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Chapter 18Origin and History of Life

Lecture Outline

BiologySylvia S. Mader

Michael Windelspecht

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Outline

• 18.1 Origin of Life• 18.2 History of Life• 18.3 Geological Factors That Influence

Evolution

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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.” The molecules of living organisms, called biomolecules,

are organic molecules.• However, the first cells had to arise from nonliving chemicals,

inorganic substances.

Studies in chemistry, evolutionary biology, paleontology, microbiology, and other branches of science help scientists develop hypotheses about life’s origins.

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Origin of Life

• The earth came into being about 4.6 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

• Four stages in life’s origins Stage 1: Organic monomers evolved from inorganic compounds. Stage 2: Organic polymers were joined to form organic

polymers. Stage 3: Organic polymers became enclosed in membranes to

form protocells or protobionts. State 4: Protobionts acquired the ability to self-replicate.

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Stages of the Origin of Life

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xxx

early Earth

Inorganic chemicals

Stage 1

Stage 2

Stage 3

Stage 4

Stage 4

abioticsynthesis

Ch

emic

al E

volu

tio

nB

iolo

gic

al E

volu

tio

nB

iolo

gic

al E

volu

tio

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Origin of Life:first self-replicating cell

extinct lineages

Common ancestorof all life on Earth orLUCA (last universalcommon ancestor)

extant organisms

RNADNA

origin ofgenetic code

proto cell

plasmamembrane

polymers

polymerization

Small organic molecules

cell

cell

energycapture

LUCA

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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 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 and Harold Urey (1953) Conducted an experiment to test the Oparin/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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Miller-Urey Experiment

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electrode

cool water in

hot water outstopcock forwithdrawing liquid

stopcock foradding gases

boiler liquid droplets

condenser

electricspark

CH4

NH3

H2

H2O

heat small organic molecules

gases

Chemical Evolution at Hydrothermal Vents

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© Ralph White/Eureka Premium/Corbis

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plume of hot waterrich in iron-nickel sulfides

hydrothermalvent

Origin of Life• In cells, monomers join to form polymers in the

presence of enzymes. Iron-Sulfur World Hypothesis

• It suggests organic molecules reacted with amino acids to form peptides in the presence of iron-nickel sulfides.

Protein-First Hypothesis• It assumes that protein enzymes arose first.• DNA genes came afterwards.

RNA-First Hypothesis• It suggests only RNA was needed to progress toward formation of the

first cell or cells.• Some viruses have only RNA genes.• DNA genes would have come 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.

They 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

hydrophilic “head”

hydrophobic “tail”

phospholipid

outside celloutside protocell

inside protocell inside cell

phospholipid bilayerfatty acid bilayer

d.c.

a. b.

2 fatty acids

individual fatty acid

Structure and Growth of the First Plasma Membrane

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.

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b.

a.

micelle

vesicle

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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, an organism that consumes preformed organic molecules.

• 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 (cont’d)

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.• Scientists speculate that it took millions of years for glycolysis,

a metabolic pathway that transforms high-energy chemical bonds into energy for a cell to do work, to evolve completely.

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Origin of Life

• A Self-Replication System RNA-first hypothesis

• The first cell would have had an 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.

18.2 History of Life

• Fossils are the 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 a

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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The History of Life

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Fossils

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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 using, for example 14C, which decays to 14N.• 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.

• Occurs at a constant rate• Some only fractions of a second• Some billions of years• Most in-between

• Many isotopes are used in absolute dating, 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

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 appeared about 3.5 BYA.

Cyanobacteria fossils have been found in ancient stromatolites.

Photosynthetic cyanobacteria added oxygen to the atmosphere.

Aerobic bacteria proliferated in the oxygen-rich atmosphere.

New metabolic pathways evolved.

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The Tree of Life

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ARCHAEA

BACTERIA

first cells

photosynthetic bacteria (produce oxygen)

aerobic bacteria

other photosynthetic bacteria (do not produce oxygen)

thermophiles

halophiles

methanogens

mitochondria

chloroplasts

3.5 BYA 543 MYA1.4 BYA2.2 BYA

Archaebacteria

Animals

Fungi

Protists

Plants

Bacteria

2

7

8

EUKARYA

heterotrophicprotists

photosyntheticprotists

1

3

4

5

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Prokaryote Fossils of the Precambrian

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a. Stromatolites b. Primaevifilum

0

20

10 m

18.9a: © Francois Gohier/Science Source; 18.9b: © Dr. J. William Schopf

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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.

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.• Among today’s protists are colonial forms in which some

cells are specialized to produce gametes needed for sexual reproduction.

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Ediacaran Fossils

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History of Life

• The Paleozoic Era It begins with the Cambrian period. It lasted over 300 million years. It includes three major mass extinction events. An extinction is the total disappearance of all

members of a species or higher taxonomic group. Mass extinction:

• Disappearance of a large number of taxa• Occurred within a relatively short time interval, a few million

years (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.

• 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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Sea Life of the Cambrian Period

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History of Life

• Invasion of land Began around 500 MYA Plants

• Seedless vascular plants date back to the Silurian period.• They 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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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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Dinosaurs of the Late Cretaceous Period

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History of Life• The Cenozoic Era

It is divided into Tertiary and Quaternary periods.

Mammals continued adaptive radiation.

• Several species took to the air.

Flowering plants were already diverse and plentiful. Primate evolution began.

• Some primates adapted to living in trees for protection from predators and to obtain food in the form of fruit.

• Ancestral apes appeared during the Oligocene epoch.

• Megafauna during Pleistocene epoch– Human hunting may have been responsible for the extinction of mammalian megafauna.

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Mammals of the Oligocene Epoch

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Woolly Mammoth of the Pleistocene Epoch

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© Gianni Dagli Orti/Corbis

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 result in continental drift.

• Modern mammalian diversity results from isolated evolution on separate continents.

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Continental Drift

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NorthAmerica Eurasia

SouthAmerica

AfricaIndia

Australia

NorthAmerica

Eurasia

AfricaIndia

Australia

Antarctica

SouthAmerica

Laurasia

Gondwana

Pangaea

(251 million years ago) (135 million years ago)

Antarctica

Present day(65 million years ago)

PALEOZOIC MESOZOIC CENOZOIC

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Geological Factors That Influence Evolution

• Mass extinctions occurred at the end of the following periods:

• Ordovician 444 MYA 75% of species

disappeared

• Devonian 360 MYA 70% of marine invertebrates

disappeared

• Permian 251 MYA

90% of species disappeared

• Triassic 20 MYA

60% of species disappeared

• Cretaceous 66 MYA

75% of species disappeared

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Mass Extinctions

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ammonoids extinct

poriferans

brachiopods

insects

birds

mammals

65.5 MYA

75% 70% 90% 60% 75%

poriferans

brachiopods

mammals

birds

dinosaurs

insects

ammonoids

dinosaurs extinct

CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN CARBONIFEROUS PERMIAN TRIASSIC JURASSIC CRETACEOUS TERTIARYQUARTE-RNARY

PRESENT

199.6 MYA251 MYA359.2 MYAMajorExtinctions

% SpeciesExtinct

443.7 MYA

STATUS TODAY

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