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ORIGIN OF LIFE
ON EARTH
D1
FUNDAMENTAL QUESTIONS THAT
HAVE PLAGUED HUMANKIND….
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Where do we come from?
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How did life start on Earth?
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What were our ancestors like millions of years
ago?
PROBLEMS FOR STARTING LIFE ON EARTH
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How could the lifeless ball of rock that the planet
Earth was 3.5 billion years ago, become home to
such lush vegetation and a wide variety of
bacteria, fungi, protists, and animals that we see
today?
There are 4 problems which needed to be
overcome for the life on Earth to exist.
FOUR PROCESSES NEEDED FOR THE
SPONTANEOUS ORIGIN OF LIFE

1. The non-living synthesis of simple
organic molecules
life is based on organic molecules (amino acids,
CHOs, lipids, nucleic acids)
early
Earth only had inorganic matter (i.e. rock,
minerals, gasses- H2, CO2, NH3, CH4, H2O)
organic
compounds may have come from
inorganic molecules via volcanoes, UV radiation
and electric discharge
or
they may have been introduced to Earth from
space

2. The assembly of these molecules into
polymers

Polymerization of these simple molecules occurred to
form more complex organic chemicals.
3. ORIGIN OF SELF-REPLICATING
MOLECULES (MAKES INHERITANCE
POSSIBLE)
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For something to be “alive” it must reproduce on
its own.
Need a self-replicating molecule
Only self-replicating molecules are able to
undergo evolution by natural selection
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DNA is the molecule most used for replication of
organisms but it is complex and requires
enzymes for its formation.
Therefore, it is unlikely that DNA developed very
early.
Also, to get the proteins required to form from
DNA, RNA is required

4. The packaging of these molecules into
membranes with an internal chemistry
different from their surroundings
Many compounds tend to dissolve in water, and
therefore will depolymerise (it makes it hard to
organize small molecules into larger ones)
 Closed membrane vesicles form spontaneously from
lipids and proteins
 The chemical composition inside of these vesicles can
be different from their surroundings
 Allows for an internal cellular metabolism

FORMATION OF EARTH
Scientific evidence a cloud of dust particles
surrounded the Sun to form Earth about 4.5
billion years ago
 Early Earth had an atmosphere that probably
contained hydrogen, water vapour, methane,
ammonia, nitrogen, and hydrogen sulfide (called
a reducing atmosphere as there was no molecular
oxygen - O2).
 Biological monomers must have formed from
chemical reactions between these compounds in
shallow waters of the oceans (called “primordial
soup” or “primeval soup” or “chemical soup”)

SOLUTIONS TO PROBLEM 1 & 2

Possible mechanisms for the formation of organic
compounds
Abiogenesis
 Panspermia


Possible Locations with conditions conducive to
abiogenesis and polymerization:
Volcanoes
 Deep sea hot water vents
 Wet/dry conditions
 Mars

MILLER AND UREY

Conducted experiments in 1953 which simulated
the conditions of early Earth.
MILLER AND UREY EXPERIMENTS
Tried to recreate the primordial soup in a glass
sphere.
 Used H2O, NH3, H2, and CH4 in a sealed loop of
glass tubes and flasks.
 They kept the system at a warm temperature
and exposed the apparatus to UV light
 This heat evaporated the water, that was then
allowed to cool and condense (the water cycle)
 Generated electric sparks to simulate lightning


After one week:
15% of the carbon was in the form of organic
compounds
 13 of the naturally occurring amino acids were
detected
 Sugars were formed
 Adenine (nitrogenous base) had formed


Problems with abiogenesis:
Miller-Urey experiment showed abiogenesis
(creating organic matter from inorganic matter)
 A limitation to the Miller-Urey primordial soup
theory is that it is difficult to explain how amino
acids and nucleotides polymerised in an aqueous
environment (which would have promoted hydrolysis)

