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The History of Life Chapter 17 The First Living Things • Alexander Oparin and John Haldane (1920s and 1930s) – Noted that an oxygen-rich atmosphere would not have permitted the spontaneous formation of complex organic molecules – Speculated that the atmosphere of early Earth contained little oxygen – Proposed that prebiotic chemical evolution gave rise to life The First Living Things • Oparin and Haldane envisioned that prebiotic chemical evolution occurred in four stages – Prebiotic synthesis and accumulation of small organic molecules – Small organic molecules combined to form larger molecules – Origin of self-replicating molecules – Packaging of molecules within some kind of enclosing membrane Organic Molecules • Stanley Miller and Harold Urey (1953) – Noted that the atmosphere of early Earth probably contained methane, ammonia, hydrogen, and water vapor, but no oxygen – Simulated early Earth’s atmosphere by mixing the above gases in a flask and adding an electrical discharge to simulate lightning • Simple organic molecules appeared after a few days Organic Molecules • Exact composition of “atmosphere” was unimportant – Must contain carbon, hydrogen, and nitrogen, and exclude oxygen • Type of energy source was unimportant – Electrical discharge, UV light, and heat were equally effective Organic Molecules Accumulate • The lack of both life and oxygen gas on early Earth allowed large quantities of organic molecules to accumulate in areas protected from UV radiation (beneath rock ledges, in oceans) • UV radiation bombarded early Earth’s surface because there was no ozone to block it • UV radiation can break apart organic molecules • Accumulated simple organic molecules combined to form complex organic molecules Organic Molecules • Similar experiments by Miller and others have produced amino acids, short proteins, nucleotides, and ATP • Complex organic molecules join together to form clusters – coacervates or aggregates • Aggregates became more complex & highly organized RNA • May have been the first self-reproducing molecule • Thomas Cech and Sidney Altman (1980s) discovered an RNA molecule (ribozyme) • Ribozymes could catalyze a chemical reaction • Naturally-occurring ribozymes have been found that catalyze reactions including – Cutting other RNA molecules – Splicing together different RNA fragments – Attaching amino acids to growing proteins RNA • Since Cech and Altman’s initial discovery researchers have synthesized ribozymes that catalyze the replication of small RNA molecules • Discovery of ribozymes led to hypothesis that RNA preceded the origin of DNA • RNA served as – The information-carrying genetic molecule – The enzyme catalyst for its own replication When Did Life Arise on Earth? • Earth formed about 4.5 billion years ago • Life arose 3.9 to 3.5 billion years ago during the Precambrian era – Oldest fossil organisms found to date are estimated to be about 3.5 billion years old Heterotroph Hypothesis • The first cells to arise in Earth’s oceans were prokaryotes, cells that lack a membrane-bound nucleus • First life forms were not able to synthesize their own organic nutrients • These primitive bacteria probably: – Obtained nutrients and energy by absorbing organic molecules from their environment – Metabolized organic molecules anaerobically (early Earth lacked oxygen gas) Capturing the Sun’s Energy • The first photosynthesizing organisms (ancestors of cyanobacteria) appeared about 3.5 billion years ago • Photosynthesis requires sunlight, CO2, and hydrogen – Earliest source of hydrogen believed to be hydrogen sulfide – Eventually, water replaced hydrogen sulfide as the source of hydrogen and photosynthesis became water-based Increased Oxygen in Atmosphere • Water-based photosynthesis resulted in the release of oxygen gas as a by-product • Initially, oxygen combined with iron in the Earth’s crust to form iron oxide • Subsequently, oxygen began accumulating in the atmosphere – Chemical analysis of rocks suggests that significant levels of atmospheric oxygen first appeared about 2.2 billion years ago Aerobic Metabolism • The accumulation of oxygen in Earth’s atmosphere probably – Exterminated many anaerobic organisms – Provided the environmental pressure for the evolution of aerobic metabolism • The evolution of aerobic metabolism was significant because aerobic organisms can harvest more energy per food molecule than anaerobic organisms Membrane-Enclosed Organelles • The first eukaryotes (cells that possess membrane-bound organelles) appeared about 1.7 billion years ago • Several organelles (mitochondria, chloroplasts, centrioles) may have arisen when primitive cells engulfed certain types of bacteria (the endosymbiont hypothesis) Evolution of Mitochondria • Anaerobic, predatory prokaryotic cell engulfs an aerobic bacterium that it failed to digest • Predatory cell and bacterium gradually enter into a symbiotic relationship • Descendants of engulfed bacterium evolve into mitochondria Evolution of Chloroplasts • Mitochondria-containing predatory prokaryotic cell engulf a photosynthetic bacterium • Predatory cell and bacterium gradually enter into a symbiotic relationship • Descendants of engulfed bacterium evolve into chloroplasts