Chapter 34. Common Origins of Life
Recommended Article : 【Biology】 Table of Contents for Biology
1. Hypotheses on the Origin of Life
2. Emergence of Primitive Organisms
3. Emergence of Prokaryotes and the Oxygen Revolution
1. Hypotheses on the Origin of Life: The realm of biochemists or physicists
⑴ Hypotheses on the Origin of Life
① Spontaneous Generation Theory: A theory that life arises spontaneously from inanimate matter (rejected)
○ Inanimate matter + vitality (life force) → living organism
○ Proposed by ancient Greeks, including Aristotle
② Biogenesis Theory: Life originates from pre-existing life → What is the origin of the first life? (Need supplementation)
○ Evidence 1: Spallanzani’s experiment
○ Evidence 2: Pasteur’s experiment (1862) : Using S-shaped flasks
③ Abiogenesis (= Chemical Evolution)
⑵ Hypotheses on the Origin of Life
① Theory of Celestial Origin : Bacteria arrived on Earth via meteorites (what about other planets?) (rejected)
○ Counterexample: Amino acids in meteorites are a mixture of L-form and D-form, while living organisms use only L-form
② Creation Theory: Life originated as described in the Bible (potential for refutation ↓) (rejected)
③ Chemical Evolution Theory: Initial life emerged from chemical substances
⑶ Hypothesis on the Origin of Life from Inorganic Matter (Oparin, A. I; 1924) : Also known as the Coacervate-Oparin hypothesis
① Synthesis of the first organic molecules in Earth’s inorganic matter
② Formation of polymers from organic monomers
③ Polymers initiate self-replication
④ Polymers with similar chemical properties aggregate, leading to the birth of the first life
2. Emergence of Primitive Organisms
⑴ Primitive Earth’s Environment
① Strongly reducing atmosphere
② Formation of oceans : Cooling of Earth led to condensation of H2O
③ Lightning, volcanic activity, and intense ultraviolet radiation
⑵ Formation of Early Organic Compounds
① Miller’s experiment : Demonstrated the possibility of natural formation of organic compounds through inorganic processes in a primitive Earth rich in reducing gases
○ Reducing gases : CH4, NH3, H2O, H2. Oxygen was scarce
○ Inorganic processes : High temperature, discharges
○ 1953 : Analysis of thin brown film on container wall and solution using paper chromatography
○ 2008 : Reanalysis of residue using improved equipment → Discovery of 20 amino acids present in living organisms
② Fox’s experiment
○ Dropping organic solution onto hot sand or mud results in the formation of polymers due to concentration increase of monomers
○ Protenoid : Polypeptides spontaneously formed from amino acids
○ Hypothesis: Nucleic acids, proteins, ATP, and other polymer organic compounds likely synthesized in the primordial ocean
⑶ RNA World
① Evidence of the First Genetic Material being RNA
○ Evidence 1: Spontaneous polymerization : RNA monomers can spontaneously form short RNA polymers
○ Evidence 2: RNA can self-replicate without protein enzymes (complementary RNA strand)
○ DNA replication is more complex and requires many enzymes compared to RNA replication
○ Evidence 3: RNA with catalytic function (ribozyme)
○ RNA’s primary structure: Chain
○ RNA’s secondary structure: Hairpin
○ RNA’s tertiary structure: Ribozyme (examples: snRNA, 23s rRNA, 28s rRNA, RNase P (RNA cleavage enzyme function), RNase E)
○ Catalytic function: Can form complex 3D structures, contains highly reactive OH group
○ Example 1: RNA splicing, RNA polymerization reaction
○ Example 2: rRNA can catalyze peptide bond formation reaction
○ Example 3: Primary transcript of rRNA can self-remove introns without enzyme help
○ Example 4: Using small RNA molecules, cutting and joining E. coli tRNA primary transcripts is possible
○ Evidence 4: Reverse transcriptase
② RNA World : Mixture of RNA monomers accidentally forms the first gene, leading to continuous formation of complementary strands
③ Coevolution of RNA and Proteins : Proteins generated by RNA provide protection and assistance to RNA
④ DNA-RNA system : Evolution of new enzymes that create DNA and RNA copies from DNA
⑷ RNA Substitution
① RNA is inherently unstable
② After the RNA world, DNA begins to form, coexisting with RNA, and synthesis shifts from RNA to DNA
⑸ Pre-life Organisms : Very difficult to implement in reality
① Coacervate: Complex organic molecules in the primitive ocean form small enclosed liquid droplets
② Microspheres: Protenoids discovered by Fox, formed when organic monomers on hot sand or mud cool down and create small liquid droplets
③ Characteristics of Pre-life Organisms
○ Selective absorption of substances
○ Coacervates or microspheres evolved further into the first primitive life forms
⑹ First Life Form
① Genetic material is enclosed in a membrane, ensuring stability through confinement
② Birth of the first life form with energy mechanisms to overcome (negative entropy)
3. Emergence of Prokaryotes and the Oxygen Revolution
⑴ First Life Form: Anaerobic nutrition-dependent organisms
⑵ Early Organisms’ Genetic Strategies
① Strategy 1: Increased utilization of genes: Almost no meaningless genetic sequences
② Strategy 2: Elimination of unnecessary repetitive sequences: Repetitive sequences are necessary for sophisticated gene expression
③ Genetic Strategies of Later Organisms
○ Tissue-specific enzymes abound
○ Decreased gene utilization
○ Increase in pseudogenes
⑶ Emergence of Simple Autotrophic Cells
① Birth of organisms that synthesize organic substances from CO2 using light as energy source due to energy depletion in the ocean
② First autotrophic cells likely used hydrogen sulfide as an electron donor with a simple photosystem
③ Examples: Purple sulfur bacteria, green sulfur bacteria, etc.