COULD COMETS HAVE BROUGHT
ORGANIC COMPOUNDS TO
EARTH?
COMETS MAY HAVE DELIVERED THE
GOODS

Panspermia: the
hypothesis that life on
Earth may have
originated by the
introduction of
organic chemicals or
even bacteria from
comets
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Comet: small body of rock, dust, and ice that orbits the
Sun
Meterorite: a solid piece of debris, from such sources as
asteroids or comets, that originates in outer space and
survives its impact with the Earth's surface.
Geological records show that our planet was
bombarded by a shower of comets and asteroids about
4 billion years ago (Late Heavy Bombardment)
Organic molecules hitchhiking on comets could survive
the impact and the impact could help to polymerize
certain amino acids into polypeptides)
COULD LIFE EXIST ON A COMET
IN THE EXTREME CONDITIONS
IN SPACE?
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Some bacteria and archaebacteria can survive in
extreme environments (Bacterial endospores
found in ice cores in Antarctica)
Cosmic radiation could provide the energy to
form complex organic molecules
By studying spectral lines of distant clouds of
cosmic dust particles, astronomers claim to have
revealed the presence of glycine, which is the
simplest amino acids. This suggest organic
molecules can form in space
POSSIBLE LOCATIONS FOR THE ORIGIN OF
LIFE

Specific locations must have existed where
polymerisation would have been promoted.

Alternating wet/dry, Deep oceans, volcanoes, mars
ALTERNATING WET DRY CONDITIONS
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On earth at a seashore or the flood plains of a
river where there is an alternation of wet –dry
conditions
The drying of clay particles could have created
catalyzing reactions and formed early organic
molecules
DEEP OCEANS
organic molecules could have formed around
hydrothermal vents, where hot water spews
from below the ocean floor
 formed when cracks in the crust of seabed expose
sea water to heat from magma below
 hot water rises and picks up minerals along the
way (looks like black smoke AKA black
smokers)
 Many communities of organisms currently live
around these vents and suggests life can be
supported there (chemosynthetic bacteria,
archaea, giant tube worms, clams etc.)

VOLCANOES
emit water vapour, gases and minerals which
could be used to form organic matter
 early Earth had many volcanoes, lightning and
was bombarded by UV rays
 the raw materials plus the conditions presented
by volcanoes could have favoured formation of
larger organic compounds

MARS AND EXTRATERRESTRIAL BODIES
alternate theory to abiogenesis is that life
formed elsewhere in space
 organic compounds are common in outer
solar system
 Earth was still too hot, Mars (smaller and
further) was cooler and allowed for
abiogenesis
 organic molecules could have been blasted
from Mars via asteroid or comet impacts
 Little direct evidence, however, meteorites
from Mars (possibly containing fossilized
bacteria) have been found in Antarctica

D1 ORIGINS OF LIFE
Part 2
SOLUTION 2
In the millions of years following the creation of
organic compounds in the primordial soup, these
compounds became more complex.
 Amino acids, monosaccharides, nucleic acids
would have undergone polymerization
 This process may have occurred in shallow rock
pools, particularly those where organic
compounds had accumulated by absorption on
the surface of clay particles
 When clay dries out and is heated, as many as
200 amino acids can spontaneously join together
in polypeptide chains.

PRECURSORS TO CELLS- SOLUTION 3
Coacervate Droplets
 microscopic sphere that forms from lipids in
water
 form spontaneously due to the hydrophobic
interactions between water and lipids
 can be selectively permeable
 not composed of phospholipids but may
incorporate proteins
 can grow and split
PRECURSORS TO CELLS-SOLUTION 3
Proteinoid Microspheres
 Related to coacervate droplets
 After amino acids are heated, mixed with hot
water, and then cooled, small protein globules are
formed
 catalytic properties
 can undergo simple division
PRECURSORS TO CELLS-SOLUTION 3
Although they are not living organisms,
coacervates and proteinoid microspheres are a
significant step toward the formation of cells.
 They solve the problem of protecting polymers
from their destructive environments.
 Could be primitive versions of the first cell
membranes

PRECURSORS TO CELLS-SOLUTION 3
Protobionts
 likely coacervate droplets or proteinoid
microspheres which surrounded polynucleotides
(i.e. RNA or DNA).
 have a membrane-like structure which can
surround organic molecules and maintain an
interval environment that is different than the
external one
 Overtime, true cell membranes evolved and other
characteristics of cells developed like;
Cellular respiration
 Asexual reproduction

PROPERTIES OF RNA- SOLUTION 4
Evolution by natural selection can only occur on
molecules that exhibit properties of variation and
heredity.
 So before life was created there needed to be a
molecule with variation and heredity – that
molecule was probably RNA

