origion and history of life
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Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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CHAPTER 22
ORIGIN AND
HISTORY OF LIFE
Prepared by
Brenda Leady, University of Toledo
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The universe began with the Big Bang about 13.7 bya
Our solar system began about 4.6 bya The Earth is 4.55 billion years old 4 bya the Earth cooled enough for outer
layers to solidify and oceans to form 4-3.5 bya life emerged
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Origin in 4 overlapping stages
1. Nucleotides and amino acids produced prior to the existence of cells
2. Nucleotides and amino acids became polymerized to form DNA, RNA and proteins
3. Polymers became enclosed in membranes
4. Polymers enclosed in membranes evolved cellular properties
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Stage 1: Origin of organic molecules
Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules
Prebiotic or abiotic synthesisFormed prebiotic soup
Several hypotheses on where and how organic molecules originated
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Reducing atmosphere hypothesisBased on geological dataExperiments simulated conditions of primitive Earth
postulated in 1950sFormed precursors, amino acids, sugars and
nitrogenous basesFirst attempt to apply scientific experiments to
understand origin of lifeSince 1950s, ideas about early Earth atmosphere
changed Similar results
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Extraterrestrial hypothesis Meteorites brought organic carbon to Earth
Includes amino acids and nucleic acid bases
Opponents argue that most of this would be destroyed in the intense heating and collision
Deep-sea vent hypothesis Biologically important molecules may have been
formed in the temperature gradient between extremely hot vent water and cold ocean water
Supported by experiments Complex biological communities found here that
derive energy from chemicals in the vent (not the sun)
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Stage 2: Organic polymers
Experimentally, prebiotic synthesis of polymers not possible in aqueous solutions
Experiments have shown formation of nucleic acid polymers and polypeptides on clay surface
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Stage 3: Formation of boundaries Protobiont/ prebiont describes first nonliving
structures that evolved into living cells 4 characteristics
1. Boundary separated external environment from internal contents
2. Polymers inside the protobiont contained information
3. Polymers inside the protobiont had enzymatic function
4. Protobionts capable of self-replication
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Living cells may have evolved from Coacervates
Droplets that form spontaneously from the association of charged polymers
Enzymes trapped inside can perform primitive metabolic functions
Microspheres Small water-filled vesicles surrounded by a macromolecular
boundary Liposomes
Vesicles surrounded by a lipid layer Clay can catalyze formation of liposomes that grow and divide Can enclose RNA
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Stage 4: RNA world
Majority of scientists favor RNA as the first macromolecule of protobionts
3 key RNA functions1. Ability to store information2. Capacity for replication3. Enzymatic function – ribozymes
DNA and proteins do not have all 3 functions
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Chemical selection Chemical within a mixture of different chemicals
has special properties or advantages that cause it to increase in number compared to other chemicals in the mixture
Hypothetical scenario with 2 steps1. One of the RNA molecules mutates and has
enzymatic ability to attach nucleotides together Advantage of faster replication
2. Second mutation produces enzymatic ability to synthesize nucleotides No reliance on prebiotic synthesis
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Bartel and Szostak Demonstrated Chemical Selection in the Laboratory
Began with synthesis of 1015 RNA molecules (long) Each RNA contained 2 regions – constant region
the same in all the molecules and a variable region Also made short RNAs that were complementary
to a portion of the long RNA and had a tag sequence to bind to beads
If the long RNAs mutated and obtained enzymatic activity, the long RNA would be held to the short RNA bound to the beads
Long RNAs that had this ability formed pool #1 More long RNAs were made that were variations
on Pool #1 Repeated several times Pool #10 showed enzymatic ability 3 million
times higher that the original random pool Results showed that chemical selection
improves the functional characteristics of a group of RNA molecules over time by increasing the proportions of those molecules with enhanced function
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Information storageDNA would have relieved RNA of informational role
and allowed RNA to do other functionsDNA is less likely to suffer mutations
Metabolism and other cellular functionsProteins have a greater catalytic potential and
efficiencyProteins can perform other tasks – cytoskeleton,
transport, etc.