⑷ Appearance of Cyanobacteria (Blue-Green Algae)
① Emergence of autotrophic cells capable of receiving electrons from water
○ Fossil of ancient cyanobacteria: Stromatolites
② Cyanobacteria thrive and the produced oxygen changes the reducing atmosphere to an oxidizing atmosphere, leading to the extinction of anaerobic organisms
○ Active oxygen: Anaerobic organisms lacking means to remove oxygen are vulnerable to attack by reactive oxygen
○ Formation of peroxisomes: Hypothesis that peroxisomes were a type of bacteria
③ Oxygen Revolution
○ About 2 billion years ago, cyanobacteria gained dominance in competition with anaerobic organisms
○ Oxygen Revolution: Cyanobacteria became dominant, resulting in a sudden increase in atmospheric oxygen
○ Reduction atmosphere turned into an oxidizing atmosphere
④ After the Oxygen Revolution
○ Oxygen-sensitive organisms go extinct: Oxygen damages proteins and nucleic acids as a strong oxidizing agent
○ Ozone formation from oxygen → Ozone layer formation
○ Birth of aerobic organisms utilizing oxygen: More ATP can be generated from the same amount of glucose
⑸ Emergence of Terrestrial Organisms: Ozone layer blocks ultraviolet radiation, allowing aquatic organisms to move to land
4. Origin of Eukaryotic Cells
⑴ First Prokaryotic Cell (Approximately 3.5 billion years ago) → First Eukaryotic Cell (Approximately 2 billion years ago)
⑵ Oxygen Revolution → Formation of Radicals
① Radicals → Destruction of biomolecules → Cells grow larger to develop mechanisms to protect biomolecules
② Cell size increases → Decreased surface area-to-volume ratio → Decreased efficiency in material exchange → Folding of cell membrane needed to increase surface area
⑶ Endosymbiotic Theory (Membrane Infolding Theory)
① Plasma membrane folds inward to form intracellular compartments
② Sequence of organelle formation : Plasma membrane infolding → Formation of nuclear membrane and vesicles → Formation of vesicles from Golgi apparatus → Formation of lysosomes from Golgi apparatus
⑷ Endosymbiotic Theory: Proposed by Lynn Margulis
① 1st Step: Plasma membrane of anaerobic archaea folds inward into cytoplasm (plasma membrane infolding)
○ Examples of anaerobic archaea: Proteobacteria
② 2nd Step: Nuclear membrane, vesicles, and Golgi apparatus become independent from the outer membrane
③ 3rd Step: Ancestor eukaryote engulfs a heterotrophic bacterium capable of anaerobic respiration but doesn’t kill it (1st intracellular symbiosis)
○ Ancestor eukaryote was vulnerable to attacks from reactive oxygen due to its anaerobic nature; surviving with an anaerobic respiration bacterium likely offered an advantage
④ 4th Step: Anaerobic respiration bacterium survives inside eukaryote, and both evolve dependently (Mitochondria formation; generation of animal cells)
⑤ 5th Step: Ancestor of algae and terrestrial plants engulfs photosynthetic prokaryote (cyanobacteria), but doesn’t kill it (2nd intracellular symbiosis)
⑥ 6th Step: Cells evolve interdependently (Chloroplast formation; generation of plant cells)
⑦ 7th Step: Repeated endosymbioses lead to the evolution of different groups of algae
⑸ Evidence for Endosymbiotic Theory: Observed in Mitochondria and Chloroplasts
① Most crucial evidence: Double membrane structure
② Circular DNA, 70S ribosomes, bacterial promoters and RNA pol, fMet-tRNA : Antibiotic sensitivity
○ Nuclear DNA in eukaryotes is linear
○ Eukaryotic cells have 80S ribosomes
○ Circular DNA in chloroplasts encodes r-proteins (large subunit DNA is inserted into nuclear DNA)
③ Functional homology of membranes: Electron transport chain and chemical composition of outer membrane
○ Cardiolipin in inner membranes
○ Facultative bacteria have electron transport chain in plasma membrane; mitochondria have electron transport chain in inner membrane
⑹ Emergence of Multicellular Eukaryotes: Adaptation to various environments due to more functions than single-celled organisms
Input: 2015.07.09 14:56