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Lecture 20
The origin of life
1. It must be capable of reproducing.
2. It must possess a genotype and a phenotype.
3. It must possess a metabolism.
4. It must be capable of evolving.
Some problems in addressing the origin of
life on earth
Some problems in addressing the origin of
life on earth
1. Narrow time window
Some problems in addressing the origin of
life on earth
1. Narrow time window
• the age of the earth is 4.55 billion years old.
Some problems in addressing the origin of
life on earth
1. Narrow time window
• the age of the earth is 4.55 billion years old.
• life first appeared about 3.8 billion years ago.
Some problems in addressing the origin of
life on earth
1. Narrow time window
• the age of the earth is 4.55 billion years old.
• life first appeared about 3.8 billion years ago.
• the planet could not have sustained life for its first 500
million years!
Some problems in addressing the origin of
life on earth
1. Narrow time window
• planet could not have sustained life for its first 500
million years!
Life on Earth - Timescale
Precambrian
(4,600 to 543 MYA) (MYA = million years ago)
-  life on earth evolved about 3,800 MYA.
-  prokaryotes evolved about 3,500 MYA
-  eukaryotes evolved about 2,100 MYA
- multicellular eukaryotes appeared about 1,500 MYA.
Life on Earth - Timescale
Theories for the evolution of life
Extraterrestrial origin of life
Panspermia
Svante August Arrhenius
(1907)
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
A.I. Oparin
1894 - 1980
J.B.S. Haldane
1892 - 1964
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
First macromolecules
Extraterrestrial origins
F. Crick
1916- 2004
Step 1. Inorganic molecules → organic
molecules
1. Extraterrestrial evidence
• the Murchison meteorite contained over 70 amino
acids!
Step 1. Inorganic molecules → organic
molecules
1. Extraterrestrial evidence
• the Murchison meteorite contained over 70 amino
acids!
• equal mixture of D and L isomers present.
First macromolecules
1953, classic experiment by Stanley Miller and Harold Urey
(Production of some organic compounds under possible primitive Earth
conditions)
1. gases representing the
primitive atmosphere
2. electrical discharge using
tungsten electrodes.
Results after a few days:
aldehydes, carboxylic acid and amino acids
First macromolecules
1953, classic experiment by Stanley Miller and Harold Urey
(Production of some organic compounds under possible primitive Earth
conditions)
1. gases representing the
primitive atmosphere
2. electrical discharge using
tungsten electrodes.
Results after a few days:
aldehydes, carboxylic acid and amino acids
S. Miller
1930- 2007
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
First polynucleotides
clay substrates
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
First polynucleotides
clay substrates
Ribozymes
Ribozyme from Tetrahymena thermophila
Evidence for an early role for RNA
Evidence for an early role for RNA
RNA is involved in:
Evidence for an early role for RNA
RNA is involved in:
1. DNA replication.
Evidence for an early role for RNA
RNA is involved in:
1. DNA replication.
2. Protein synthesis.
Evidence for an early role for RNA
RNA is involved in:
1. DNA replication.
2. Protein synthesis.
3. Ribonucleoside triphosphates (ATP, GTP) are the
energy currency of cells.
Evidence for an early role for RNA
RNA is involved in:
1. DNA replication.
2. Protein synthesis.
3. Ribonucleoside triphosphates (ATP, GTP) are the
energy currency of cells.
4. Deoxyribonucleotides are synthesized from RNA
precursors.
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
Step 4. Replicating systems → protobionts
Step 4. Replicating systems → protobionts
Replicating systems → protobionts
• heating and cooling mixtures of amino acids can form
spherical proteinoids.
Replicating systems → protobionts
• heating and cooling mixtures of amino acids can form
spherical proteinoids.
• mixtures of lipids and proteins can form liposomes.
Replicating systems → protobionts
• heating and cooling mixtures of amino acids can form
spherical proteinoids.
• mixtures of lipids and proteins can form liposomes.
• liposomes can “reproduce” by budding off smaller
units.
Replicating systems → protobionts
• heating and cooling mixtures of amino acids can form
spherical proteinoids.
• mixtures of lipids and proteins can form liposomes.
