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12.3 Origin of Life KEY CONCEPT The origin of life on Earth remains a puzzle. What is Evolution 12.3 Origin of Life Earth was very different billions of years ago. • There have been many hypotheses of Earth’s origins. • The most widely accepted hypothesis of Earth’s origins is the nebula hypothesis. 12.3 Origin of Life Several sets of hypotheses propose how life began on Earth. • There are two organic molecule hypotheses. – Miller-Urey experiment electrodes “atmosphere” water “ocean” heat source – meteorite hypothesis amino acids 12.3 Origin of Life • There are different hypotheses of early cell structure. – iron-sulfide bubbles hypothesis 12.3 Origin of Life • There are different hypotheses of early cell structure. – lipid membrane hypothesis 12.3 Origin of Life • A hypothesis proposes that RNA was the first genetic material. – Ribozymes are RNA molecules that catalyze their own replication. – DNA needs enzymes to replicate itself. 12.3 Origin of Life KEY CONCEPT Single-celled organisms existed 3.8 billion years ago. 12.3 Origin of Life Microbes have changed the physical and chemical composition of Earth. • The oldest known fossils are a group of marine cyanobacteria. – prokaryotic cells – added oxygen to atmosphere – deposited minerals 12.3 Origin of Life • Fossil stromatolites provide evidence of early colonies of life. 12.3 Origin of Life Eukaryotic cells may have evolved through endosymbiosis. • Endosymbiosis is a relationship in which one organism lives within the body of another. • Mitochondria and chloroplasts may have developed through endosymbiosis. 12.3 Origin of Life The evolution of sexual reproduction led to increased diversity. • Genetic variation is an advantage of sexual reproduction. • Sexual reproduction may have led to the evolution of multicellular life.