Origin & History of Life CHAPTER 19 Part 1 Origin & History of Life

I. Life Began on a Young Earth A. “Big Bang” Theory 1. ~ 10-20 billion years ago (bya) compacted universe blew apart with a “big bang” 2. Solar system & planets arose from condensing dust B. Early Earth 1. Began ~ 4.6 bya 2. Initially cold planet turns hot & molten 4/9/2019

C. Origin of Earth’s Atmosphere 1. Early atmosphere was probably mostly water vapor, N2, CO2, and small amounts of H2, H2S and CO. 2. Probably had little free oxygen making it a reducing atmosphere. a. That might have been a good thing since oxygen tends to attach to organic molecules, preventing them from joining into larger molecules. 4/9/2019

3. At first atmosphere was probably so hot that water was a vapor that formed dense, thick clouds. a. As Earth cooled, water vapor condensed to form liquid water & rain began to fall, probably for millions of years. 4/9/2019

II. How could life begin? A. Early Ideas & Experiments 1. Oparin & Haldane (1920s) a. Suggested that first organic molecules could have been produced from atmospheric gases in the presence of strong energy sources • heat from volcanoes & meteorites, radioactivity, lightning, ultraviolet radiation, sunlight 4/9/2019

2. Miller & Urey (1950s) a. Set out to test Oparin’s ideas b. Set up an apparatus to test idea  Put H2, CH4, and NH3 inside chamber with water  Heated it up  Ran electric sparks through  Water condensed c. Results: Got variety of organic compounds including amino acids d. Showed organic chemical could form from simple inorganic chemicals 4/9/2019

Miller’s & Urey’s Apparatus & Experiment

3. Other researcher’s results a. Others have created all 20 amino acids, as well as sugars, lipids, the nitrogenous bases present in DNA, RNA and even ATP b. Early atmosphere probably did not have much ammonia (NH3) c. Ammonia might have been abundant, however, in hydrothermal vents on ocean floor where catalysts can turn N2 to NH3 4/9/2019

Chemical Evolution at Hydrothermal Vents

III. How did polymers form? A. Sidney Fox 1. Amino acids can polymerize (form polypeptides) when exposed to dry heat. 2. Amino acids could have collected in shallow puddles. 3. Heat of sun could have caused them to form proteinoids (small polypeptides with catalytic properties) 4/9/2019

4. When Fox dropped proteinoids into water in his lab, they formed microspheres (structures composed of protein but with many properties of cells) 5. Fox proposed the Protein-first Hypothesis: a. Protein enzymes arose first b. DNA genes came later 4/9/2019

1. Believes clay is especially helpful in polymerization B. Graham Cairns-Smith 1. Believes clay is especially helpful in polymerization 2. Clay attracts small organic molecules & clay contains iron & zinc which may have served as organic catalysts 3. Clay has tendency to collect energy from radioactive decay & discharge it when temperature or humidity change 4/9/2019

4. Cairns-Smith suggests that RNA. nucleotides & amino acids became 4. Cairns-Smith suggests that RNA nucleotides & amino acids became associated in a way that polypeptides were ordered by RNA and also helped to synthesize RNA. a. This hypothesis suggests that both polypeptides & RNA arose at the same time. 4/9/2019

C. RNA-first hypothesis 1. Suggests that only RNA was needed to progress toward the formation of the first cells. a. Cech & Altman won Nobel Prize in 1989 for their discovery that RNA can be both a substrate and an enzyme b. Some RNA, called ribozymes, act as catalysts c. Some viruses have RNA genes 4/9/2019

2. How could RNA world work? a. RNA could have carried out processes we associate with DNA • RNA genes inside early cells • RNA directs protein synthesis • Eventually reverse transcription could have happened and RNA might have been used as a template to create DNA which then became genetic material 4/9/2019

IV. First Protocells Evolve A. Structures with a membrane would need to exist before true cells B. What could be origin of the first membranes? 1. If lipids are added to microspheres they produce a lipid-protein membrane a. These microspheres resemble bacteria and can divide. 4/9/2019

b. Semipermeable boundary may form around the droplets. 2. Oparin showed complicated mixtures of macromolecules can give rise to complex units called coacervate droplets. a. These have a tendency to absorb and incorporate substances from their surroundings. b. Semipermeable boundary may form around the droplets. 4/9/2019

3. Liposomes a. Lipids extracted from egg yolks, when placed in water, naturally organize themselves into double-layered bubbles called liposomes. b. These could have engulfed early molecules with enzymatic abilities & protected them from surroundings. 4/9/2019

Protocell Anatomy Microspheres Liposomes

1. In order to grow, protocells would need nutrition. C. Protocell Nutrition 1. In order to grow, protocells would need nutrition. 2. Organic molecules carried by rain into oceans could have provided food. 3. Suggests that first protocells were heterotrophs 4/9/2019

4. If first protocells evolved by. hydrothermal vents, however, first 4. If first protocells evolved by hydrothermal vents, however, first cells might have carried out chemosynthesis a. Chemoautotrophs obtain energy by oxidizing inorganic compounds like hydrogen sulfide which is abundant at such vents. 4/9/2019

V. Evolution of Chemical Paths A. First protocells might have used preformed ATP. B. As ATP supplies dwindled, however, natural selection would have favored any cells that could get energy from carbohydrates 1. Glycolysis is common to all living things & probably evolved very early on 4/9/2019

Origin of First Cells

VI. Evolution of Autotrophy A. Heterotrophs would have eventually run out of food B. Any organism that could synthesize organic compounds from inorganic compounds (autotrophy) would have gained a selective advantage 1. First chemosynthesis, 2. Second, modern photosynthesis 4/9/2019

D. Evolution of Atmosphere 1. Oxygen started to accumulate in water and atmosphere 2. Forms ozone layer (O3) a. Formed shield from UV light 3. This changed the Earth in a way that would prevent the origin of life (atmosphere now an oxidizing one) 4. Aerobic respiration now would have been possible 4/9/2019