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Transcript
CHAPTER 22:ORIGIN AND HISTORY OF LIFE 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 Origin in 4 overlapping stages Nucleotides and amino acids produced prior to the existence of cells Nucleotides and amino acids became polymerized to form DNA, RNA and proteins Polymers became enclosed in membranes Polymers enclosed in membranes evolved cellular properties Stage 1: Origin of organic molecules Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules Prebiotic or abiotic synthesis Formed prebiotic soup Several hypotheses on where and how organic molecules originated Reducing atmosphere hypothesis Based on geological data Experiments simulated conditions of primitive Earth postulated in 1950s Formed precursors, amino acids, sugars and nitrogenous bases First attempt to apply scientific experiments to understand origin of life Since 1950s, ideas about early Earth atmosphere changed Similar results 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) 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 Stage 3: Formation of boundaries Protobiont/ prebiont describes first nonliving structures that evolved into living cells 4 characteristics Boundary separated external environment from internal contents Polymers inside the protobiont contained information Polymers inside the protobiont had enzymatic function Protobionts capable of self-replication 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 1 Microspheres Small water-filled vesicles surrounded by a macromolecular boundary Liposomes Vesicles surrounded by a lipid layer Clay can catalyze formation of liposomes Can enclose RNA that grow and divide Stage 4: RNA world Majority of scientists favor RNA as the first macromolecule of protobionts 3 key RNA functions Ability to store information Capacity for replication Enzymatic function – ribozymes DNA and proteins do not have all 3 functions 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 steps One of the RNA molecules mutates and has enzymatic ability to attach nucleotides together Advantage of faster replication Second mutation produces enzymatic ability to synthesize nucleotides No reliance on prebiotic synthesis 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 Advantages of DNA/RNA/protein world Information storage DNA would have relieved RNA of informational role and allowed RNA to do other functions DNA is less likely to suffer mutations Metabolism and other cellular functions Proteins have a greater catalytic potential and efficiency Proteins can perform other tasks – cytoskeleton, transport, etc. History of life on Earth Geological time scale Origin 4.55 bya to present Precambrian- first 3 eons Changes in living organisms the result of 2 Genetic changes Environmental changes Can allow for new types of organisms Responsible for many extinctions Major environmental changes Climate/temperature Atmosphere Land masses Flood Glaciation Volcanic eruptions Meteoric impacts 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 Fossils Recognizable remains of past life on Earth Paleontologists study fossils Many rocks with fossils are sedimentary Sediments pile up and become rock Organisms buried quickly and hard parts replaced by minerals Older rock is deeper and older organisms are deeper in the rock bed 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 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 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 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 3 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 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 Phanerzoic Eon Proliferation of multicellular eukaryotic life extensive during Phanerzoic Eon (543 mya to today) Paleozoic Era Mesozoic Era Cenozoic Era Phanerzoic Eon, Paleozoic Era 543-248 mya Cambrian period Ordovician period Silurian period Devonian period Carboniferous period Permian period 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 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 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 4 Had to evolve adaptations to drying out Spiders and centipedes Earliest vascular plants 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 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 prevalent First flying insects Amphibians prevalent Amniotic egg emerges - reptiles 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 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 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 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 5 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 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 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 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 6