1. Raritan Valley Community College
Section 22.1
4 Overlapping Stages
LECTURE
SLIDES
Prepared by
Aditya Aiyer
Raritan Valley Community College
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Section 22.1 Overview Origin of Life Stage 1*
Reduced Atmosphere Hypothesis
Extraterrestrial Hypothesis
Deep Sea Vent Hypothesis
Stage 2*
Stage 3*
Stage 4*

3. Origin of Life The universe created 13.7 billion years ago
Our solar system created 4.6 billion years ago
The Earth formed 4.55 billion years ago
50 million years
Earth cooled enough for outer layers to solidify and oceans to form
Life begins to emerge

4. Stage 1: Origin of Organic Molecules
There were specific conditions on early Earth may conducive to prebiotic formation of organic molecules
In prebiotic/abiotic synthesis, methane, carbon dioxide, water vapor, and hydrogen gases formed where no oxygen oxidized these gases or living organisms metabolized these gas
Scientists referred to this accumulation as a “prebiotic soup”

5. Stage 1 cntd
Scientists formulated hypotheses on where and how organic molecules first formed
Widely debated hypotheses include the Reducing Atmosphere, Extraterrestrial, and Deep Sea Vent Hypotheses

6. Reducing atmosphere hypothesis
Reducing atmosphere rich in water vapor, H2, CH4, and NH3 gases
Ammonia and Methane gases reduced other molecules
Oxygen cannot oxidize gases and no living organisms can metabolize molecules
Formed organic monomers essential to macromolecule formation: amino acids, sugars and nitrogenous bases
First attempt to apply scientific experiments to understand origin of life
Since 1950s, about early Earth atmosphere changed where prebiotic synthesis could be replicated under neutral conditions

7. Electrical discharge Electrodes Gases H2O To vacuum H2 CH4 Cold water
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Electrical discharge
Electrodes
Gases
H2O
To vacuum H2
CH4
Cold water
NH3
Condenser
Precipitating
droplets
Boiling water
Trap
Sample containing
organic molecules
such as amino acids

8. Extraterrestrial hypothesis
Asteroids and comets brought organic carbon compounds to Earth
Includes amino acids and nucleic acid nitrogen bases
Some scientists argue that most of organic content would be disintegrated in intense heating and collision against atmosphere
Deep-sea vent hypothesis
Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water
Most tested and accepted among the three hypotheses

9. b: © Woods Hole Oceanographic Institution/Visuals Unlimited
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cold H2O
Hot H2S gas
Hot
H2O
Vent
H2O temperature
suitable for organic
chemistry
Ocean floor
Crack in
Earth’s crust
(a) Deep-sea vent hypothesis
(b) A deep-sea vent community
b: © Woods Hole Oceanographic Institution/Visuals Unlimited

10. Stage 2: Complex Organic Molecule Synthesis
Experimentally, prebiotic synthesis of polymers is not possible in aqueous solutions
Hydrolysis competes with polymerization
Certain experiments have shown formation of complex nucleic acids and proteins on a solid surface, particularly clay

11. Stage 3: Formation of boundaries
Protobiont
Collection of prebiotically synthesized organic macromolecules surrounded by a boundary, such as a lipid bilayer or membrane, that allowed it to have chemical properties different from that of its surrounding environment

12. 4 defining characteristics of protobionts
- Inner contents of protobiont and environment separated by a boundary like membrane
- Nucleic acid polymers like RNA inside protobionts store information
- Nucleic acid polymers like RNA had enzymatic functions within protobionts
- Nucleic polymers like RNA allow protobionts the capability of self-replication

13. Living cells may have evolved from Two Different Protobiont Structures
Coacervates
Clusters of charged polymers
Inner enzymes in coarcervates could perform rudimentary metabolism
Liposomes
Vesicles surrounded by a lipid bilayer
Clay, or solid surface, can catalyze formation of liposomes that grow and divide
Liposomes can enclose RNA, as RNA is considered to be inner contents of protobionts

14. Solid droplet of protein and carbohydrate
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Skin of water
(a) Coacervates
57 µm
Solid droplet of protein
and carbohydrate
Hollow sphere
of phospholipid filled with water
Phospholipid bilayer
(b) Liposomes
200 nm
a: © A. I. Oparin. From the Origin of Life, New York: Dover, 1952; b: © Mary Kraft

15. Stage 4: RNA was Inner Content of Protobionts
Most scientists believe RNA to be the first macromolecule contained in protobionts
RNA has 3 major functions
- Ability to store information
- Capacity for self-replication
- Enzymatic function as ribozymes
DNA and proteins do not share all 3 of these important functions

16. Chemical selection
Original RNA population gradually changes over time by mutations, ultimately producing a new mutant RNA population with a different chemical composition
One example comprises of 2 mutations
- One mutation caused first mutant RNA population to acquire enzymatic function to attach nucleotides, thereby increasing rate of replication
- Another mutation caused new mutant RNA population to acquire enzymatic function to create nucleotides, which does not require prebiotic synthesis of nucleotides