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
⑴ Hypotheses about the origin of life
① Spontaneous generation: the doctrine that life arises on its own from non-living matter (rejected)
○ Non-living matter + “vital force” → living organisms
○ Advocated by Aristotle and other ancient Greeks.
② Biogenesis: life arises from pre-existing life → then what about the very first life (the origin of life)? (needs elaboration)
○ Evidence 1. Spallanzani’s experiment
○ Evidence 2. Pasteur’s experiment (1862): used an S-shaped “swan-neck” flask
③ Neo-spontaneous generation (= chemical evolution)
⑵ Hypotheses about the origin of life
① Panspermia: bacteria arrived on Earth carried by meteorites (then what about other planets?) (rejected)
○ Counterexample: amino acids in meteorites occur as a mixture of L- and D-forms, whereas living organisms use only L-forms.
② Creationism: life originated as conveyed in the Bible (low falsifiability) (rejected)
③ Chemical evolution: the first life arose from chemical substances.
⑶ Hypothesis of life’s origin from inorganic matter (Oparin, A. I.; 1924), also called the Haldane–Oparin hypothesis
① The first organic molecules were synthesized from Earth’s inorganic materials.
② Polymers were formed from organic monomers.
③ Some polymers began self-replication.
④ As these polymers aggregated with other polymers according to their chemical properties, the first life emerged.
2. Emergence of Primitive Organisms
⑴ Environment of the primitive Earth
① Strongly reducing atmosphere
② Formation of the oceans: as the Earth cooled, H2O condensed.
③ Lightning, volcanic activity, and ultraviolet radiation were far more intense.
⑵ Formation of the first organic compounds
① Miller’s experiment: demonstrated the possibility that, on a prebiotic Earth rich in reducing gases, organic molecules could arise abiotically.
○ Reducing gases: CH4, NH3, H2O, H2; molecular oxygen was scarce.
○ Abiotic processes: high temperatures and electrical discharges.
○ 1953: analyzed the thin reddish-brown film on the inner wall of the vessel and the solution by paper chromatography.
○ 2008: reanalyzed the archived residues with improved instruments → found 20 amino acids used by living organisms.
② Fox’s experiments
○ When solutions of organic molecules are dripped onto hot sand or clay, some monomers polymerize as their concentration increases.
○ Proteinoids: polypeptides formed by the spontaneous polymerization of amino acids.
○ It is inferred that macromolecular organics such as nucleic acids, proteins, and ATP were synthesized in the primordial ocean.
⑶ RNA World: Proposed model as DNA has no autocatalytic function
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: Possible because RNA can form complex 3D structures, and 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 Replacement of RNA
① RNA is inherently unstable.
② After the RNA world, DNA began to be produced; following a period of coexistence with RNA, systems shifted toward RNA being synthesized from DNA.
⑸ Proto-life (protobionts): in practice, extremely difficult to realize.
① Coacervates: small membrane-encased liquid droplets formed when complex organics in the primordial ocean aggregated.
② Microspheres: small droplets observed by Fox, formed when proteinoids are placed in hot water and then slowly cooled.
③ Features of proto-life
○ Selective uptake of substances
○ It is inferred that coacervates or microspheres further evolved into the first primitive life forms.
⑹ The first living organisms
① Genetic material happened to become enclosed by a membranous structure, gaining stability.
② As energy mechanisms capable of counteracting entropy (i.e., supplying “negative entropy”) were acquired, the first life emerged.
3. Emergence of Prokaryotes and the Oxygen Revolution
⑴ Earliest life forms: heterotrophs that performed anaerobic respiration
⑵ Genetic strategies of early life
① Strategy 1. Increase gene utilization: little to no nonfunctional/noncoding sequence
② Strategy 2. Remove unnecessary repeats: only the repetitive sequences needed for fine-tuned gene expression were retained.
③ Genetic strategies of later life
○ Many tissue-specific enzymes
○ Lower gene utilization
○ Increase in repetitive sequences
⑶ Emergence of simple autotrophic cells
① As the oceans’ available energy was depleted, organisms arose that used light to synthesize organic matter from CO2.
② The earliest autotrophs are thought to have had simple photosystems and to have used hydrogen sulfide as the electron donor.
