Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 18
Life’s Origin and Early Evolution
(Sections 18.1 - 18.7)
Albia Dugger • Miami Dade College
18.1 Looking for Life
• Astrobiologists study properties of the ancient Earth that
allowed life to arise, survive, and diversify
• Presence of cells in deserts and deep below Earth’s surface
suggests life may exist in similar settings on other planets
• astrobiology
• The scientific study of life’s origin and distribution in the
universe
Chile’s Atacama Desert
• Astrobiologists study Earth’s extreme habitats to determine
the range of conditions that living things can tolerate
18.2 Earth’s Origin
and Early Conditions
• Physical and geological forces produced Earth, its seas, and
its atmosphere
• Earth and other planets formed more than 4 billion years ago
• Early in Earth’s history, there was little oxygen in the air,
volcanic eruptions were common, and there was a constant
hail of meteorites
From the Big Bang
to the Early Earth
• According to the big bang theory, the universe formed in an
instant 13 to 15 billion years ago
• Over millions of years, gravity drew the gases together and
they condensed to form giant stars
• big bang theory
• Model describing formation of the universe as a nearly
instant distribution of matter through space
An Early Sun
• What the cloud of dust, gases, rocks, and ice around the early
sun may have looked like
Conditions on the Early Earth
• An oxygen-free atmosphere allowed assembly of organic
compounds necessary for life (oxygen would destroy the
compounds as fast as they formed)
• As Earth’s surface cooled, rocks formed — rains washed
mineral salts into early seas where life began
Early Earth
• When volcanic activity and meteor strikes were common
Key Concepts
• Setting the Stage for Life
• Earth formed about 4 billion years ago from matter
distributed in space by the big bang (the origin of the
universe)
• The early Earth was an inhospitable place, where
meteorite impacts and volcanic eruptions were common
and the atmosphere held little or no oxygen
18.3 The Source of
Life’s Building Blocks
• All living things are made from organic subunits: simple
sugars, amino acids, fatty acids, and nucleotides
• Where did the subunits of the first life come from? There are
several possibilities:
1. Lightning-fueled atmospheric reactions
2. Reactions at deep-sea hydrothermal vents
3. Meteorites from space
Lightning-Fueled
Atmospheric Reactions
• In 1953, Stanley Miller and Harold Urey showed that reactions
in Earth’s early atmosphere could have produced building
blocks for the first life
• Provideed indirect evidence that organic compounds selfassemble spontaneously under conditions like those in
Earth’s early atmosphere
Miller-Urey Experiment
• Mix of water, hydrogen
(H2), methane (CH4), and
ammonia (NH3)
• Sparks simulated
lightning
• Amino acids formed
Miller-Urey Experiment
electrodes
to
vacuum
pump
CH4
NH3
H2O
H2
spark
discharge
gases
water out
condenser
water in
water droplets
boiling water
water containing
organic compounds
liquid water in trap
Fig. 18.4, p. 285
ANIMATION: Miller's reaction chamber
experiment
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Reactions at Hydrothermal Vents
• Reactions in the hot, mineral-rich water near deep-sea
hydrothermal vents also produce organic building blocks
• Experiments combining hot water with carbon monoxide (CO)
potassium cyanide (KCN) and metal ions formed amino acids
• hydrothermal vent
• Rocky, underwater opening where mineral-rich water
heated by geothermal energy streams out
A Hydrothermal Vent
• Mineral-rich water heated by
geothermal energy streams
out of the vent
• Precipitation causes
minerals to form a chimneylike structure around the
vent
Delivery From Space
• The presence of amino acids, sugars, and nucleotide bases in
meteorites that fell to Earth suggests that such molecules may
have formed in interstellar clouds of ice, dust, and gases and
been delivered to Earth by meteorites
Key Concepts
• Building Blocks of Life
• All life is composed of the same organic subunits
• Simulations of conditions on the early Earth show that
these molecules could have formed by reactions in the
atmosphere or sea
• Organic subunits also form in space and could have been
delivered to Earth by meteorites
ANIMATION: Building blocks of life
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18.4 From Polymers to Cells
• Similarities in structure, metabolism, and replication among all
life indicate descent from a common cellular ancestor
