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Life’s Origin and Early Evolution Chapter 18 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011. 18.1 Looking for Life Biochemical, genetic, and metabolic similarities among Earth’s species imply that all like evolved from a common ancestor that lived billions of years ago Astrobiology • The scientific study of life’s origin and distribution in the universe • Study extreme habitats determines the range of conditions that living organisms can tolerate • High pressure, high temperature, dry Looking for Life All metabolic reactions on Earth involve interactions that occur in water solutions • Conclusion liquid water is considered an essential requirement for life Life on Mar’s, Is so… • it supports the hypothesis that life on Earth arose as a consequence of processes that occurred throughout the universe • Find extraterrestrial microbes… this could mean intelligent life in the universe is possible 18.2 Beginnings: The Big Bang Earth formed approximately 4.6 billion years ago Big Bang Theory Big Bang Theory • Model describing the formation of the universe as a nearly instant distribution of matter through space • The universe began 13-15 billion years ago • The Event: • all existing matter and energy appeared and exploded outward from a single point Big Bang Theory The Event: • Single elements of hydrogen and helium formed • Over millions of years, gravity drew the gases together and formed giant stars and eventually galaxies • Asteroids collided and merged together • The heavier the asteroid (pre-planetary rock) the more gravitational pull they exerted and the more material they gathered • 5 billion years ago the sun was formed • 4.6 billion years ago Earth and planets in our solar system formed Conditions on Early Earth Scientists study stars and space to discover clues about how our universe originated Organic compounds spontaneously self-assemble under conditions possible on the early Earth • First Air Contained water vapor, carbon dioxide, and gaseous hydrogen and nitrogen • LITTLE OR NO OXYGEN • Later, life began in salty runoff pooled in early seas 18.3 Stanley Miller’s Experiment Proposed that reactions in Earth’s early atmosphere could have produced building blocks for the first life Experiment • Procedure: Placed water and gases (methane, ammonia, and hydrogen gas) into a reaction chamber • Observation: As the mix circulated, sparks from electrodes simulated lightning • Result: Within weeks, a variety of amino acids and other small molecules formed 18.3 Stanley Miller’s Experiment Reactions at Hydrothermal Vents Hydrothermal vent • Rocky, underwater opening where mineral-rich water heated by geothermal energy steams out • Reactions at vents can produce organic building blocks • Experiment: • Hot water + carbon monoxide + potassium cyanide + metal ions (like those near vents) • Result: amino acid formation Key Concepts: ABIOTIC SYNTHESIS OF ORGANIC COMPOUNDS When Earth first formed about 4 billion years ago, conditions were too harsh to support life Over time, its crust cooled, seas formed, and organic compounds of the sort now found in living cells may have formed spontaneously or arrived in meteorites 18.4 From Polymers to Cells All cells have a • plasma membrane, • genome of DNA that is transcribed into RNA and protein • All cells replicated and pass on copies of genetic material to descendants Self-replicating genetic systems require proteins (including enzymes) and nucleic acids Origin of Metabolism Proteins and nucleic acids may self-assemble when certain conditions are met • Clay-template hypothesis • Clay (- charge) attach + charge molecules • Low tide, evaporation concentrated the subunits • Concentration and energy from the sun caused the molecules to bond together as polymers • Hydrothermal vent hypothesis • High pressure, high temperature environment • Iron sulfide in rocks donated electrons to dissolved carbon monoxide start reactions that lead to the formation of larger organic compounds Protocells: Origins of the Plasma Membrane? Laboratory produced protocell •Lipid bilayer encloses RNA •Cell grows by adding fatty acids and nucleotides •Mechanical force causes protocell division Origin of the Plasma Membrane Protocell • Membranous sac that contains lipid-enclosed collections of interaction molecules • It is able to take up materials and replicate itself • Hypothesized to have formed prior to the earliest life forms membrane-bound proto-cells living cells Self-replicating system enclosed in a selectively permeable, protective lipid sphere DNA RNA formation of protein-RNA systems, evolution of DNA enzymes and other proteins formation of lipid spheres Proposed sequence for the evolution of cells spontaneous formation of lipids, carbohydrates, amino acids, proteins, nucleotides under abiotic conditions Fig. 18.7, p.295 Origins of Self-Replicating Genetic Systems Hypothesis: RNA world • RNA stores genetic information, but breaks apart easily and mutates often • Ribozymes: Catalytic RNAs Switch from RNA to DNA • Makes the genome more stable • Defense against viruses that attack RNA-based cells 18.5 Life’s Early Evolution 3.8 billion years ago, oxygen levels in atmosphere and seas were low • Early prokaryotic cells probably were anaerobic • Stromatolites • Dome-shaped structures composed of layers of bacterial cells and sediment • Cyanobacteria and photosynthetic bacteria grow here Divergence separated bacteria from ancestors of archaeans and eukaryotes Stromatolites The Oxygen Atmosphere Cyanobacteria evolved an oxygen-releasing, noncyclic pathway of photosynthesis • Noncyclic photosynthesis arose through mutations and modified the cyclic pathway • Changed Earth’s atmosphere Increased oxygen favored aerobic respiration • ATP-forming metabolic pathway • Key innovation in evolution of eukaryotic cells The Oxygen Environment Consequences of Life 1. Oxygen interferes with self-assembly of complex organic compounds. Life no longer could arise from nonliving materials 2. Oxygen put organisms that thrived in aerobic conditions at an advantage. Aerobic respiration allowed the evolution of multicelled eukaryotes 3. Some oxygen molecules broke apart and then recombined as ozone (O3). The ozone layer reduced the amount of solar UV radiation. • Without the ozone life could not have moved onto land Rise of Eukaryotes Eukaryotic cells branched off from archaean lineage Lipids are biomarkers for eukaryotes • Biomarker molecule produced only by a specific type of cell (molecular signature) Red alga 1.2 billion years ago • Oldest species to reproduce sexually 870 mya sponge-like animals 543 mya Animal diversity increased greatly during a great adaptive radiation. • All major animal lineages, including vertebrates were represented in the seas Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS Laboratory experiments and advanced computer simulations support the hypothesis that forerunners of living cells arose through known physical and chemical processes, such as tendency of lipids to assemble into membranelike structures when mixed with water Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS (cont.) First cells probably were anaerobic prokaryotes • Some gave rise to bacteria, others to archaeans and to ancestors of eukaryotic cells Photosynthetic bacteria started releasing free oxygen into the atmosphere • Oxygen accumulated over time and became a global selection pressure 18.4 Where Did Organelles Come From? Eukaryotic internal membranes may have evolved through infoldings of cell membrane DNA infolding of plasma membrane Fig. 18.10a, p.298 Fig. 18.10b, p.298 Endosymbiosis Endosymbiosis One cell lives and reproduces inside another Host and guest cells come to depend upon one another for essential metabolic processes Mitochondria and chloroplasts may have evolved by endosymbiosis Key Concepts: HOW THE FIRST EUKARYOTIC CELLS EVOLVED A nucleus, ER, and other membrane-enclosed organelles are among the defining features of eukaryotic cells Some organelles may have evolved from infoldings of the plasma membrane Mitochondria and chloroplasts probably are descendants of bacterial cells that became modified after taking up residence in host cells 18.7 Time Line: Life’s Origin and Evolution Fig. 18.12b, p.300 Key Concepts: VISUAL PREVIEW OF THE HISTORY OF LIFE Key events in life’s origin and early evolution can be correlated with the geologic time scale A time line for milestones in the history of life offers insight into shared connections among all organisms Animation: Milestones in the history of life Animation: Miller's reaction chamber experiment Animation: Origin of organelles