• liposomes can “reproduce” by budding off smaller
units.
• if an RNA-protein based metabolism evolved, natural
selection can again occur.
Theories for the evolution of life
The chemical theory (Oparin-Haldane theory)
inorganic molecules
↓
organic molecules
↓
biological polymers
↓
replicating systems
↓
protobionts
↓
true cells
Protobionts → true cells
Protobionts → true cells
• natural selection would have acted to biochemical
sophistication of protobionts.
Protobionts → true cells
• natural selection would have acted to biochemical
sophistication of protobionts.
• DNA became the “repository” of the genetic
information.
Protobionts → true cells
• natural selection would have acted to biochemical
sophistication of protobionts.
• DNA became the “repository” of the genetic
information.
Why?
Protobionts → true cells
• natural selection would have acted to biochemical
sophistication of protobionts.
• DNA became the “repository” of the genetic
information.
Why?
1. DNA is more stable than RNA.
Protobionts → true cells
• natural selection would have acted to biochemical
sophistication of protobionts.
• DNA became the “repository” of the genetic
information.
Why?
1. DNA is more stable than RNA.
2. RNA freed to act only in one arena (catalysis).
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
1. Stromatolites
Shark Bay, Western Australia
Stromatolites
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
1. Stromatolites
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
1. Stromatolites
• are bun-shaped structures made by cyanobacteria.
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
1. Stromatolites
• are bun-shaped structures made by cyanobacteria.
• fossil stromatolites are abundant 3.5 bya.
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
1. Stromatolites
• are bun-shaped structures made by cyanobacteria.
• fossil stromatolites are abundant 3.5 bya.
• recently “re-discovered” in shallow, hypersaline
environments (e.g., Hamelin pool in Shark Bay,
western Australia).
Stromatolites
Stromatolites are the oldest known fossils, more than 3 billion
years.
They are colonial structures formed by photosynthesizing
cyanobacteria (blue green algae) and other microbes.
Cyanobacteria were likely responsible for the creation of earth's
oxygen atmosphere.
They were the dominant lifeform on Earth for over 2 billion
years.
Today they are nearly extinct, living a precarious existence in
only a few localities worldwide.
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
2. C isotope ratios suggest early
photosynthesis
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
2. C isotope ratios suggest early
photosynthesis
• 12CO2 is preferentially fixed in photosynthesis than
the heavier 13CO2.
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
2. C isotope ratios suggest early
photosynthesis
• 12CO2 is preferentially fixed in photosynthesis than
the heavier 13CO2.
• isotopic ratios of 13C to 12C indicate that autotrophic
bacteria fixing carbon via the Calvin cycle by 3.5 bya.
What is the evidence for prokaryotic life
3.5 to 4.0 BYA?
2. C isotope ratios suggest early
photosynthesis
• 12CO2 is preferentially fixed in photosynthesis than
the heavier 13CO2.
• isotopic ratios of 13C to 12C indicate that autotrophic
bacteria fixing carbon via the Calvin cycle by 3.5 bya.
• where did the O2 go? Into the oceans to form
banded ironstone sediments.
The origin and early evolution of the
eukaryotes
The origin and early evolution of the
eukaryotes
1. The universal gene-exchange pool hypothesis
The origin and early evolution of the
eukaryotes
1. The universal gene-exchange pool hypothesis
• Archaea, Bacteria, and Eucarya evolved only after
extensive lateral gene transfer ceased.
The universal gene-exchange pool hypothesis
The origin and early evolution of the
eukaryotes
1. The universal gene-exchange pool hypothesis
• Archaea, Bacteria, and Eucarya evolved only after
extensive lateral gene transfer ceased.
2. The ring of life hypothesis
The origin and early evolution of the
eukaryotes
1. The universal gene-exchange pool hypothesis
• Archaea, Bacteria, and Eucarya evolved only after
extensive lateral gene transfer ceased.
2. The ring of life hypothesis
• Eucarya evolved from a fusion of a bacterium and an
archean.
The ring of life hypothesis
Endosymbiotic hypothesis
Lynn Margulis
1938-2011
Endosymbiotic hypothesis
Respiration - First use of Oxygen
Multicellular organisms