis, the protocell was a
heterotroph.
• If the protocell evolved at hydrothermal events, it
could have carried out chemosynthesis.
– Chemosynthesis is the synthesis of organic molecules by
the oxidation of inorganic compounds.
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Origin of Life
• Evolution of the protocell
• Nutrition (continued)
 Natural selection would have favored cells that could
extract energy from carbohydrates to produce ATP.
• Oxygen was not available.
• The protocell may have carried on a form of fermentation.
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Origin of Life
• A Self-Replication System
 RNA-first hypothesis
• The first cell would have had RNA gene that directed protein
synthesis
• Reverse transcription could have led to DNA genes
• RNA was responsible for both DNA and protein formation
• Eventually protein synthesis would have been carried out according
to the central dogma, with information flowing from DNA to RNA to
protein
 Protein-first hypothesis
• The protocell would have developed a plasma membrane and
enzymes
• Then DNA and RNA synthesis would have been possible
• After DNA genes evolved, protein synthesis would have been carried
out according to the central dogma
 After DNA formed, the genetic code had to evolve
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Stages of the Origin of Life
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Biological Evolution
extant organisms
Common ancestor
of all life on Earth or
LUCA (last universal
common ancestor)
LUCA
extinct lineages
Origin of Life:
first self-replicating cell
x
x
x
Stage 4
Biological Evolution
cell
Stage 4
cell
DNA
RNA
origin of
genetic code
proto cell
Stage 3
plasma
membrane
polymers
Stage 2
Chemical Evolution
polymerization
Small organic molecules
energy
capture
Stage 1
abiotic
synthesis
Inorganic chemicals
early Earth
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18.2 History of Life
• Fossils - Remains and traces of past life
• Paleontology is the study of the fossil record
• Most fossils are traces of organisms embedded
in sediment
 Sediment is produced by weathering and erosion of
rocks
 Sediment becomes a recognizable stratum in the
stratigraphic sequence
 Strata of the same age tend to contain the similar
fossil assemblages (index fossils) that can be used
for relative dating
 This helps geologists determine relative dates of
embedded fossils despite upheavals (relative dating)
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History of Life
• Absolute dating assigns an actual date to a fossil
• One absolute dating method relies on radiometric
(radioactive) dating techniques
• Half-life:
 The length of time required for half the atoms to change into
another stable element
 Unaffected by temperature, light, pressure, etc.
 All radioactive isotopes have a dependable half-life
• Some only fractions of a second
• Some billions of years
• Most in between
• Many isotopes are used, and their combined half-lives
make them useful over all periods of interest
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History of Life
• The geologic timescale
 divides the history of the earth into
• Eras
• Periods
• Epochs
 Derives from accumulation of data from the age of
fossils in strata all over the world
 Life arose during the Precambrian
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Geologic Timescale: Cenozoic
and Mesozoic Eras
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Geologic Timescale: Paleozoic
and Precambrian Eras
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History of Life
• The Precambrian includes about 87% of the geological
timescale
 Little or no atmospheric oxygen in the early atmosphere
 Lack of ozone shield allowed UV radiation to bombard Earth
• The first cells came into existence in aquatic
environments
 Prokaryotes
 Cyanobacteria fossils have been found in ancient stromatolites
 Photosynthetic cyanobacteria added oxygen to the atmosphere
 Aerobic bacteria proliferated in the oxygen-rich atmosphere
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The Tree of Life
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thermophiles
Archaebacteria
halophiles
3
ARCHAEA
methanogens
7
Animals
5
1
Fungi
heterotrophic
protists
4
first cells
EUKARYA
6
Protists
photosynthetic
protists
mitochondria
8
Plants
chloroplasts
2
BACTERIA
aerobic bacteria
Bacteria
photosynthetic bacteria (produce oxygen)
other photosynthetic bacteria (do not produce oxygen)
3.5 BYA
2.2 BYA
1.4 BYA
543 MYA
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Prokaryote Fossil of the
Precambrian
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10 mm
0
a. Stromatolites
b. Primaevifilum
a: Courtesy J. William Schopf; b: © Francois Gohier/Photo Researchers, Inc.
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History of Life
• Eukaryotic Cells Arise
 About 2.1 BYA
• Most are aerobic
• Contain a nucleus as well as other membranous organelles
 Endosymbiotic Theory
• Mitochondria were probably once free-living aerobic
prokaryotes.
• Chloroplasts were probably once free-living photosynthetic
prokaryotes.
• A nucleated cell probably engulfed these prokaryotes that
became various organelles.
 Cilia and flagella may have originated from slender
undulating prokaryotes that attached to the host cell.