• Experiments demonstrate how traits and processes seen in
all living cells could have begun with physical and chemical
reactions among nonliving collections of molecules
Steps on the Road to Life
Steps on the Road to
Life
Inorganic molecules
self-assemble on Earth and
in space
Organic monomers
self-assemble in aquatic
environments on Earth
Organic polymers
interact in early metabolism
self-assemble as vesicles
become the first genome
Protocells in an RNA world
Are subject to selection that favors
a DNA genome
Stepped Art
DNA-based cells
Fig. 18.6, p. 286
Origin of Metabolism
• Proteins that speed metabolic reactions might have first
formed when amino acids stuck to clay, then bonded under
the heat of the sun
• Or, metabolism may have begun in rocks near deep-sea
hydrothermal vents when iron sulfide in the rocks donated
electrons to dissolved carbon monoxide
Iron Sulfide-Rich Rocks
• Cell-sized chambers
formed by simulations
of conditions near
hydrothermal vents
• Could have served as
environments for first
metabolic reactions
Origin of the Cell Membrane
• Membrane-like structures and vesicles form when proteins or
lipids are mixed with water
• They serve as a model for protocells, which may have
preceded cells
• protocell
• Membranous sacs that contain interacting organic
molecules; hypothesized to have formed prior to the
earliest life forms
Laboratory-Produced Protocells
• One type consists of a bilayer membrane of fatty acids that
holds strands of RNA
• Ribonucleotides diffuse into the protocell and become
incorporated into complementary strands of RNA
• Vesicle enlarges by incorporating additional fatty acids
• Another type consists of RNA-coated clay surrounded by fatty
acids and alcohols
Laboratory-Produced Protocells
• Fatty acids and RNA (left); RNA and clay (right)
Field-Testing a Hypothesis
• No vesicle-like
structures formed when
David Deamer poured a
mix of small organic
molecules and
phosphates into a hot
acidic pool in Russia
Origin of the Genome
• Protein synthesis depends on DNA, which is built by proteins;
how did this cycle begin?
• An RNA world, a time in which RNA was the genetic material,
may have preceded DNA-based systems
• RNA world
• Hypothetical early interval when RNA served as the
genetic information
RNA World
• RNA is part of ribosomes that carry out protein synthesis
• Discovery of ribozymes (RNAs that function as enzymes)
supports the RNA world hypothesis
• A later switch from RNA to DNA would have made the
genome more stable
Key Concepts
• The First Cells Form
• All cells have enzymes that carry out reactions, a plasma
membrane, and a genome of DNA
• Experiments provide insight into how cells arose through
physical and chemical processes, such as the tendency of
lipids to form membrane-like structures when mixed with
water
18.5 Life’s Early Evolution
• Fossils and molecular comparisons among living species
inform us about the history of life on Earth
• The first cells evolved when oxygen levels in the atmosphere
and seas were low, so they probably were anaerobic
Origin of Bacteria and Archaea
• Early divergence separated bacteria from ancestors of
archaeans and eukaryotes
• An oxygen-releasing, noncyclic pathway of photosynthesis
evolved in one bacterial lineage (cyanobacteria) that, over
generations, formed stromatolites
• Over time, oxygen released by cyanobacteria changed
Earth’s atmosphere
Stromatolites
• stromatolite
• Dome-shaped structures composed of layers of bacterial
cells and sediments
• Each layer formed when a mat of living cells trapped
sediments
• Descendant cells grew over the sediment layer, then
trapped more sediment, forming the next layer
Stromatolites
• Artist’s depiction:
stromatolites in an ancient
sea
• Cross-section of fossilized
stromatolite
Fossils of Early Life
• Possible bacterial cells 3.5 billion years old, and fossils of two
types of cyanobacteria approximately 850 million years old
Effects of Increasing Oxygen
1. Oxygen interferes with self-assembly of complex organic
compounds – prevented evolution of new life from nonliving
molecules
2. Presence of oxygen gave organisms that thrived in aerobic
conditions an advantage
3. Formation of an ozone layer in the upper atmosphere
protected Earth’s surface from high levels of solar ultraviolet
(UV) radiation
The Rise of Eukaryotes
• Lipids (biomarkers for eukaryotes) in 2.7-billion-year-old
rocks suggest when eukaryotic cells may have branched off
from the archaean lineage
• biomarker
• Molecule produced only by a specific type of cell; a
molecular signature
The Rise of Eukaryotes (cont.)
