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Origin of Life
WHEN Did Life Evolve?
• Earth was molten during Hadean
• cooled somewhat by 3.9 billion years ago
• water could remain liquid at the surface
Origin of the Oceans
• outgassing and photochemical dissociaton both produce abundant water vapor
• water vapor condenses and falls as rain
• oceans accumulated quickly at first
• decreased in growth as volcanic activity
slowed
• salts added over 3.9 billion years
as material eroded from continents
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oxygen breaks down organic molecules
life must have evolved before oxygen abundant
Early Archean sediment dark-colored = unoxidized
oxidized (rusted) sediment appears about 3.5 billion
years ago
• life must have evolved before 3.5 bya
WHEN Did Life Evolve?
• oldest (possible) life-forms
~3.5 billion years old
• life likely evolved between 3.9 and 3.5
billion years ago
Monomers
• building blocks of life
• must form for life to
evolve
• how did monomers first
form?
Amino Acids
Monomer Synthesis
• Miller and Urey - created “Early Earth
Apparatus”
• early Earth atmosphere in the top
• electrodes to produce “lightning”
• primordial pond in the bottom
Results:
• after a week, primordial pond became
primordial soup
• 12 of 20 most common amino acids
synthesized + other stuff
• next step: polymerization
Polymers
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•
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chains of molecules
proteins
carbohydrates
lipids
nucleic acids
Requirements for
Polymerization
• energy source: to drive reactions
• protection: from too much energy
• concentration: to bring materials together so
they can react together
• catalysts: to make reactions happen faster
and more efficiently
Energy Source
Energy induces chemical reactions
• volcanoes
• lightening
• cosmic rays
• UV radiation
Protection
• too much energy can be a bad thing!
• early organic material protected by: rock
ledges, under ice, under thin film of
sediment, just under surface of water
Concentration
• when chemicals are dispersed in water,
reactions are less likely to happen
• concentration brings reactants together
• evaporation
• freezing
• scums
• droplets, bubbles
• clay
Concentration - clay
• clay forms from sedimentary
particles called platelets
• platelets are:
– very small
– flat
– with negative charge
on
surface
Clay
• organic molecules are attracted to clay
surface
• concentrate and align
• Examples: bentonite (kitty litter, mud
masks), kaolinite (Kaopectate)
Catalysts
• decrease amount of energy needed for
chemical reactions
• so increase the rate of chemical reactions
• catalysts in the body = enzymes
• Inorganic catalysts often metal ions
Making Cells
• what is required to make a cell?
• DNA: passes on genetic code
• cell membrane:
made of phospholipids
Phospholipids
• each molecule has a hydrophobic
end
and a hydrophilic end
• when surrounded by water, fatty acids
form shapes where the hydrophobic ends
protected on the inside
Protobionts
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•
•
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Phospholipid spheres that form naturally
polymers and enzymes concentrated inside
reactions occur inside
protobionts:
– maintain their structure
– increase in size over time
– divide when too large
– selectively absorb and release compounds
– metabolize starch
– store and release energy
Are Protobionts Alive?
• No
• they can’t replicate themselves
Replication
• process by which organisms make
genetic copies of themselves
• asexual reproduction
• sexual reproduction
Origin of Heredity
• many different types of protobionts
• those best able to accumulate organic
molecules, grow, and divide become most
common
• but “competition” is useless unless traits can
be passed on/inherited
• polymers that can replicate themselves:
DNA and RNA
Origin of Heredity
• short strands (50 nucleotides) of RNA
assemble naturally
• replicate themselves if more monomers
available
• zinc, copper act as catalysts
WHERE Did Life First Evolve?
Desirable features:
– energy source
– protection
– chance for concentration
– warm (chemical reactions work faster)
– wet (life requires liquid water)
– good supply of chemicals/nutrients
energy source?
protection?
concentration?
nutrients?
warm?
wet?
Archaebacteria
sun, earth
water
small pools
minerals
hot
you betcha
Hot Springs
Mud Flats and Tide Pools
energy source? sun
protection?
water, mud
concentration? Clay, evaporation
nutrients?
fewer
warm?
can be
wet?
yes
alternate between
wet and dry
Deep Ocean
• boiling water from vents
through crust
• very stable environment
• very protected
• lots of clay for
concentration
• lots of minerals
• warm -> hot
• Archaebacteria
Aliens?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• some meteorites up to 6% organic
molecules
• comets up to 20% organic sludge
• Halley’s Comet contains: C, O, H, N, CO2,
ice, nucleic acid bases, amino acids and
formaldehyde
• 74 amino acids identified so far from
meteorites
Did Life as we Know it Arrive
from Space?