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History of Life
• Support for the Endosymbiotic Theory
 Mitochondria and chloroplasts are similar in size to
bacteria
 Mitochondria and chloroplasts have their own DNA and
make some of their own proteins
 Mitochondria and chloroplasts divide by binary fission
 The outer membranes of mitochondria and
chloroplasts differ
• The outer membrane resembles a eukaryotic membrane
• The inner membrane resembles a prokaryotic membrane
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History of Life
• Multicellularity Arises
 About 1.4 BYA
 Separating germ cells from somatic cells may
have contributed to the diversity of organisms.
 Early multicellular organisms lacked internal
organs and could have absorbed nutrients
from the sea.
 It’s possible that they practiced sexual
reproduction
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Ediacaran Fossils
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a.
b.
a: Courtesy J. William Schopf; b: © Francois Gohier/Photo Researchers, Inc.
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History of Life
• The Paleozoic Era
 Begins with the Cambrian Period
 Lasted over 300 million years
 Includes three major mass extinction events
• Disappearance of a large number of taxa
• Occurred within a relatively short time interval
(compared to geological time scale)
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History of Life
• The abundance of fossils of animals of the Cambrian
period may be due to the evolution of outer skeletons
• The ancestry of all modern animals can be traced to the
Cambrian period based on molecular clock data
• Molecular Clock:
 Based on hypothesis that
• Changes in base-pair sequences of certain DNA segments occur at
a fixed rate, and
• The rate is not affected by natural selection or other external factors
 When these base-pair sequences are compared between two
species:
• Count the number of base-pair differences
• Count tells how long two species have been evolving separately
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History of Life
• Invasion of land
 Plants
• Seedless vascular plants date back to the Silurian period
• Later flourished in Carboniferous period
 Invertebrates
• Arthropods were the first animals on land
• Outer skeleton and jointed appendages pre-adapted them to
live on land
 Vertebrates
• Fishes first appeared in the Ordovician period
• Amphibians first appeared in the Devonian period and
diversified during the Carboniferous period
 A mass extinction occurred at the end of the Permian
period
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Swamp Forests of the
Carboniferous Period
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a.
b.
c.
a: © The Field Museum, #CSGEO75400c; b: © John Cancalosi/Peter Arnold, Inc.; c: © John Gerlach/Animals Animals/Earth Scenes;
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History of Life
• The Mesozoic Era
 Triassic Period
• Nonflowering seed plants became dominant
 Jurassic Period
• Dinosaurs achieved enormous size
• Mammals remained small and insignificant
 Cretaceous Period
• Dinosaurs declined at the end of the Cretaceous period due
to a mass extinction
• Mammals:
– Began an adaptive radiation
– Moved into habitats left vacated by dinosaurs
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History of Life
• The Cenozoic Era
 Mammals continued adaptive radiation
 Flowering plants were already diverse and
plentiful
 Primate evolution began
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18.3 Geological Factors That
Influence Evolution
• Continental drift
 The positions of continents and oceans are
not fixed
 Plate tectonics
• Earth’s crust consists of slab-like plates
• Tectonic plates float on a lower hot mantle layer
• Movements of plates results in continental drift
• Modern mammalian diversity results from
isolated evolution on separate continents
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Continental Drift
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North
America
Laurasia
Eurasia
North
America
Eurasia
Africa
Africa
India
South
America
India
South
America
Australia
Australia
Antarctica
(251 million years ago)
PALEOZOIC
(135 million years ago)
(65 million years ago)
MESOZOIC
Antarctica
Present day
CENOZOIC
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Geological Factors that
Influence Evolution
• Mass extinctions occurred
at the end of the following
periods:
• Permian
• Ordovician
• Triassic
 444 MYA
 75% of species disappeared
• Devonian
 360 MYA
 70% of marine invertebrates
disappeared
 251 MYA
 90% of species disappeared
 20 MYA
 60% of species disappeared
• Cretaceous
 66 MYA
 75% of species
disappeared
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Mass Extinctions
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STATUS TODAY
mammals
mammals
birds
birds
dinosaurs
dinosaurs extinct
insects
insects
ammonoids
brachiopods
poriferans
ammonoids extinct
brachiopods
poriferans
CAMBRIAN
Major
Extinctions
% Species
Extinct
ORDOVICIAN
SILURIAN
DEVONIAN
CARBONIFEROUS
PERMIAN
TRIASSIC
443.7 MYA
359.2 MYA
251 MYA
75%
70%
90%
JURASSIC
199.6 MYA
60%
CRETACEOUS
TERTIARY
QUARTE
-RNARY
PRESENT
65.5 MYA
75%
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