Chapter 26 The Origin and Evolution of Life In the Beginning . . . A. The universe is expanding but long, long ago it was greatly compressed, then: the Big Bang! B. About 4.5 billion years ago a cloud of gas and dust began to cool to form our solar system. I. Conditions on the Early Earth A. Origin of the Earth 1. About 4.5 billion years ago remnants of exploding stars began to condense into planets around the sun. 2. The earth was initially very hot, but cooled to form an outer mantle and partially-molten core. 3. Within 200 million years life had originated on its surface, but how? a. What were the prevailing conditions on the earth at this time? b. Could large organic molecules have formed spontaneously and then evolved into the molecular systems of life? c. Can we devise experiments to test whether living systems could have emerged by chemical evolution? B. The First Atmosphere 1. The first atmosphere probably consisted of gaseous hydrogen, nitrogen, carbon monoxide and carbon dioxide. 2. Gaseous oxygen and water were not thought to be present. 3. When the crust cooled the water condensed, rains began, and pools of chemicals began to form. C. Synthesis of Organic Compounds 1. Evidence of neighboring bodies in our solar system indicates that precursors for building biological molecules were present on the primitive earth. 2. Energy in the form of sunlight, lightning, and heat from the earth’s crust was also present. a. Stanley Miller used a lab apparatus to demonstrate synthesis of amino acids from hydrogen, methane, ammonia, and water under abiotic conditions. b. Even if molecules were formed spontaneously, they would have quickly hydrolyzed unless clay templates served to hold the molecules together for condensation reactions. II. Emergence of the First Living Cells A. Origin of Agents of Metabolism 1. During the early history of the earth, enzymes, ATP, and other molecules could have assembled spontaneously. Remember the "RNA world" and the abiotic origins of life due to lack of oxygen on the earth. 2. The participation of these and other entities in metabolic pathways could have been facilitated by clay templates that brought them together in the same place and time. B. Origin of Self-Replicating Systems 1. From accumulated organic compounds emerged replicating systems consisting of DNA, RNA, and proteins. 2. Ribonucleotides may have then stuck to the clay and eventually replaced clay as a template. 3. An RNA world may have preceded DNA’s dominance as the main informational molecule. 4. How DNA entered the picture is not yet clear, but we do know that some reactions were more probable than others&endash;not random. C. Origin of the First Plasma Membranes 1. The metabolism in living cells cannot occur without a barrier against the chemical actions on the outside. 2. Proto-cells were probably membrane-bound sacs containing nucleic acids that served as templates for proteins. 3. Sidney Fox heated amino acids to form protein chains, which when allowed to cool self-assembled into small spheres that were selectively permeable.   III. Origin of Prokaryotic and then Eukaryotic Cells: Recal lthe Endosymbiotic Theory of Eukaryotic Cell Evolution A. The Archean eon (3.9 to 2.5 billion years ago) was the time of macromolecule synthesis plus the origin of anaerobic prokaryotes. 1. The original prokaryote line split into Three Domains: Archaea, Bacteria, and a line leading to Eukarya (eukaryotes). 2. Evolution of the cyclic pathway of photosynthesis in Bacteria (Cyanobacteria) tapped a renewable source of energy&endash;sunlight; large accumulations of these cells are seen today as fossils known as stromatolites. B. In the Proterozoic eon (2.5 billion to 550 million years ago), the noncyclic pathway evolved in first in eubacteria and then later in eukaryotic cells (algae, fungi); oxygen accumulated, and aerobic respiration evolved. Where Did Organelles Come From?