Transcript Origin of life - River Dell Regional School District

BIOLOGY Life on Earth
WITH PHYSIOLOGY Tenth Edition
Audesirk Audesirk Byers
The History
of Life
Lecture Presentations by
Carol R. Anderson
Westwood College, River Oaks Campus
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Chapter 17 At a Glance
How Did Life Begin?
What Were the Earliest Organisms Like
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17.1 How Did Life Begin?
 Until the 19th century, most people thought that new
members of species sprang up all the time through
spontaneous generation both from nonliving
matter and other, unrelated forms of life
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17.1 How Did Life Begin?
 Medieval beliefs reflected the concept of spontaneous
generation
– Maggots were thought to arise from meat
– Microbes were thought to arise from broth
– Mice were thought to arise from mixtures of sweaty
shirts and wheat
 The maggots-from-meat idea was disproved by
Francesco Redi in 1668
– No maggots developed when he kept flies away from
uncontaminated meat
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17.1 How Did Life Begin?
 The broth-to-microorganism idea was disproved by
Louis Pasteur and John Tyndall in the mid-1800s
– Microorganisms did not appear in sterile broth unless
the broth was first exposed to existing
microorganisms in the surrounding environment
– Pasteur and Tyndall’s work demolished the notion of
spontaneous generation
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Figure 17-1 Spontaneous generation refuted
no growth
The broth in a flask
is boiled to kill preexisting
microorganisms
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The long, S-shaped neck
allows air, but not microorganisms
to enter the flask
growth
If the neck is later broken off,
outside air can carry
microorganisms into the broth
17.1 How Did Life Begin?
– Pasteur and Tyndall’s work did not address the
question of how life on Earth originated in the first
place
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17.1 How Did Life Begin?
 The first living things arose from nonliving ones
– Modern scientific ideas about the origin of life began
to emerge in the 1920s
– Alexander Oparin in Russia and John B. S. Haldane in
England showed that spontaneous formation of the
complex organic molecules necessary for life would not
be permitted in today’s oxygen-rich atmosphere
– Oxygen reacts readily with other molecules, disrupting
chemical bonds
– An oxygen-rich environment tends to keep molecules
simple
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17.1 How Did Life Begin?
– Oparine and Haldane speculated that the atmosphere
of early Earth contained little oxygen because an
oxygen-rich atmosphere would not have permitted the
spontaneous formation of complex organic molecules
– Oxygen’s high reactivity with chemical bonds would
have prevented large molecules from forming
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17.1 How Did Life Begin?
– Organic molecules can form spontaneously under
prebiotic conditions
– In 1953, Stanley Miller and Harold Urey set out to
simulate the first stage of prebiotic evolution in the
laboratory
– They noted that the atmosphere of early Earth probably
contained methane (CH4), ammonia (NH3), hydrogen
(H2), and water vapor (H2O), but no oxygen
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17.1 How Did Life Begin?
– They simulated early Earth’s atmosphere by mixing the
gases in a flask and adding an electrical discharge to
simulate lightning
– Simple organic molecules appeared after a few days
– The experiment showed that small molecules likely
present in the early atmosphere can combine to form
larger organic molecules if electrical energy is
present
– Similar experiments by Miller and others have
produced amino acids, short proteins, nucleotides,
ATP, and other molecules characteristic of living
things
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Figure 17-2 The experimental apparatus of Stanley Miller and Harold Urey
An electric spark simulates
a lightning storm
electric spark
chamber
CH4
NH3
H2
H2O
Energy from the spark
powers reactions among
molecules thought to be
present in Earth’s early
atmosphere
Boiling water adds
water vapor to the
artificial atmosphere
condenser
cool water
flow
boiling chamber
water
Organic molecules
appear after a few
days
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When the hot gases in
the spark chamber are
cooled, water vapor
condenses and any
soluble molecules
present are dissolved
17.1 How Did Life Begin?
Modern geochemists believe the early atmosphere was
somewhat different from that modeled in Miller and Urey’s
experiments
– Additional experiments with more realistic (but still
oxygen-free) simulated atmospheres have also yielded
organic molecules
– The experiments proved that electricity is not the only
suitable energy source
– Other sources include heat and ultraviolet (UV) light,
which have been shown to drive the formation of organic
molecules in experimental simulations of prebiotic
conditions
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17.1 How Did Life Begin?
– Additional organic molecules probably arrived from
space when meteorites and comets crashed into the
Earth’s surface
– Analysis of present-day meteorites recovered from
impact craters on Earth has revealed that some
meteorites contain relatively high concentrations of
amino acids and other simple organic molecules
– When small molecules known to be present in space
were placed under space-like conditions of very low
temperature and pressure and bombarded with UV light,
larger organic molecules were produced
–
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17.1 How Did Life Begin?
