Chapter 19: History of Life on Earth Section 1: How Did Life Begin? I. The Basic Chemicals of Life A. 1920s B. Earth s early oceans contained large amounts of organic molecules C. Molecules formed spontaneously in chemical reactions that were activated by energy from: 1. Sun 2. Volcanic eruptions 3. Lightening D. Early atomosphere was rich in 1. Hydrogen gas (H 2 ) 2. Other hydrogen-containing gases a. Water vapor (H 2 O) b. Ammonia (NH 3 ) c. Methane (CH 4 ) E. Scientists thought if the atmosphere lacked oxygen, a variety of organic compounds made up of these elements could form Section 2: The Age of Earth Fossil Record: the history of life in the geologic past as indicated by the traces or remains of living things Relative Dating: a method of determining whether an event or object, such as a fossil, is older or younger than other events or objects 1
Radiometric Dating: a method of determining the absolute age of an object, often by comparing the relative percentages of a radioactive isotope and a stable isotope Half-life: the time required for half of a sample of a radioactive substance to decay Geologic Time Scale: the standard method used to divide the Earth s long natural history into manageable parts Mass Extinction: an episode during which large numbers of species become extinct I. The Fossil Record A. Includes all fossil remains of living things on Earth B. Fossil Record can tell us 1. Geological distribution of organisms 2. When they lived on Earth C. Chronicles the diversity of life on Earth D. Provides evidence of intermediate life forms E. Suggests how organisms are related to each other F. Although our examination will never be complete, it presents strong evidence for evolution II. How Fossils Form A. Many fossils are found in sedimentary rock B. Form when organisms or traces of organisms are rapidly buried in fine sediments that are deposited by: 1. Water 2. Wind 3. Volcanic eruptions Fossil Examples- Volcanoes 2
Fossil Examples-Coral Fossils once Under water C. Environments that cause fossil formation are 1. Wet lowlands 2. Slow-moving streams 3. Lakes 4. Shallow Seas 5. Areas near volcanoes that spew ash Agate Fossil River Bed D. Many species have lived in environments where fossils do not form E. Even in environments where fossils do form, a dead body may not be buried in sediment before it decays or is eaten III. Analyzing Fossil Evidence A. Earth s surface changes constantly 1. Rocks are eroding and laid down as sediment 2. Sediment forms layers of sedimentary rock called strata 3. Law of Superposition a. Older strata are covered by younger strata 4. Geological events such as earthquakes can affect how strata are arranged Superposition Video 3
Example of Law of Superposition Cross Cutting Example B. Paleontologists use both relative and absolute dating methods to date fossils Relative Age C. Types of Fossils 1. Mineralized fossils are the most common type 2. In some cases, an organism breaks down, leaving a hollow space called a mold a. Mold may fill with minerals, preserving shape of organism Mineralized Fossils Fossil Mold 4
Amber Fossils 3. In rare cases, fossils are preserved in hardened plant sap, or amber a. In these fossils, soft parts of tissue are preserved in detail D. Relative Age 1. Relative dating is used to estimate the ages of fossils found within strata a. Cannot reveal a fossil s age in years b. Can reveal the order that strata and the fossils within them were laid down over time E. Paleontologists organize fossils into a sequence based on the relative age of the strata in which the fossil was found Example of Index Fossils F. Index Fossils 1. Fossils of an organism that was common and had widespread geographic distribution during a certain time in Earth s history 2. Used to estimate the age of other strata that contain the same type of fossil 3. Scientists have compared patterns of strata and index fossils within them to make the geologic time scale 5
G. Absolute Age 1. Radiometric dating estimates the age in years of an object by measuring certain radioactive isotopes that the object contains 2. Isotope is a form of an element whose atomic mass differs from that of other atoms of the same element 3. Radioactive isotopes, or radioisotopes, are unstable isotopes that break down and give off energy in the form of charged particles, or radiation 4. Breakdown is called radioactive decay Carbon-14 H. Half-life 1. Time required for half of a sample of a radioisotope to decay 2. Each radioisotope has a specific half-life 3. The rate at which a radioisotope decays is not affected by external factors Radiometric Dating Video IV. Geologic Time Scale I. Carbon-14 1. Half-life is 5600 or 5730 years 2. Used to measure the age of carboncontaining materials that are younger than 50,000 or 75,000 years old 3. To find the age of older materials, scientists must measure other radioisotopes 4. Carbon 14 Video A. Divisions of Geologic Time 1. Earth has existed for more than 4 billion years 2. Precambrian a. Beginning of Earth to about 542 million years ago (mya) 6