Variation – in a population of RNA molecules there
are a number of molecules with different base
sequences
 Heredity – the RNA molecule can produce copies (or
slightly modified copies) of itself
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
Eventually RNA would be replaced by DNA and
enzymes
RNA WORLD HYPOTHESIS-SOLUTION 4
RNA World Hypothesis: theory that RNA stored
genetic info and acted as a catalyst in a very
primitive self-replicating system
 Ribozymes

Small sequences of RNA that can act as enzymes
 Can be used to polymerize nucleotides and made to
cleave chemical bonds including peptide bonds
 Example: The ribosome – catalyses a peptide bond to
create proteins

HOW LIFE CHANGED OUR PLANET
20% of our atmosphere is now oxygen, but early
Earth had very little O2.
 Early prokaryotes therefore had to rely on other
forms of respiring

Methanogens- metabolize methane
 Others fed on other inorganic material like H2S to
obtain energy
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As bacteria reproduced, food became scarce
 It is believed that because of this food shortage,
bacteria that contained chlorophyll and could
photosynthesize (i.e. related to cyanobacteria)
would be selected for.

PHOTOSYNTHESIZING BACTERIA
photosynthesis is a significant event in history of
Earth
 Bacteria could now rely on the sun as the main
energy source, which produced O2 as by-product
 However, O2 is toxic to anaerobic bacteria and
killed many anaerobic bacteria
 Anaerobic bacteria that survived would live in
mud or places protected from the new oxygenrich atmosphere.
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The ability of an organism to make its own food
gives it a distinct advantage over those that
cannot.
As a result, photosynthetic bacteria proliferated
and produced more and more oxygen
FROM PROKARYOTES TO EUKARYOTES
Prokaryotes are relatively simple compared to
eukaryotes, but many chemical pathways (e.g.
Glycolysis) are similar between the two types of
cells.
 3.8-2 bya, bacteria (prokaryotic cells) were the
only organisms on Earth
 The first fossils of cells with a nucleus
(eukaryotes) is from around 2 bya.
 How did prokaryotes develop into eukaryotes?
 Endosymbiosis is the most popular theory of how
eukaryotic cells formed

ENDOSYMBIOTIC THEORY
Endo = into
 Symbiosis = interaction between two different
organisms living in close physical association,
typically to the advantage of both.

Suggests that chloroplasts and mitochondria (and
possibly even the nucleus) are derived from free
living prokaryotes being engulfed by larger
prokaryotes.
 These smaller prokaryotes survived in the
cytoplasm and eventually evolved into the
organelles.

ENDOSYMBYOSIS
The host cell would provide protection for the
smaller prokaryotic cell
 The engulfed cell would be beneficial to the host
if it was photosynthetic (providing food) for the
host or able to metabolize food efficiently and
produce energy for the host.
 Rather than being digested, the prokaryotes were
kept alive inside the host cell in exchange for
their services
 Explains how membrane bound organelles such
as chloroplasts and mitochondria became part of
eukaryotic cells.

EVIDENCE FOR ENDOSYMBYOSIS
DNA
 both contain own DNA that is different from
nuclear DNA but similar to bacterial DNA (i.e.
circular)
Double membrane
 both are surrounded by two membranes
 inner membrane could belong to original engulfed
prokaryote, outer one could be the vesicle in
which it was engulfed
EVIDENCE FOR ENDOSYMBYOSIS
Replication
 both are self-replicating (do so separately from
nucleus)
 both divide similarly to bacteria (i.e. binary
fission)
Ribosomes
 mitochondrial and chloroplast ribosomes are
similar to prokaryotic ribosomes (i.e. 70S vs 80S
in eukaryotes
EVIDENCE FOR ENDOSYMBYOSIS
Internal structure and chemistry
 chloroplast and mitochondria chemistry and
structure very similar to bacteria
Size
 chloroplasts, mitochondria and bacteria all
similar in size (1-10 m)
PROBLEMS WITH ENDOSYMBIOSIS
Inheritance: The ability to engulf another cell
and have it survive in the cytoplasm does not
guarantee that the host cell can pass it on to its
offspring the genetic code to synthesize the newly
acquired organelle
 Gene transfer: The genes for making
mitochondrial and chloroplast proteins have been
transferred to and are controlled by nucleus
 Independence: When chloroplasts or
mitochondria are removed from a cell, they
cannot survive on their own.
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