The ozone layer is a region high in today’s atmosphere
that is enriched with ozone molecules
– The ozone molecules form when incoming solar energy
splits some O2 molecules in the outer atmosphere into
individual oxygen (O) atoms
– The oxygen (O) atoms react with O2 to form O3 (ozone)
– Before the formation of the ozone layer, UV
bombardment must have been fierce
– UV radiation can provide energy for the formation of
organic molecules, but can also break them apart
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17.1 How Did Life Begin?
– Sites beneath rock ledges or at bottoms of even fairly
shallow seas, would have been protected from UV
radiation
– In these locations, organic molecules may have
accumulated
– Clay may have catalyzed the formation of larger organic
molecules
– In the next stage of evolution, simple molecules combined
to larger molecules
– One possibility is that small molecules accumulated on
surfaces of clay particles may have a small electrical
charge that attracts dissolved molecules of the opposite
charge
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17.1 How Did Life Begin?
– Initially, these molecules might had formed on clay at
the bottom of early Earth’s oceans or lakes, but have
now become the building blocks of the first living
organisms
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17.1 How Did Life Begin?
 RNA may have been the first self-reproducing
molecule
 RNA is simpler.
– DNA was probably not the earliest informational
molecule
– DNA replication requires large complex protein
enzymes
– The instructions for building these enzymes are coded in
DNA
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17.1 How Did Life Begin?
– chicken-and-egg interdependency makes DNA an
unlikely candidate for self-replication
– The current DNA-based system of information storage
likely evolved from an earlier system
– RNA can act as a catalyst
– RNA is a prime candidate for the first self-replicating
informational molecule
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17.1 How Did Life Begin?
– Thomas Cech and Sidney Altman (1980s), working
with the single-celled organism called Tetrahymena,
discovered a cellular reaction that was catalyzed by a
protein, by a small RNA molecule
– Cech and Altman named their catalytic RNA molecule
ribozyme
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Figure 17-3 A ribozyme
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17.1 How Did Life Begin?
– RNA can act as a catalyst
– Since Cech and Altman’s initial discovery, dozens of
naturally occurring ribozymes have been found that
catalyze reactions, including
– Cutting other RNA molecules
– Splicing together different RNA fragments
– Attaching amino acids to growing proteins
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17.1 How Did Life Begin?
– Earth may once have been an RNA world
– Discovery of ribozymes led to the hypothesis that RNA
preceded the origin of DNA, in an “RNA world”
– According to this view, the current era of DNA-based life
was preceded by one in which RNA served as the
information-carrying genetic molecule and the catalyst for
its own replication
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17.1 How Did Life Begin?
This first self-reproducing ribozyme probably wasn’t very
good at its job and produced copies with lots of errors
– Natural selection acted on these errors to improve the
function of these early ribozymes
– With increased speed and accuracy of replication, these
variant ribozymes reproduced, copying themselves and
displacing less efficient molecules
– Molecular evolution continued
– By some unknown chain of events, RNA gradually
receded into its present role as intermediary between
DNA and protein enzymes
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17.1 How Did Life Begin?
 Membrane-like vesicles may have enclosed
ribozymes
– Self-replicating molecules on their own do not
constitute life
– In all living cells such molecules are contained within
some kind of enclosing membrane
– Chemists have shown that if water containing proteins
and lipids is agitated to simulate waves beating
against ancient shores, the proteins and lipids
combine to form hollow vesicles
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17.1 How Did Life Begin?
– Certain vesicles (protocells) may have contained
organic molecules, including ribozymes, and would
have been the precursors of living cells
– The membranes would have served to confiscate the
molecules of life and to protect them from extraneous
ribozymes
– ….BIG JUMP---- REPRODUCTION
– After sufficient time, these protocells may have
developed the ability to divide and pass on copies of
their enclosed ribozymes to daughter protocells
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17.2 What Were the Earliest Organisms Like?
 Earth formed about 4.5 billion years ago and was
hot
– Meteorites smashed into the forming planet, and the
kinetic energy of these extraterrestrial rocks was
converted into heat on impact
 Geological evidence suggests that Earth cooled
enough for water to exist in liquid form 4.3 billion
years ago
 The oldest fossil organisms found so far are in rocks
that are approximately 3.4 billion years old
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17.2 What Were the Earliest Organisms Like?
 Life arose 3.9 billion years ago in what is called the
Precambrian era
 Geologists and paleontologists have devised a
hierarchical naming system of eras, periods, and
epochs to delineate the immense span of geological
time
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Figure 17-4 Early Earth
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Table 17-1, 1 of 4
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Table 17-1, 2 of 4
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Table 17-1, 3 of 4
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Table 17-1, 4 of 4
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17.2 What Were the Earliest Organisms Like?