• it’s possible
• small, simple organisms in a dormant state
may be able to survive a ride from another
planet in space
What we do know is • up to 15% of organic material in early
oceans may have come from space
Earliest Life Forms
Likely:
• prokaryotic
• Obligate anaerobes
• chemoheterotrophs:
– use organic molecules for energy
– fermentation
3.4 byo, South Africa
• source of organic molecules primordial soup
modern
The First Energy Crunch
• organic molecules become depleted
• competitive advantage goes to - Autotrophs
• Cyclic psyn first autotrophs – did not make
•
O2 or sugars
• Then later Non-cyclic psyn….2 types
Anaerobic Non-cyclic
Photosynthesis
• light-absorbing pigments (like chlorophyll)
probably already present
• light provides energy to remove e- from H2S
• S released as a by-product
energy from sunlight
6CO2
carbon
dioxide
+
6H2S
hydrogen
sulfide
C6H12O6 + 6S
sugar
sulphur
Aerobic Non-cyclic
Photosynthesis
• some cyanobacteria evolved to use H2O instead of
H2S as e- source
• O2 released as a by-product
• problem: O2 breaks bonds of organic molecules
•
(i.e. - it’s toxic)
energy from sunlight
6CO2
carbon
dioxide
+
6H2O
water
C6H12O6 + 3O2
sugar
oxygen
The Oxygen Revolution
(Pollution)
Indirect (non-fossil) evidence
for the presence of
cyanobacteria ~ 3.5 bya
Oxygen
• the modern atmosphere contains about
21% oxygen
• where did the oxygen come from?
• Oxygen Revolution
• Cyanobacteria - the first major global
polluters
Oxygen Crisis
• eventually, oxygen built up in oceans
• then outgassed from oceans into the
atmosphere
• corrosive (toxic) atmosphere led to first
major mass extinction
• anaerobic bacteria restricted to refuges
without abundant oxygen (stagnant water,
deep soils, etc.)
Aerobic Bacteria
• some bacteria evolved antioxidant
mechanisms
• allowed those bacteria to tolerate rising O2
levels
• some bacteria even evolved to use O2
How Far Back Does the Fossil
Record Extend?
3.5 byo - Australia
Warrawoona Group, Australia
Maybe?
Modern cyanobacteria
for comparison
Likely
3.1 - 3.2 byo - South Africa
Fig Tree Group
Earliest (Undisputed)
Evidence of Life Stromatolites
2.2 byo Michigan
Stromatolites
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dome-shaped, layered structures
Up to 3.5 byo
became very abundant by 2.2 bya
consist of layers of bacteria
upper layers aerobic, photosynthetic
lower layers anaerobic
produce abundant oxygen
how do we know?
They are still alive today in special
environments, notably Shark Bay, Australia
Tide In
Tide Out
Formation of Stromatolites
Cyanobacteria form a mat
on top of sediment
1 cm
A new layer of sediment
is deposited on top
Bacteria grow up
through new layer
Stromatolites provide
evidence for the
occurrence of
cyanobacteria in the
fossil record.
Modern
Ancient
• if we use stromatolites to infer
presence of cyanobacteria
• we might expect to find fossils
bacteria-like organisms in them...
the
of
And we often find them . . .
Gunflint Chert (~2.0 billion years old), Canada
O2 in
Atmos.
O2
added
The rise of cyanobacteria and the building
up of oxygen in the Earth’s atmosphere had
three significant effects:
1. Aerobic photoautotrophs became producers that
fuel the food chains of surface world
2. Earth experienced the first mass extinction
3. an oxygen-rich atmosphere set the stage for the
appearance of complex life (aerobic respiration)
Eukaryotic Cells
• First well established fossils 1.7 bya
• Mid-Proterozoic eon
• Early protists
•
unicellular
•
with nucleus
•
•
100x larger
than bacteria
•
10 – 100 μm
Origin of Nucleus & E. R.
• In-folding of cell
membrane
• Similar to
bacterial psyn
membrane
• Surrounded DNA
• Formed ER
Origin of Chloroplasts &
Mitochondria
• Endosymbiotic Theory – symbiotic bacteria
became dependent on host eukaryote cell
• Evidence:
• 1) separate DNA
• 2) bacteria-like ribosomes
• 3) inner membrane = bacterial membrane
• 4) replicate independently