Advantages of DNA/RNA/protein world
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History of life on Earth
Geological time scaleOrigin 4.55 bya to present
Precambrian- first 3 eons
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Changes in living organisms the result of Genetic changesEnvironmental changes
Can allow for new types of organisms Responsible for many extinctions
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Major environmental changes
Climate/temperature Atmosphere Land masses Flood Glaciation Volcanic eruptions Meteoric impacts
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Mass extinctions
5 large mass extinctions Near end of Ordovician, Devonian,
Permian, Triassic, and Cretaceous periods Geologic time periods are often based on
these events
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Fossils
Recognizable remains of past life on Earth Paleontologists study fossils Many rocks with fossils are sedimentary
Sediments pile up and become rockOrganisms buried quickly and hard parts replaced
by minerals Older rock is deeper and older organisms are
deeper in the rock bed
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Radioisotope dating
Fossils can be dated using elemental isotopes in accompanying rock
Half-life – length of time required for exactly one-half of original isotope to decay
Measure amount of a given isotope as well as the amount of isotope produced when the isotope decays
Usually igneous rock dated Expect fossil record to underestimate actual
date species came into existence
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Prokaryotic cells arose during Archaeon Eon
Archaeon Eon when diverse microbial life flourished in primordial oceans
First known fossils 3.5 bya First cells prokaryotic
Bacteria and Archaea are similar but different All life forms prokaryotic during Archaeon Eon Hardly any free oxygen so organism were anaerobic First cells were heterotrophs Autotrophs evolved as supply of organic molecules
dwindled
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Stromatolites
Autotrophic cyanobacteria were preserved when heterotrophic ancestors were not
Form stromatolites- layered structure of calcium carbonate
Cyanobacteria produce oxygen as a waste product of photosynthesis
Spelled doom for many prokaryotic groups that were anaerobic
Allowed the evolution of aerobic species
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The Origin of Eukaryotic Cells During the Proterozoic Eon Involved a Union Between Bacterial and Archaeal Cells
Origin of first eukaryotic cell matter of debate In modern eukaryotes, DNA found in nucleus,
mitochondria and chloroplasts Examine properties of this DNA and modern
prokaryotes Nuclear genome – both bacteria and archaea
contributed substantially Symbiotic relationship – 2 species live in direct contact Endosymbitoic – one organism lived inside another
Data supports this origin
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Proterzoic Eon
Multicellular eukaryotes arise 1.5 bya 2 possible origins
Individuals form a colony Single cell divides and stays stuck together
Volvocine green algae display variations in the degree of multicellularity
Multicellular animals emerge toward the end of the eon
First animals invertebrates Bilateral symmetry facilitates locomotion
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Phanerzoic Eon
Proliferation of multicellular eukaryotic life extensive during Phanerzoic Eon (543 mya to today)
Paleozoic Era Mesozoic Era Cenozoic Era
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Phanerzoic Eon, Paleozoic Era
543-248 mya Cambrian period Ordovician period Silurian period Devonian period Carboniferous period Permian period
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Phanerzoic Eon, Paleozoic Era, Cambrian Period
543-490 mya Warm and wet with no ice at poles Cambrian explosion – abrupt increase in diversity of
animal species Cause unknown – shell evolution, atmospheric oxygen?
All existing major types of marine invertebrates plus many other that no longer exist
Although new species have arisen since, no major reorganizations of body plans
First vertebrates 520 mya
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Phanerzoic Eon, Paleozoic Era, Ordovician Period
490-443 mya Warm temperatures and atmosphere very moist Diverse group of marine invertebrates including
trilobites and brachiopods Primitive land plants and arthropods first invade land Toward end, abrupt climate change (large glaciers)
resulting in mass extinction Over 60% of existing marine invertebrates became
extinct
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Phanerzoic Eon, Paleozoic Era, Silurian Period
443-417 mya Relatively stable climate Glaciers largely melted No new major invertebrates Significant new vertebrates and plants Many new fish Coral reefs appeared Large colonization by terrestrial plants and animals
Had to evolve adaptations to drying out Spiders and centipedes Earliest vascular plants
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Phanerzoic Eon, Paleozoic Era, Devonian Period
417-354 mya Generally dry across north but southern hemisphere
mostly covered by cool, temperate oceans Major increase in number of terrestrial species Ferns, horsetails and seed plants (gymnosperms) emerge Insects emerge Tetrapods – amphibians emerge Invertebrates flourish in the oceans Age of Fishes Near end, prolonged series of extinctions eliminate many
marine species
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Phanerzoic Eon, Paleozoic Era, Carboniferous Period
354-290 mya Rich coal deposits formed Cooler, land covered by forested swamps Plants and animals further diversified
Very large plants and trees prevalentFirst flying insectsAmphibians prevalentAmniotic egg emerges - reptiles
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Phanerzoic Eon, Paleozoic Era, Permian Period
290-248 mya Continental drift formed supercontinent Pangaea Interior regions dry with seasonal fluctuations Forest shift to gymnosperms Amphibians prevalent but reptile became dominant First mammal-like reptiles appeared At the end, largest known mass extinction event
90-95% of all marine species and large proportion of terrestrial species eliminated
Glaciations or volcanic eruptions blamed
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Phanerzoic Eon, Mesozoic Era
Permian extinction marks boundary between Paleozoic and Mesozoic eras
Age of Dinosaurs Consistently hot climate, dry terrestrial
environments, little if any ice at poles
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Phanerzoic Eon, Mesozoic Era, Triassic Period
248-206 mya Reptiles plentiful First dinosaurs First true mammals Gymnosperms dominant land plant Volcanic eruptions led to global warming
and mass extinctions near the end
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Phanerzoic Eon, Mesozoic Era, Jurassic Period
206-144 mya Gymnosperms continued to be dominant Dinosaurs dominant land animal Some attained enormous size First known bird Mammals present but not prevalent
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Phanerzoic Eon, Mesozoic Era, Cretaceous Period
144-65 mya Dinosaurs still dominant on land Earliest flowering plants, angiosperms Another mass extinction at the end of the period Dinosaurs and many other species died out Large meteorite/asteroid or volcanic eruptions
blamed
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Phanerzoic Eon, Cenozoic Era
Spans most recent 65 million years Tropical conditions replaced by a colder,
drier climate Amazing diversification of birds, fish,
insects, and flowering plants
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Phanerzoic Eon, Cenozoic Era, Tertiary Period
65-1.8 mya Mammals that survived expanded rapidly Birds and terrestrial insects diversified Angiosperms become the dominant land plant Fish diversified Sharks become abundant Whales appeared Hominids appeared about 7 mya
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Phanerzoic Eon, Cenozoic Era, Quaternary Period
1.8 mya to present Periodic ice ages cover much of Europe
and North America Widespread extinction of many species Certain hominids become more human-
like Homo sapiens appears 130,000 years ago
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