Chapter 19: 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 4.6 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 4.6 billion years ago remnants of exploding stars began to condense into planets around our 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. 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. 3. An intriguing example of such is the possibility that the porphyrin ring (a component of both chlorophyll and cytochromes) was the electron transporter of the first metabolic pathways. 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. Protein chains, which when allowed to cool self-assembled into small spheres that were selectively permeable.   III. Origin of Prokaryotic and Eukaryotic Cells 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 archaebacteria, eubacteria, and a line leading to eukaryotes. 2. Evolution of the cyclic pathway of photosynthesis in eubacteria 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. IV. Focus on Science: Where Did Organelles Come From? V. Life in the Paleozoic Era A. During the Cambrian period, nearly all of the major phyla evolved; most organisms lived on or near the sea floor (trilobites were a dominant group). B. In the Ordovician period, the Gondwana continent drifted southward, shallow marine environments were formed, reef organisms flourished, and glaciers formed to trigger the first mass global extinction. C. In the Silurian and Devonian periods, Gondwana drifted northward, reef organisms recovered, predatory fishes flourished, and amphibians and stalked plants were moving onto land. D. In the Carboniferous period, major radiations of plants and animals occurred as land masses were alternately flooded and drained; coal deposits formed. E. In the Permian period, insects, amphibians and reptiles flourished; formation of Pangea supercontinent caused greatest of all mass extinctions. VI. Life In the Mesozoic Era A. Speciation on a Grand Scale 1. Early in the Cretaceous, the supercontinent Pangea broke up, favoring divergences and speciation on a grand scale. 2. Flowering plants became dominant; marine invertebrates, fishes, and insects underwent spectacular radiations. B. Rise of the Ruling Reptiles 1. Early in the Triassic, the first dinosaurs evolved from a reptilian lineage 2. Later in the era, superplumes caused global temperatures to increase, which led to a proliferation of photosynthetic organisms. 3. At the close of the era, the dinosaurs disappeared perhaps due to the consequences of an asteroid impact in Mexico.  VII. Focus on Science: Horrendous End to Dominance VIII. Life in the Cenozoic Era A. The breakup of Pangea resulted in major changes in land mass configurations, climates, and adaptive zones. 1. During the Paleocene epoch, warmer and wetter climates favored tropical forests which reached to the polar regions. 2. Later epochs saw a gradual cooling trend resulting in vegetation that favored the rise of grazers and browsers. B. The activities of human civilization begun about 50,000 years ago may have accelerated the pace of extinction. IX. Summary of Earth and Life History