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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