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CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
24
Early Life and
the Diversification
of Prokaryotes
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Figure 24.1
Overview: The First Cells
• Earth formed 4.6 billion years ago
• The oldest fossil organisms are prokaryotes dating back to 3.5
billion years ago
• Prokaryotes are single-celled organisms in the domains
Bacteria and Archaea
© 2014 Pearson Education, Inc.
Concept 24.1: How did the first cells
form?
• Chemical and physical processes on early Earth may
have produced very simple cells through a sequence of
stages
1. Abiotic synthesis of small organic molecules
2. Joining of these small molecules into
macromolecules
3. Packaging of molecules into protocells, membranebound droplets that maintain a consistent internal
chemistry
4. Origin of self-replicating molecules
© 2014 Pearson Education, Inc.
Synthesis of Organic Compounds on
Early Earth
• Earth’s early atmosphere likely contained water vapor
and chemicals released by volcanic eruptions (nitrogen,
nitrogen oxides, carbon dioxide, methane, ammonia,
and hydrogen)
• No free oxygen
• As Earth cooled, water vapor condensed into oceans,
and most of the hydrogen escaped into space
© 2014 Pearson Education, Inc.
Chemical Hypothesis
Theory
• In the 1920s, A. I. Oparin and J. B. S. Haldane
hypothesized that the early atmosphere was a
reducing environment
• In 1953, Stanley Miller and Harold Urey
conducted lab experiments that showed that
the abiotic synthesis of organic molecules in a
reducing atmosphere is possible
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Chemical Hypothesis Theory
• Miller-Urey-type experiments demonstrate that
organic molecules could have formed with
various possible atmospheres
• Instead of forming in the atmosphere, the first
organic compounds may have been synthesized
near volcanoes or deep-sea vents
• Organic molecules have also been found in
meteorites
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Other theories
Extraterrestrial Theory
• Organic compounds
from space
• Murchison meteorite
contained amino acids
• Organic compounds in
comets etc.
• Org. in space
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Clay Theory
• Small organic molecules
polymerize when they
are concentrated on hot
sand, clay, or rock
• RNA monomers have
been produced
spontaneously from
simple molecules
Formation of Protocells
• Spontaneous vesicle formation with hydrophobic
lipids enclosing hydrophilic organics
Can be accelerated with clay
• Adding clay can increase the rate of vesicle
formation
• Vesicles exhibit simple reproduction and
metabolism and maintain an internal chemical
environment
© 2014 Pearson Education, Inc.
Relative turbidity, an
index of vesicle number
Figure 24.4
0.4
Precursor molecules plus
montmorillonite clay
0.2
Precursor
molecules only
0
0
40
20
Time (minutes)
60
Vesicle
boundary
1 m
(a) Self-assembly
20 m
(b) Reproduction
(c) Absorption of RNA
Protocells
• Replication and metabolism are key properties of life and may
have appeared together
• Protocells may have been fluid-filled vesicles with a
membrane-like structure
• In water, lipids and other organic molecules can
spontaneously form vesicles with a lipid bilayer
• RNA first genetic material and catalytic molecule
• For example, ribozymes can make complementary copies of short
stretches of RNA
© 2014 Pearson Education, Inc.
• Natural selection has produced self-replicating RNA
molecules
• RNA molecules that were more stable or replicated more
quickly would have left the most descendant RNA molecules
• The early genetic material might have formed an “RNA
world”
© 2014 Pearson Education, Inc.
• Vesicles with RNA capable of replication would
have been protocells
• RNA could have provided the template for DNA, a
more stable genetic material
© 2014 Pearson Education, Inc.
First life form - Prokaryote
• Prokaryotes are the most abundant organisms
on Earth
• There are more in a handful of fertile soil than the
number of people who have ever lived
• Prokaryotes thrive almost everywhere, including
places too acidic, salty, cold, or hot for most other
organisms
• Some prokaryotes colonize the bodies of other
organisms
© 2014 Pearson Education, Inc.
Fossil Evidence of Early Life
• Many of the oldest fossils are stromatolites, layered
rocks that formed from the activities of prokaryotes
up to 3.5 billion years ago
• Ancient fossils of individual prokaryotic cells have
also been discovered
• For example, fossilized prokaryotic cells have been
found in 3.4-billion-year-old rocks from Australia
• Heterotroph hypothesis – first cell was anaerobe and
heterotroph.
© 2014 Pearson Education, Inc.
5 cm
30 m
Figure 24.5
1.1-billion-year-old
fossilized stromatolite
3-billion-year-old
fossil of a cluster of
nonphotosynthetic
prokaryote cells
10 m
Stromatolites
Nonphotosynthetic bacteria
1.5-billion-year-old fossil
of a cyanobacterium
Possible
earliest
appearance
in fossil record
4
Cyanobacteria
3
2
Time (billions of years ago)
1
0
Sequence of eventS in evolution
• Chlorophyll evolved – autotrophic prokaryote
• The cyanobacteria that form stromatolites were
the main photosynthetic organisms for over a
billion years
• Began the release of oxygen into Earth’s
atmosphere
• Surviving prokaryote lineages either avoided or
adapted to the newly aerobic environment.
• Aerobic prokaryotes evolved
© 2014 Pearson Education, Inc.
Concept 24.2: Diverse structural and metabolic
adaptations have evolved in prokaryotes
• Most prokaryotes are unicellular, although some species
form colonies
• Most prokaryotic cells have diameters of 0.5–5 µm,
much smaller than the 10–100 µm diameter of many
eukaryotic cells
• Prokaryotic cells have a variety of shapes
• The three most common shapes are spheres (cocci),
rods (bacilli), and spirals
© 2014 Pearson Education, Inc.
1 m
(a) Spherical
(b) Rod-shaped
3 m
1 m
Figure 24.6
(c) Spiral
Cell-Surface Structures
• A key feature of nearly all prokaryotic cells is their cell
wall, which maintains cell shape, protects the cell, and
prevents it from bursting in a hypotonic environment
• Eukaryote cell walls are made of cellulose or chitin
• Bacterial cell walls contain peptidoglycan, a network
of modified sugars cross-linked by polypeptides
© 2014 Pearson Education, Inc.
Figure 24.11
1 m
0.2 m
Respiratory
membrane
Thylakoid
membranes
(a) Aerobic prokaryote
(b) Photosynthetic prokaryote
Nutritional and Metabolic Adaptations
© 2014 Pearson Education, Inc.
The Role of Oxygen in Metabolism
• Prokaryotic metabolism varies with respect to O2
• Obligate aerobes require O2 for cellular
respiration
• Obligate anaerobes are poisoned by O2 and use
fermentation or anaerobic respiration, in which
substances other than O2 act as electron acceptors
• Facultative anaerobes can survive with or without
O2
© 2014 Pearson Education, Inc.
Concept 24.3: Rapid reproduction, mutation,
and genetic recombination promote genetic
diversity in prokaryotes
• Prokaryotes have considerable genetic variation
• Three factors contribute to this genetic diversity
• Rapid reproduction
• Mutation
• Genetic recombination
© 2014 Pearson Education, Inc.
Figure 24.18
Euryarchaeotes
Crenarchaeotes
UNIVERSAL
ANCESTOR
Nanoarchaeotes
Domain Archaea
Korarchaeotes
Domain
Eukarya
Eukaryotes
Proteobacteria
Spirochetes
Cyanobacteria
Gram-positive
bacteria
Domain Bacteria
Chlamydias
Archaea
• Archaea share certain traits with bacteria and other
traits with eukaryotes
© 2014 Pearson Education, Inc.
Figure 24.UN02
Eukarya
Archaea
Bacteria
Table 24.2a
Table 24.2b
Concept 25.1: Eukaryotes arose by endosymbiosis more
than 1.8 billion years ago
Early eukaryotes were unicellular
 Eukaryotic cells have organelles and are
structurally more complex than prokaryotic cells
 A well-developed cytoskeleton enables eukaryotic
cells to have asymmetrical forms and to change
shape