• Fossils with sexual spores may also be evidence of early
eukaryotes (only eukaryotes reproduce sexually)
• Protists were the first eukaryotic cells, and their fossils date
back a little more than 2 billion years
• Diversification of protists gave rise to ancestors of plants,
fungi, animals
Fossil History of Eukaryotes
• Possible oldest eukaryote (2.1 billion years old); an early alga;
and fossils of red alga (1.2 billion years old)
Fossil History of Eukaryotes
Fig. 18.10a, p. 289
Fossil History of Eukaryotes
Fig. 18.10b, p. 289
Fossil History of Eukaryotes
Fig. 18.10c, p. 289
Key Concepts
• Life’s Early Evolution
• The first cells were probably anaerobic
• An early divergence separated bacteria from archaeans
and ancestors of eukaryotic cells
• Evolution of oxygen-producing photosynthesis in bacteria
altered Earth’s atmosphere, creating conditions that
favored aerobic organisms
18.6 Evolution of Organelles
• Scientists study modern cells to test hypotheses about how
organelles evolved in the past
• By one hypothesis, internal membranes typical of eukaryotic
cells may have evolved through infoldings of plasma
membrane of prokaryotic ancestors
• Existence of some bacteria with internal membranes supports
this hypothesis
Origin of the Nucleus
• In eukaryotes, DNA resides in a nucleus that protects the
genome from physical or biological threats
• The nuclear envelope consists of a double layer of membrane
with protein-lined pores that control flow of material into and
out of the nucleus
• The nucleus and endomembrane system probably evolved
when the plasma membrane of an ancestral cell folded inward
Model: Origin of Nuclear Envelope
and Endoplasmic Reticulum
Model: Origin of Nuclear Envelope
and Endoplasmic Reticulum
infolding of plasma
membrane in prokaryotic
ancestor
ER
nuclear envelope
of early eukaryote
Fig. 18.11, p. 290
Bacteria with Internal Membranes
Bacteria with Internal Membranes
A Marine bacterium (Nitrosococcus oceani)
with highly folded internal membranes visible
across its midline.
Fig. 18.12a, p. 290
Bacteria with Internal Membranes
B Freshwater bacterium (Gemmata obscuriglobus) with DNA enclosed by a two-layer
membrane (indicated by the arrow).
Fig. 18.12b, p. 290
Mitochondria and Chloroplasts
• Mitochondria and chloroplasts resemble bacteria, and likely
evolved by endosymbiosis
• endosymbiosis
• One species lives and reproduces inside another
• Over generations, host and guest cells come to depend
upon one another for essential metabolic processes
Support for Endosymbiotic Hypothesis
• Rickettsia prowazekii,
an aerobic bacterium
that infects human cells
• Like mitochondria,
these bacteria take up
pyruvate from the
cytoplasm and break it
down by aerobic
respiration
Additional Evidence For Endosymbiosis
• Some modern protists have bacterial symbionts inside them
• Microbiologist Kwang Jeon grew amoebas infected by a rodshaped bacterium – eventually, the amoebas came to rely on
the bacteria for some life-sustaining function
• We also have evidence to support the hypothesis that
cyanobacteria can become organelles
Support for Endosymbiotic Hypothesis
• Protist with green photosynthetic organelles that resemble
cyanobacteria
Support for Endosymbiotic Hypothesis
photosynthetic organelle
with a bacteria-like cell wall
mitochondrion
nucleus
B Cyanophora paradoxa, one of the flagellated protists called glaucophytes. Its
photosynthetic structures and break it down by aerobic respiration. resemble
cyanobacteria. They even have a wall similar in composition to the wall around a
cyanobacterial cell.
Fig. 18.13b, p. 291
Key Concepts
• Eukaryotic Organelles
• A nucleus, ER, and other membrane-enclosed organelles
are defining features of eukaryotic cells
• Some organelles may have evolved from infoldings of the
plasma membrane
• Mitochondria and chloroplasts probably descended from
bacteria that lived inside other cells
18.7 Time Line for
Life’s Origin and Evolution
18.7 Time Line for
Life’s Origin and Evolution (cont.)
18.7 Time Line for Life’s Origin and Evolution
Hydrogen-rich,
oxygen-poor atmosphere
Atmospheric oxygen level
begins to increase
Archaean lineage
Aerobic respiration in some groups
3
6
7
Ancestors of eukaryotes
3
Endomembrane
system, nucleus
evolve
5
2 Origin
of cells
3
1
6
Aerobic respiration in some groups
Bacterial lineage
3.8 billion
years ago
-producing photosynthesis
4
3.2 billion
years ago
2.7 billion
years ago
Fig. 18.14, p. 292
18.7 Time Line for Life’s Origin and Evolution
Atmospheric oxygen reaches current levels;
ozone layer gradually forms
11 Archaea
11 Eukarya
10
Animals
8
9
Origin of fungi
Fungi
Heterotrophic protists
Protists with chloroplasts that
evolved from algae
Endosymbiotic
origin of
mitochondria
Protists with chloroplasts that
evolved from bacteria
Endosymbiotic
origin of
chloroplasts
of lineage leading to plants Plants
10
11 Bacteria
Oxygen-producing
photosynthetic bacteria
Other photosynthetic
bacteria
Heterotrophic bacteria
1.2 billion
years ago
900 million
years ago
435 million
years ago
Fig. 18.14, p. 293
ANIMATION: Milestones in the history of life
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Looking For Life (revisited)
• Compared to Mars,
Earth is just the right
size to sustain life
• If Earth were smaller, it
would not have enough
gravity to keep the
atmosphere from
drifting off into space