VIEWS: 3 PAGES: 30 POSTED ON: 7/23/2012
Life’s Origin and Early Evolution Chapter 20 Impacts, Issues Looking for Life in All the Odd Places Life can adapt to nearly any environment with sources of carbon and energy – including extreme temperatures, pH, salinity, or pressure Origin of the Universe and Our Solar System Big bang model • The universe began in an instant, with a big bang about 13 to 15 billion years ago • The universe is still expanding Earth formed from dust and debris orbiting the sun, about 4.6 billion years ago Conditions on the Early Earth Earth’s early atmosphere came from gas released by volcanoes, and was low in oxygen Rain washed minerals and salts out of rocks to form early seas Early Earth Origin of the Building Blocks of Life Small organic molecules that serve as building blocks of life can be formed by nonliving mechanisms Stanley Miller’s experiments showed that amino acids form in reaction chambers designed to simulate early Earth Stanley Miller’s Experiments 20.1 Key Concepts Origin of Organic Compounds When Earth formed more than 4 billion years ago, conditions were too harsh to support life Over time, its crust cooled and seas formed Organic compounds of the sort now found in living cells may have self assembled in the seas or arrived in meteorites 20.2 How Did Cells Emerge? We will never know for sure how the first cells came to be, but we can investigate the possible steps on the road to life Origins of Proteins and Metabolism All living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselves Concentration of molecules on clay particles or in tiny rock chambers near hydrothermal vents may have helped start metabolic reactions Possible Origins of Complex Organic Compounds Clay templates on tidal flats; iron-sulfide rocks at hydrothermal vents on the deep ocean floor Origin of the Plasma Membrane Vesicle-like structures with outer membranes form spontaneously when some organic molecules are mixed with water Protocell • Any membrane-enclosed sac of molecules that captures energy, concentrates materials, engages in metabolism, and replicates itself Models for Protocells Selectively permeable vesicles and RNA-coated clay Origin of Genetic Material An RNA-based system of inheritance may have preceded DNA-based systems RNA world • A time when RNA both stored genetic information and functioned like an enzyme in synthesis Ribozymes • Synthetic, self-replicating RNAs Hypothesis: Chemicals to Living Cells 20.2 Key Concepts Origin of Cells In all living cells, proteins catalyze metabolic reactions, a plasma membrane encloses the cell, and DNA is the molecule of inheritance Laboratory experiments provide insight into how cellular components and processes could have evolved 20.3 Life’s Early Evolution Fossils and molecular comparisons among modern organisms inform us about the early history of life The Golden Age of Prokaryotes Life that arose 3-4 billion years ago was probably anaerobic and prokaryotic Early divergence separated ancestors of bacteria, archaeans and eukaryotes The first photosynthetic cells were bacteria that used the cyclic pathway (does not produce O2) The Proterozoic Era 2.5 billion years ago, the oxygen-producing, non-cyclic pathway of photosynthesis began evolving in cyanobacteria (stromatolites) Oxygen accumulation in the air and seas halted spontaneous formation of molecules of life, formed a protective ozone layer, and spurred evolution of organisms using aerobic respiration Fossil Prokaryotic Cells The Rise of Eukaryotes Oldest eukaryotic fossils: 2.1 billion years 20.4 Where Did Organelles Come From? Eukaryotic cells have a composite ancestry, with different components derived from different prokaryotic ancestors A nucleus and other organelles are defining features of eukaryotic cells Origins of Nucleus, ER, and Golgi Body The nucleus and ER may have arisen through modification of infoldings of plasma membrane Evolution of Mitochondria and Chloroplasts Mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cells Endosymbiosis • The symbiont lives out its life inside a host • Eventually, host and symbiont become incapable of living independently Evidence of Endosymbiosis 20.5 Time Line for Life’s Origin and Evolution 20.6 About Astrobiology Astrobiology • Study of origins, evolution, and persistence of life on Earth as it relates to life in the universe Studying conditions on other planets provides clues to how life arose on Earth • Examples: Unmanned missions to Mars and Europa; telescopic studies Lessons of Chile’s Atacama Desert What we learn about life on Earth informs our ideas about the possibility of life elsewhere 20.3-20.5 Key Concepts Early Evolution The first cells were prokaryotic Eukaryotes arose after evolution of the noncylic pathway of photosynthesis in some prokaryotes put oxygen into the air Mitochondria and chloroplasts are descendants of bacteria that lived in other cells Major Events in the History of Life
"Lifes Origin and Early Evolution"