③ Examples: purple sulfur bacteria, green sulfur bacteria, etc.
⑷ Emergence of cyanobacteria
① Autotrophs capable of extracting electrons from water appeared.
○ Fossils of primitive cyanobacteria: stromatolites
② As cyanobacteria flourished, the oxygen they produced transformed a reducing atmosphere into an oxidizing one and caused anaerobic organisms to go extinct.
○ Reactive oxygen species (ROS): anaerobes lacking defenses against oxygen were vulnerable to highly reactive ROS.
○ Origin of peroxisomes: there is a hypothesis that peroxisomes were once bacteria.
③ Oxygen Revolution (Great Oxidation Event)
○ About 2 billion years ago, cyanobacteria gained an advantage over anaerobes.
○ “Oxygen Revolution”: as cyanobacteria became dominant, atmospheric oxygen rose abruptly.
○ Shift from a reducing to an oxidizing atmosphere.
④ After the Oxygen Revolution
○ Oxygen-sensitive organisms went extinct: oxygen is a powerful oxidant that damages proteins and nucleic acids.
○ Ozone (O3) formed from O2 → the ozone layer developed.
○ Emergence of organisms performing aerobic respiration using oxygen: more ATP could be generated from the same amount of glucose.
⑸ Appearance of terrestrial life: with the ozone layer blocking ultraviolet radiation, organisms living in the sea moved onto land.
4. Origin of Eukaryotic Cells
⑴ First prokaryotic cells (≈ 3.5 billion years ago) → first eukaryotic cells (≈ 2.0 billion years ago)
⑵ Oxygen Revolution → radical formation
① Radicals → destruction of biomolecules → to protect biomolecules, cells increased in size.
② Increase in cell size → decrease in surface area–to–volume ratio → reduced efficiency of material exchange → necessitated increasing surface area via folding of the cell membrane.
⑶ Membrane invagination hypothesis (membrane evolution hypothesis)
① The plasma membrane invaginated and layered inward, forming intracellular organelles.
② Order of organelle formation: membrane invagination → formation of the nuclear envelope and endoplasmic reticulum → Golgi apparatus formed from the ER → lysosomes formed from the Golgi.
⑷ Endosymbiotic theory (proposed by Lynn Margulis)
① 1st. The cell membrane of an anaerobic archaeon folded inward into the cytoplasm (membrane folding).
○ Example of an anaerobic archaeon: Proteobacteria.
② 2nd. The nuclear envelope, endoplasmic reticulum, and Golgi apparatus became independent of the outer membrane.
③ 3rd. Ancestral eukaryotes engulfed free-living aerobic bacteria but did not kill them (primary endosymbiosis).
○ Because the ancestral eukaryotes were anaerobic and were attacked by reactive oxygen species, they would have survived better by forming a symbiosis with aerobic bacteria that consumed oxygen.
④ 4th. The aerobic bacteria survived within the eukaryotes, and both evolved dependently on each other (formation of mitochondria; origin of animal cells).
⑤ 5th. In ancestors of algae and land plants, photosynthetic prokaryotes (cyanobacteria) were engulfed but not digested (secondary endosymbiosis).
⑥ 6th. The cells co-evolved dependently (formation of chloroplasts; origin of plant cells).
⑦ 7th. Multiple additional symbioses gave rise to different algal lineages.
⑸ Evidence for the endosymbiotic theory: observed in mitochondria and chloroplasts
① Most crucial evidence: double-membrane structure.
② Circular DNA; 70S ribosomes; bacterial promoters and RNA polymerase; fMet-tRNA → antibiotic sensitivity.
○ Eukaryotic nuclear DNA is linear.
○ Eukaryotic cells have 80S ribosomes.
○ Chloroplast circular DNA encodes the Rubisco small subunit (the large-subunit DNA is inserted into nuclear DNA).
③ Functional homology of membranes: electron transport chains and the chemical composition of the outer membrane.
○ Cardiolipin in the inner membrane.
○ Aerobic bacteria have electron transport chains in the plasma membrane, and mitochondria have electron transport chains in the inner membrane.
⑹ Emergence of multicellular eukaryotes
① Able to adapt to diverse environments because they can possess more functions than unicellular organisms.
Input: 2015.07.09 14:56