7.2
Origins of Life on Earth
1. Geological Evidence for the Origin of Life
Earth's Formation:
Earth formed approximately 4.6 billion years ago (bya).
The environment was too hostile for life until 3.9 bya.
Earliest fossil evidence for life dates to 3.5 bya.
This evidence provides a plausible range of dates for the origin of life.
2. Models of the Origin of Life
Several models explain how life originated on Earth:
Primitive Earth Model:
Provided inorganic precursors from which organic molecules could be synthesized.
Factors included:
Presence of available free energy.
Absence of significant atmospheric oxygen (O2).
Extraterrestrial Transport Model:
Organic molecules may have been delivered to Earth by meteorites or other celestial events.
Chemical Experiments and Evidence:
Experiments show it is possible to form complex organic molecules from inorganic ones in the absence of life.
Organic molecules (monomers) serve as building blocks for:
Formation of amino acids and nucleotides.
Joining of monomers produces polymers capable of replicating, storing, and transferring information.
3. The RNA World Hypothesis
Proposes that RNA was the earliest genetic material, supporting the evolution of life.
4. Key Understandings in Cell Biology
Cell Division:
Cells can only form from pre-existing cells.
First cells arose from non-living materials.
Endosymbiotic Theory:
Explains the origin of eukaryotic cells.
Genetic Code:
The 64 codons have the same meanings nearly universally across organisms, with minor variations hinting at the common origin.
5. Assessment Statements and Core Material
5.1 Processes Needed for the Spontaneous Origin of Life
Four Essential Processes:
Non-living synthesis of simple organic molecules.
Assembly of these molecules into polymers.
Origin of self-replicating molecules making inheritance possible.
Packaging of these molecules into membranes with an internal chemistry different from their surroundings.
5.2 Miller-Urey Experiments
Purpose:
Investigate the origin of organic compounds under early Earth conditions.
Results:
They synthesized several organic molecules, including amino acids, supporting the hypothesis of the chemical origins of life.
5.3 Cometary Delivery of Organic Compounds
Impact of Comets:
Comets may have delivered organic compounds and water to early Earth, especially during heavy bombardment about 4,000 million years ago.
6. Possible Locations for Organic Compound Synthesis
Examples of environments conducive to organic synthesis include:
Deep-sea hydrothermal vents.
Volcanic locations.
Potential extraterrestrial sites.
7. RNA Properties for Life Origin
Two Critical Properties:
Self-replicating ability.
Catalytic activity, enabling early biochemical reactions.
8. Protobionts and Early Cellular Life
Protobionts:
Preceded living cells with differing internal chemical environments (e.g., coacervates, microspheres).
They possess the potential for simple reproduction and metabolism.
9. The Role of Ribozymes
Ribozymes:
RNA molecules capable of catalyzing reactions and self-splicing.
Support the RNA World Hypothesis regarding the origin of life.
10. Panspermia Hypothesis
Suggests life could have originated from organic compounds transported by comets or meteorites (e.g., Murchison meteorite).
Recent findings indicate that components of nucleic acids have been discovered in meteorites.
11. Alternative Hypotheses
TNA World Hypothesis:
Proposes the existence of TNA (Threose Nucleic Acid), which may have played a key role alongside RNA and DNA.
TNA distinguished by its sugar backbone and potential advantages in molecular formation.
12. Summary and Conclusion
Life's origin involves complex reactions that potentially began with simple organic molecules, evolving through various processes leading to self-replicating systems and ultimately cellular structures.
Research continually uncovers new insights into the origins of life, reinforcing and refining existing models.