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The Fossil Record of Early Eukaryotes
Chemical evidence for the presence of
eukaryotes dates back to 2.7 billion years ago
 The earliest fossils of eukaryotic cells are 1.8
billion years old

© 2014 Pearson Education, Inc.
Origin of Mitochondria and Plastids
Endosymbiont theory proposes that
mitochondria and plastids were formerly small
prokaryotes that began living within larger cells
 An endosymbiont is a cell that lives within a host
cell
 Prokaryote ancestors to mitochondria and
plastids probably entered the host cell as
undigested prey or internal parasites

© 2014 Pearson Education, Inc.
Endosymbiont theory
The relationship between endosymbiont and
host cells was mutually beneficial
 Anaerobic host cells benefited from
endosymbionts’ ability to take advantage of
an increasingly aerobic world
 Heterotrophic host cells benefited from the
nutrients produced by photosynthetic
endosymbionts
 In the process of becoming more
interdependent, the host and endosymbionts
would have become a single organism

© 2014 Pearson Education, Inc.
Figure 25.3
Cytoplasm
DNA
Ancestral
prokaryote
Plasma
membrane
Endoplasmic
reticulum
Engulfing
of aerobic
bacterium
Engulfing
of photosynthetic
bacterium
Nucleus
Nuclear
envelope
Mitochondrion
Mitochondrion
Ancestral
heterotrophic
eukaryote
Plastid
Ancestral
photosynthetic
eukaryote
Endosymbiont theory

Key evidence supporting an endosymbiotic
origin of mitochondria and plastids:
◦ Inner membranes are similar to plasma
membranes of prokaryotes
◦ Division is similar in these organelles and some
prokaryotes
◦ DNA structure is similar to that of prokaryotes
◦ These organelles transcribe and translate their
own DNA
◦ Their ribosomes are more similar to prokaryotic
than eukaryotic ribosomes
© 2014 Pearson Education, Inc.
Endosymbiont theory

DNA sequence analysis indicates that
mitochondria arose from an alpha
proteobacterium

Eukaryotic mitochondria descended from a
single common ancestor

Plastids arose from an engulfed
cyanobacterium

Some photosynthetic protists may have been
engulfed to become endosymbionts
themselves
© 2014 Pearson Education, Inc.