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The Origin of Life
(생명의 기원)
Chapter 24
검은 연기를 내뿜는 심해 열수구.
그림 24.1. 지구로부터 7,000광년 떨어진 독수리 성운. 삽입된
그림은 35억년 된 암석에서 찾은 원핵세포로 추정되는 구조.
그림 24.2. Cellular life(세포성 생물)의 진화에 대한 timeline.
24.1 The Formation of Molecules
Necessary for Life
 Conditions on primordial Earth led to the
formation of organic molecules
 Oparin-Haldane hypothesis initiated scientific
investigations into the origin of life
 Chemistry simulation experiments support the
Oparin-Haldane hypothesis
 Scientists have new theories about the sites for
the origin of life
Early Life
 Earth is 4.6 billion years old
• Protocells(원시세포) precede actual cells
• First cell fossils 3.5 billion years old
 Life came from nonlife, but not spontaneously
• Chemistry and physics same in living, nonliving
• Hypothesized extraterrestrial life origin still
originally from nonliving
Early Earth
 Earth’s early properties essential for life
• Gravity high enough to retain atmosphere
• Water in liquid form (distance from Sun)
• Organic molecules from natural energy sources
 Oparin-Haldane hypothesis
• Early earth had reducing atmosphere(수소, 메탄,
암모니아, 물)
• Allows complex organic molecules to form and
persist (prebiotic soup of organics)
그림 24.3. 생명을 유지하기에는 너무 뜨거운 초기 지구를 보여주는 그림.
Miller-Urey Experiments
 Organic molecules formed in reducing atmosphere
• All organic molecules for life formed experimentally
• Alternative hypothesis: Early earth atmosphere
oxidizing, no organic molecules formed
 Alternative hypotheses
• Hydrothermal vents[(심해)열수구], extraterrestrial
(지구대기권 밖) origins
• Still require liquid water
그림 24.4. 원시지구를 가상한 조건에서 유기분자가 자연적으로 합성될 수
있다는 것을 보여주는 밀러-유레이 실험장치. 일주일 가동했을 때, 탄소의
15%가 다양한 유기화합물로 전환되었음.
24.2 The Origin of Cells
 Protocells formed with some properties of life
 Living cells may have developed from protocells
 Prokaryotic cells were the first living cells
 Subsequent events increased the oxidizing
nature of the atmosphere
Early Macromolecules
 Organic molecules are not alive by themselves
• Macromolecules and aggregration needed
 Macromolecule formation by subunits
• Evaporation of water concentrates subunits
• Dehydration synthesis(탈수 합성; 축합) connects
subunits with H and OH removal
Organic Molecule Aggregates
 Clays(점토; 진흙) facilitate organic molecule
• Layered structure(층을 이룬 구조; 얇은 무기질 층)
absorbs molecules and facilitates interactions,
stores potential energy
• No lipid bilayer assembly
 Phospholipids(인지질)assemble into bilayers
(2중층) in water
• Formed spontaneously into vesicles
• Can incorporate proteins and make new vesicles
그림 24.5. 모의 원시 환경에서 합성 인지질로부터 만들어진 다양한 크기와
형태의 소낭들을 보여주는 현미경 사진. 확대해 보면 막이 지질 2중층으로
구성된 것을 볼 수 있음.
Living Cells from Protocells
 Cells need energy-harvesting pathways
• First reactions: Direct redox
• Later: Stepwise redox reactions, more efficient,
uses intermediate carriers
 Cells need information transfer
• RNA first hypothesis: Ribozymes then proteins
• Protein first hypothesis: Enzymes then DNA, RNA
Prokaryotic First Cells
 Approximately one billion years for development
and evolution of first prokaryotic cells
 Features required of first prokaryotes
• Membrane bound
• Nuclear region with DNA transcribed to RNA
• Cytoplasmic region with RNA translated to amino
• Cytoplasmic region for energy transformation
• DNA replication and reproduction
Oxidizing Atmosphere
 Earliest photosynthesis hypothesized to use H2S
• No oxygen exhaust
 H2O more abundant, favored by natural selection
• Oxygen exhaust
 Oxygenic photosynthesis increased oxygen in
• Oxygen allows better energy harvest from
• Stromatolites[남세균(녹조류) 화석을 포함한 층상
석회석] at least 3 billion years old
그림 24.6. 호주 서부 Shark Bay의 낮은 조수에 노출된 스트로마톨라이트.
남세균에 의한 광합성의 결과 산소가 대기에 축적되기 시작함.
24.3 Origins of Eukaryotic Cells
 Endosymbiont hypothesis: Mitochondria and
chloroplasts evolved from ingested prokaryotes
 Several lines of evidence support the
endosybiont hypothesis
 Eukaryotic cells may have evolved from a
common ancestral line shared with archaeans
24.3 (cont.)
 Multicellular eukaryotes probably evolved in
colonies of cells
 Life may have been the inevitable consequence
of the physical conditions of the primitive Earth
Endosymbiont Hypothesis (1)
 Eukaryotes developed from symbiotic
relationships between prokaryotes
 Nonphotosynthetic prokaryotes feed on organic
• Aerobes(호기성 생물) fully exploit stored energy
(저장 에너지 활용)
• Predatory anaerobes(포식성의 혐기성 생물) eat
aerobes to get extra energy
그림 24.7. 내부공생설.
Endosymbiont Hypothesis (2)
 Symbiosis between predatory anaerobe and
aerobe (when aerobe not consumed)
• Aerobe provides efficient energy transformation
• Anaerobe provides food supply
 Other evolutionary steps in eukaryotic formation
• Endocytosis(세포내섭취) (infolding of plasma
• Nuclear membrane and endomembrane system
그림 24.8. 세포내섭취에 의해 세포질로 이동된 원형질막이 핵막과
소포체로 형성되었을 것으로 추정.
Endosymbiont Hypothesis (3)
 Final steps in endosymbiont hypothesis
• Gene transfer from aerobe to anaerobe nucleus
• Host anaerobe and aerobe become completely
 Same steps for plastid (chloroplast)
endosymbiosis (plants and algae)
• Occurs after mitochondria evolution
• Cyanobacteria plastid origins
Endosymbiont Evidence
 Mitochondria and chloroplasts are structurally
and biochemically similar to prokaryotes
• Circular DNA, prokaryotic rRNA and ribosomes
 150 living genera(현존하는 속) [11 phyla (문)
중에서] have endosymbiotic species
 1.3 billion years for endosymbiotic evolution
• Reflects complexity of adaptations
그림 24.9. Cyanophora paradoxa (남세균과 유사한 엽록체를 갖고
있는 원생생물).
Archaeans and Eukaryotes
 All three domains(영역; 계) ancestral
• Same genetic code, transcription and translation
 Archaeans(고세균 영역) have features of
Bacteria[(진정)세균 영역] and Eukaryotes
(진핵생물 영역)
• Circular DNA, no nucleus, no organelles
• Introns
• Unique rRNA sequences, cell wall, plasma
Multicellular Eukaryotes
 Unicellular eukaryotes before multicellular
• Molecular clock 800-1000 million years ago,
fossils 600-800 million
 First multicellular eukaryotes colonies
• Coordinated activities (adaptation to environment)
• Coordination requires gene expression signals to
change cell phenotypes
 Multicellularity evolved independently in algae,
fungi, plants and animals
Life Inevitable on Earth?
 Earth conditions may have made life inevitable
• Reducing atmosphere
• Moderate gravity
• Distance from sun