 The first organisms were anaerobic prokaryotes
– The first cells to arise in the Earth’s oceans were
prokaryotes, cells that lack a membrane-bound
nucleus
– These primitive bacteria probably obtained nutrients
and energy by absorbing organic molecules from their
environment
– Since early Earth lacked oxygen gas, these first cells
metabolized organic molecules anaerobically
– SIMPLE to COMPLEX…
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17.2 What Were the Earliest Organisms Like?
 Some organisms evolved the ability to capture the
sun’s energy
– Eventually, some cells evolved the ability to use the
energy of sunlight for synthesis of complex, highenergy molecules
– This marked the evolution of photosynthesis
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17.2 What Were the Earliest Organisms Like?
– Photosynthesis requires a source of hydrogen, and
photosynthetic bacteria began use water (H2O)—
Earth’s most abundant source of hydrogen
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17.2 What Were the Earliest Organisms Like?
– Photosynthesis converts water and carbon dioxide to
energetic molecules of sugar, releasing oxygen as a
by-product
– This new method for capturing energy introduced
significant amounts of free oxygen into the atmosphere
for the first time
– Initially, oxygen combined with iron in the Earth’s crust
to form iron oxide (rust)
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17.2 What Were the Earliest Organisms Like?
– Initially, newly liberated oxygen was quickly
consumed by reactions with other molecules in the
atmosphere and in the Earth’s crust
– Iron was a common reactive atom in the Earth’s crust,
combining with the new oxygen to form iron oxide (rust)
– As a result, iron is abundant in rocks formed during that
time
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17.2 What Were the Earliest Organisms Like?
– Subsequently, oxygen began accumulating in the
atmosphere
– Chemical analysis of rocks suggests that significant
levels of atmospheric oxygen first appeared about 2.3
billion years ago
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17.2 What Were the Earliest Organisms Like?
– Subsequently, oxygen began accumulating in the
atmosphere
– Scientists suggest that bacteria probably similar to
cyanobacteria produced the oxygen
– It is very likely that we are breathing some of the
recycled oxygen molecules expelled more than 2 billion
years ago
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17.2 What Were the Earliest Organisms Like?
 Aerobic metabolism arose in response to dangers
posed by oxygen
– Oxygen is potentially dangerous to living things
because of its ability to react with organic molecules,
breaking them down
– The accumulation of oxygen in the atmosphere of early
Earth probably exterminated many anaerobic
organisms
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17.2 What Were the Earliest Organisms Like?
– The next great advance in the age of microbes was
the ability to use oxygen in metabolism
– This ability provides a defense against the chemical
action of oxygen
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17.2 What Were the Earliest Organisms Like?
– The evolution of aerobic metabolism was significant
because aerobic organisms can harvest more energy
per food molecule than anaerobic organisms
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17.2 What Were the Earliest Organisms Like?
 Some organisms acquired membrane-enclosed
organelles
– As prokaryotes began to proliferate, some developed
the ability to engulf smaller ones
– The ability to compartmentalize functions inside the
cell through internal membrane formation greatly
improved the efficiency of the early cells
– The first eukaryotes (organisms composed of one or
more eukaryotic cells) appeared about 1.7 billion years
ago
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17.2 What Were the Earliest Organisms Like?
– Mitochondria and chloroplasts may have arisen from
engulfed bacteria
– The endosymbiosis hypothesis proposes that early
eukaryotic cells acquired the precursors of mitochondria
and chloroplasts by engulfing certain types of bacteria
– These cells and the bacteria trapped inside them
gradually entered into a symbiotic relationship, a close
association between different types of organisms
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Figure 17-5 The probable origin of mitochondria and chloroplasts in eukaryotic cells
aerobic
bacterium
An anaerobic,
predatory prokaryotic cell
engulfs an aerobic
bacterium
Descendants of the
engulfed bacterium evolve
into mitochondria
photosynthetic
bacterium
The mitochondriacontaining cell engulfs a
photosynthetic bacterium
Descendants of the
photosynthetic bacterium
evolve into chloroplasts
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17.2 What Were the Earliest Organisms Like?
 Some organisms acquired membrane-enclosed
organelles
– Choroplasts and Mitochondria have own DNA and
self-replicating
– Living intermediates (organisms alive today that are
similar to hypothetical ancestors)
– The amoeba Pelomyxa palustris lacks a mitochondria
but hosts a permanent population of aerobic bacteria
that carry out much of the same role
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17.2 What Were the Earliest Organisms Like?
 The evidence for the endosymbiont hypothesis is
strong
– Modern cells, called living intermediates, host
bacterial endosymbionts
– The amoeba Pelomyxa palustris harbors aerobic
bacteria
– A variety of corals, some clams, a few snails, and at
least one species of Paramecium harbor photosynthetic
bacteria
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Figure 17-6 Symbiosis within a modern cell
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