Transcript Document

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 16
The Origin and Evolution
of Microbial Life:
Prokaryotes and Protists
Modules 16.1 – 16.6
From PowerPoint® Lectures for Biology: Concepts & Connections
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EARLY EARTH AND THE ORIGIN OF LIFE
16.1 Life began on a young Earth
• Planet Earth formed some 4.6 billion years ago
• Heat generated by the impact of meteorites, radioactive
decay, and compaction of gravity thawed earth and
turned it into a molten mass.
• The more dense material became the core (nickel and
iron) and the less dense material the thin crust.
• The first atmosphere was most likely H2 gas which
quickly escaped into space.
• Vents from volcanoes belched gases that now formed a
new atmosphere.
•
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• This new early atmosphere probably contained H2O, CO, CO2,
N2, and possibly some CH4, but little or no O2
• Volcanic activity, lightning, and UV radiation were intense.
• The first seas were created when the earth cooled enough for
water vapor to condense.
• It was in this environment that it is theorized that life began.
Figure 16.1A
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• Fossilized prokaryotes date back 3.5 billion years from
stromatolites from western Australia.
• They were photosynthetic autotrophs whose metabolism
was too complex to represent the first organism.
• Thus, life most likely arose approximately 3.9 billion years
ago.
•
Figure 16.1B, D
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• It is hypothesized that
life may have
developed from
nonliving materials as
early as 3.9 billion
years ago.
• How this occurred
was hypothesized by
the Russian
biochemist, A.I.
Oparin (1923), and the
British geneticist, B.S.
Haldane..
Figure 16.1C
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Earliest animals; diverse algae
Earliest multicellular eukaryotes?
Billions of years ago
• The earliest entities
may have been
aggregates of
molecules which
would have then
evolved into
aggregates of
polymers with simple
metabolism and self
replication ability.
= 500 million years ago
Earliest eukaryotes
Accumulation of atmospheric
O2 from photosynthetic
cyanobacteria
Oldest known prokaryotic fossils
Origin of life?
Formation of Earth
16.2 How did life originate?
• They hypothesized that small organic molecules must have appeared
first
– This probably happened when inorganic chemicals were energized
by lightning or UV radiation.
– The early atmosphere had no oxygen, a strong oxidizing agent that
tends to disrupt chemical bonds by attracting electrons from them.
– Before the early prokaryotes added oxygen to the air, earth most
likely had a reducing atmosphere, donating electrons, bringing
simple molecules together to combine into more complex ones.
– Oparin and Haldane never tested their hypothesis , but in 1953,
Stanley Miller and Harold Urey did at the University of Chicago.
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The Present Controversy
• Question: Was the early Earth atmosphere reducting or
oxidizing? In some books, it is said that reducting, in
others the opposite. What is the latest scientists'
opinion? Answer:
•
The oxidizing or reducing state of the early atmosphere is not well
understood. Fifty years ago it was thought to be reducing, with lots
of hydrogen, methane and ammonia. Later studies tended toward
mildly oxidizing, with carbon dioxide as the major gas. Recently
both carbon dioxide and methane have been suggested as necessary
to provide sufficient greenhouse effect to warm the Earth at a time
when the Sun was about 30% dimmer than it is today. Thus the
latest scientists' opinion is that the early atmosphere was probably
neither strongly reducing nor strongly oxidizing,with carbon
dioxide and nitrogen the main gases. This opinion may well change,
however, as we learn more about the ancient environment of our
planet.
•
David Morrison
NAI Senior Scientist
(NASA Astrobiology Institute) 30 May 2006
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16.3 Talking About Science: Stanley Miller’s
experiments showed that organic materials
could have arisen on a lifeless earth
http://www.accessexcellence.org/WN/NM/mill
er.html
Figure 16.3A
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• Simulations
of such
conditions
have produced
amino acids,
sugars, lipids,
and nucleotide
bases as well
as ATP.
atmosphere
CH4
Water vapor
Electrode
lightning
Condenser
Causing rain
Cold
water
H2O
sea
Cooled water
containing
organic
compounds
Sample for
chemical analysis contained 20 amino acids, Figure 16.3B
sugars, lipids, nucleotides and even ATP.
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16.4 The first polymers may have formed on hot
rocks or clay
• The organic monomers had to form polymers.
• These monomers could have polymerized on
hot rocks or clay through dehydration
synthesis.
– This could have produced such polymers as
polypeptides and short nucleic acids.
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• Surrounding membranes may have protected some of these
polymers and macromolecules as they evolved rudimentary
metabolism (protobiont).
• Stanley Fox (1912) did research on structures of this nature.
• These structures form spontaneously in the lab from
solutions of organic molecules.
– Microspheres composed of may protein molecules are
organized into a membrane like structure.
– Coacervates are collections of droplets that are composed
of molecules of different types which may include: lipids,
amino acids, sugars and nucleic acids.
Microspheres and coacervates have a number of lifelike
qualities: take up substances from the surroundings,
increase in size, form buds, fuse with similar structures.
This proves a membrane bound structure did not need
genetic information to form. –The Debate is to the nature
of the membrane sac!!! Protein or Lipid??????????????
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Coacervates
Membrane
RNA
Polypeptide
Which organic molecules came first???
Another raging debate!!!!!!!!?
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Figure 16.6B, C
16.5 The first genetic material and enzymes may
both have been RNA..The RNA World
• The first genes may have been RNA molecules
(Thomas Cech-University of Colorado)
– These molecules could have catalyzed their own
replication in a prebiotic RNA world
2
Monomers
1
Formation of short RNA
polymers: simple “genes”
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Assembly of a
complementary RNA
chain, the first step in
replication of the
original “gene”
Figure 16.5
16.6 Molecular cooperatives enclosed by
membranes probably preceded the first real
cells-- A Peptide-Nucleic Acid World
• These molecules might have
acted as rough templates for
the formation of polypeptides
– These polypeptides may
have in turn assisted RNA
replication
Some argue the proteins
came first …for without
enzymes (most of which
are proteins), nothing
could replicate!!!!! A
Protein World
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RNA
Self-replication
of RNA
Self-replicating RNA
acts as template on
which polypeptide
forms.
Polypeptide
Figure 16.6A
Polypeptide acts
as primitive
enzyme that
aids RNA
replication.
The Evolution of prokaryotes
• The early protobionts which relied on molecules present in
the primitive soup (primitive heterotrophs) were gradually
replaced by organisms that could produce their own
needed compounds (chemoautotrophs).
• The diversification of these autotrophs led to the
emergence of true heterotrophs that relied on the
autotrophs.
• These inhabitants (prokaryotes) were the earth’s sole
inhabitants from 3.5 to about 2 billion years ago.
• They began to transform the atmosphere as atmospheric
oxygen began to appear about 2.7 billion years ago as the
result of prokaryotic photosynthetic autotrophs.
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How Ancient Bacteria Changed the World
• Biological and geologic history are closely
intertwined
• Fossilized mats of prokaryotes
2.5 billion years old mark a
time when photosynthetic
bacteria were producing O2
that made the atmosphere
aerobic
– These fossilized mats, as you recall, are
called stromatolites.
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