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Chapter 15 Early Earth and the Origin of Life Life on Earth originated between 3.5 and 4.0 billion years ago. The Earth formed about 4.5 billion years ago. The first organisms were prokaryotes. Evidence of ancient prokaryotic life has been found in rocks called stromatolites- banded domes of sedimentary rock Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park About 3 billion years ago, the prokaryotes split into two distinct groups: Bacteria Archea 2.5 billion years ago, the production of oxygen by photosynthetic prokaryotes created an aerobic atmosphere Oldest fossils that are definitely eukaryotic are 1.7 billion years old Eukaryotes probably evolved from a symbiotic relationship between prokaryotes. Endosymbiosis ://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter4/animat ion_-_endosymbiosis.html Those eukaryotic cells evolved into the ancestors of modern protists, fungi, plants and animals. Oldest animal fossils date back to 700 million years ago. The first cells The early Earth’s atmosphere consisted of very little oxygen. There was a lot of lightning, volcanic activity, and ultraviolet radiation. Chemical Evolution 1. Abiotic synthesis of small organic molecules such as amino acids and nucleotides. a. 1920’s - Oparin and Haldane independently hypothesized that the reactions that took place on the early Earth could create organic molecules. b. This cannot happen today due to the electronegative nature of oxygen. c. Early atmosphere- much more reducing (electron adding) d. 1953-Miller & Urey- simulated conditions of the early Earth e. Experiment produced a variety of amino acids, sugars, lipids, nucleotides and ATP. Stanley Miller 2. Joining of monomers into polymers such as protein and nucleic acids. a. How would this occur without enzymes? b. In experiments, polymerization does occur when solutions of monomers are dropped onto hot sand, clay or rock. In 1957 Sidney Fox demonstrated that dry mixtures of amino acids could be encouraged to polymerize upon exposure to moderate heat. When the resulting polypeptides, or proteinoids, were dissolved in hot water and the solution allowed to cool, they formed small spherical shells about 2 μm in diameter—microspheres. 3. Aggregation of abiotically produced molecules in droplets called protobionts. a. Maintain an internal environment different from their surroundings. b. Exhibit some properties associated with life- metabolism and excitability c. Coacervates - self-assemble when a suspension of macromolecules is shaken d. Microspheres- polypeptides with a semipermeable protein membrane 1. Undergo shrinking and swelling due to osmosis 2. Store energy in the form of membrane potential. e. Liposomes- form spontaneously when the organic molecules include lipids 1. Form lipid bilayer membrane 2. Grow by engulfing other liposomes and may split to form two Protobionts 4. Origin of heredity a. RNA was probably the first genetic material b. RNA appears to have the ability to selfreplicate short sequences (5-10 bp) c. RNA may act as an enzyme = ribozymes d. Once RNA became enclosed in membranes, these protobionts would have a form of heredity. e. These protobionts may be selected for survival. f. RNA may have directed the sequencing of amino acids to form primitive enzymes. Debate about the origin of life A. Scientific evidence for what could have happened. B. Alternative scientific theories: 1. Panspermia- meteorites brought organic molecules formed in outer space. 2. Nucleic acid genes may have been preceded by simpler hereditary systems.