chapter 18 life’s origin and early evolution (sections 18.1 - 18.7)

Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

www.cengage.com/biology/starr

Chapter 18 Life’s Origin and Early Evolution

(Sections 18.1 - 18.7)

http://www.cengage.com/biology/Starr/Concepts8e

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 self-assemble 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

Fig. 18.4, p. 285

boiling water

gases

water in

spark discharge

electrodes

water droplets

water containing organic compounds

liquid water in trap

CH4

NH3

H2O H2

to vacuum pump

condenser

water out

Miller-Urey Experiment

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 chimney-like 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



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

Fig. 18.6, p. 286

Stepped Art

Organic monomers

self-assemble on Earth and in space

Inorganic molecules

Organic polymers

self-assemble in aquatic environments on Earth

DNA-based cells

Are subject to selection that favors a DNA genome

Protocells in an RNA world

interact in early metabolism

self-assemble as vesicles

become the first genome

Steps on the Road to Life

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)

Fig. 18.10a, p. 289

Fossil History of Eukaryotes

Fig. 18.10b, p. 289


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

Fig. 18.11, p. 290

nuclear envelopeof early eukaryote

ER

infolding of plasma membrane in prokaryotic ancestor

Model: Origin of Nuclear Envelope and Endoplasmic Reticulum

Bacteria with Internal Membranes

Fig. 18.12a, p. 290

A Marine bacterium (Nitrosococcus oceani) with highly folded internal membranes visible across its midline.


Fig. 18.12b, p. 290

B Freshwater bacterium (Gemmata obscuri- globus) with DNA enclosed by a two-layer membrane (indicated by the arrow).


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 rod-shaped 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


• Protist with green photosynthetic organelles that resemble cyanobacteria

Fig. 18.13b, p. 291

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.

mitochondrion nucleus

photosynthetic organelle with a bacteria-like cell wall


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.)

Fig. 18.14, p. 292

Hydrogen-rich, oxygen-poor atmosphere

Atmospheric oxygen level begins to increase

Aerobic respiration in some groupsArchaean lineage

Ancestors of eukaryotes

Endomembrane system, nucleus evolve

Origin of cells

-producing photosynthesis

3.8 billion years ago



Aerobic respiration in some groupsBacterial lineage1

2

3

4

5

6

7

3

3 6


Fig. 18.14, p. 293

Endosymbiotic origin of

mitochondria

Endosymbiotic origin of

chloroplasts

Atmospheric oxygen reaches current levels; ozone layer gradually forms


900 millionyears ago

435 million years ago

Heterotrophic bacteria

Oxygen-producing photosynthetic bacteria

Plants

Protists with chloroplasts that evolved from bacteria

Protists with chloroplasts that evolved from algae

Heterotrophic protists

Fungi

Animals

Eukarya

Archaea

of lineage leading to plants

Origin of fungi

Other photosynthetic bacteria

Bacteria11

11

11

10

10

9

8


ANIMATION: Milestones in the history of life



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

chapter 18 life’s origin and early evolution (sections 18.1 - 18.7)

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