Chapter 14 The History of Life 14.1 Fossil Evidence of Change
Earth’s Early History • Earth formed 4.6 billion years ago from molten material. • Cooled for 500 million years and formed a solid crust on the surface. Surface is rich in lighter elements, as more dense elements were pulled by gravity toward Earth’s center. • Atmosphere of Earth consisted of H20, CO2, SO2, CO, H2S, HCN, N2 and H2
Clues in Rocks • A fossil is any preserved evidence of an organism. • Most organisms decompose before they have a chance to become a fossil. • Those organisms that are most likely to become fossilized are those that are covered quickly by sediment (no O2) and have hard parts. • Only one in a million organism becomes a fossil.
Fossil Formation • Fossils form in sedimentary rocks; sediments cover the organism. • Igneous rocks form from molten rock material; metamorphic rocks when other rocks are exposed to heat and pressure; both processes would destroy any fossil material in these rock types.
Relative Dating • Determine age of fossils by comparing them with those in other layers • Law of Superposition states that younger rock layers are deposited on top of older rock layers • Only true of undisturbed rock layers
Radiometric Dating • Dating method that used the decay of radioactive isotopes to measure the age of a rock. • Isotope is a form of an element that has the same atomic number but a different atomic mass (mass number) • Half life is the amount of time it takes for ½ of the original radioactive isotope to decay or change into its decay product
Radiometric Dating • Commonly used radioactive elements are Uranium-238 (U238), Potassium-40 (K40) and Carbon-14 (C14) • Some radioactive isotopes used for radiometric dating are found only is igneous or metamorphic rocks (not sedimentary) so isotopes cannot be used to date rocks that contain fossils. Relative dating of fossils from known igneous or metamorphic rocks are used. • C14 can be used to date fossils directly if they are less than 60,000 years old
Radiometric decay Rate of C14 to N14 Half life 5730 years
Radiometric decay Rate of U238 to Pb205 Half life 4520 million years (4.510 billion)
The Geologic Time Scale • Model that expresses the major geological and biological events in Earth’s history. • Largest time classification is the eon. • Eons are divided into eras • Eras are divided into periods • Periods are divided into epochs • See page 397
Plate Tectonics • During the Mesozoic era the location of the Earth’s plates underwent a major shift in position and are continuing to move today
Chapter 14 The History of Life 14.2 The Origin of Life
Origin: Early Ideas • Spontaneous generation is the idea that life arises from nonlife. • People believed that it rained frogs, mice spontaneous arose out of grain, and from mud came insects, fish and worms.
Francisco Redi 1668 • First controlled experiment to disprove spontaneous generation in which flies arose out of meat. • He hypothesized that flies, not meat, gave rise to flies
Louis Pasteur 1850’s • Experiment showed that sterile broth remained free of microorganisms until exposed to air. • Flasks still exist today free of microorganisms. • Theory of spontaneous generation rejected and replaced with theory of biogenesis (life from life)
Origin: Modern Ideas • If life can only come from preexisting life (theory of biogenesis) then how did the first life-form appear? • Three ideas: • Divine creation • Extraterrestrial • Series of chemical events
Origin: Modern Ideas • Simple organic molecules could have been formed from a mixture of gases present in the early atmosphere when sparked by lightening. • Theory proposed by Oparin and Haldren in 1920’s • Called the “primordial soup”. The oceans as a “soup” of chemicals that could have combined to form life
Origin: Modern Ideas • Simple organic molecules have been formed in the laboratory • Experiments of Miller and Urey in 1953. • Amino acids, sugars and nucleotides have been formed by refining this experiment.
Origin: Modern Ideas • Another ideas is that the organic reactions that preceded life’s emergence began at deep-sea vents where sulfur forms the base of a unique food chain.
Origin: Modern Ideas In order for life to form from nonlife (chemicals) three requirements must be met: • Making proteins • Genetic code • Molecules to cells
Making Proteins • In order for life to exist proteins must be formed. • Stable proteins can be formed on clay particles
Genetic Code • A coding system for protein is a requirement for life. • All living things have DNA and RNA. • Since some RNA sequences have changed little over time, scientists consider RNA to be the first genetic coding system. • RNA can also act like an enzyme and could have carried out some early life processes • Thought that RNA replication took place on a clay crystal.
Molecule to Cells • Cells have a membrane. • Scientists have tested ways of enclosing molecules in membranes, allowing early metabolic and replication pathways to develop. • How cells formed has yet to be explained.
Cellular Evolution • The first cells were prokaryotes similar to current day archaebacteria. • Today archaebacteria live in extreme environments similar to what may have been experienced on early Earth. • Photosynthesis began by cyanobacteria (not archae). Until 1.8 billion years ago no oxygen existed in the atmosphere. • Fossils of these bacteria have been found and dated at 3.5 billion years old.
Once cyanobacteria produced sufficient oxygen to form the ozone layer, conditions were right for the evolution of eukaryotic cells. Cellular Evolution Ozone layer protects Earth from harmful radiation
Cellular Evolution • Eukaryotic cells appeared in the fossil record about 1.8 billion years ago. (2 billion years after archae) • The endosymbiont theory suggests that some photosynthetic bacteria became engulfed within a larger bacteria and became the chloroplast; other engulfed bacteria became the mitochondria.
Evidence for the Endosymbiont Theory • Both mitochondria and chloroplasts contain their own circular DNA just like prokaryotes. • Both mitochondria and chloroplasts contain ribosomes that closely resemble archae ribosomes. • Like prokaryotic cells both mitochondria and chloroplasts reproduce by fission, independent from the rest of the cell.