Download The Origin of Life (생명의 기원) Chapter 24

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
aggregates
• 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
• ATP
 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
acids/proteins
• 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
atmosphere
• Oxygen allows better energy harvest from
respiration
• 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
molecules
• 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
membrane)
• 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
dependent
 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
membrane
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