3. From the end of Precambrian time to the present a. Three eras 1) Paleozoic Era 2) Mesozoic Era 3) Cenozoic Era b. There eras are further divided into periods c. Humans appeared during the Quarternary Period Geologic Time Scale B. Mass Extinction 1. Large numbers of species become extinct 2. Fossil Record shows many in Earth s History 3. Factors that contribute to mass extinctions a. World wide geologic changes b. World wide weather changes 4. Have contributed to overall biodiversity on Earth 5. After a mass extinction, opportunities open for new lifeforms to emerge 6. Used to mark the divisions of geologic time a. Large mass extinctions mark boundaries between eras b. Smaller mass extinctions mark divisions between periods 7
Mass Extinctions Plate Tectonics I. Continental Drift A. Wegner s Hypothesis 1. German Scientist 2. 1912 B. Continental Drift 1. Continents once formed part of a single landmass called a supercontinent 2. Pangea Alfred Wegener Pangea Example 3. Supercontinent began breaking up 200 million years ago 4. Crumpling of the crust produced mountain ranges 8
C. Fossil Evidence 1. If continents had been joined, fossils of the same plants and animals should be found in areas that were connected 2. Mesosaurus found in South America and Western Africa a. Small land reptile b. Unlikely able to swim Atlantic Ocean 3. Found Evidence of Land Bridges D. Evidence from Rock Formations 1. Ages and types of rocks in coastal areas of Western Africa and Eastern South America matched closely 2. Appalachian Mountains extend north along eastern coast of North America 3. Mountains of similar age and structure are found in Greenland, Scotland and northern Europe II. Theory of Plate Tectonics A. Theory that explains why and how continents move B. Study of the formation of features in the Earth s crust Plate Tectonic Video Second Plate Tectonic Video C. How Continents Move 1. Lithosphere a. Earth s crust and rigid upper part of mantle b. Broken into blocks called tectonic plates 9
D. Tectonic Plates 1. 15 major plates 2. Earthquakes a. Sudden shifts along boundaries that occur when tectonic plates move 3. Volcanoes a. Form when plate movements generate magma erupting onto Earth s surface b. Ring of Fire 1) Zone of active volcanoes encircles Pacific Ocean video E. Types of Plate Boundaries 1. Divergent Boundaries a. 2 plates move away from each other b. Magma rises to surface as plates move apart c. Magma cools & forms new ocean lithosphere d. Forms Mid-ocean Ridges (under sea mountain ranges) http://www.wwnorton.com/college/geo/egeo/flash/2_7.swf http://www.wwnorton.com/college/geo/egeo/flash/2_11.swf http://www.wwnorton.com/college/geo/egeo/flash/2_5.swf 2. Convergent Boundaries a. 2 plates collide b. Subduction Zone 1) region along a plate boundary where one plate moves under another c. 1st type of collision 1) Ocean lithosphere collides with continent lithosphere 2) Ocean sinks below continent 3) Ocean plate heats up as it sinks forms volcanic mountains d. 2nd type of collision 1) 2 continental plates 2) neither plate sinks 3) forms mountain ranges e. 3rd type of collision 1) 2 ocean plates 2) one sinks under 3) forms deep ocean trench 10
3. Transform Boundaries a. 2 plates slide past each other b. Scrape in series of sudden spurts of motion c. Results in Earthquakes http://www.classzone.com/books/earth_science/terc/content/visualizations/es1105/es1105page01.cfm?chapter_no=visualization Type of Boundary Description Example http://www.classzone.com/books/earth_science/terc/content/visualizations/es0804/es0804page01.cfm?chapter_no=visualization Divergent Convergent Transform Plates moving away from each other to form rifts and midocean ridges Plates moving toward each other and colliding to form ocean trenches, mountain ranges, volcanoes and island arcs Plates sliding past each other while moving in opposite directions North American and Eurasian plates at the Mid-Atlantic Ridge South American and Nazca plates at the Chilean trench along the west coast of South America North American and Pacific plates at the San Andreas Fault in California III. Causes of Plate Motion A. Mantle Convection 1. Convection a. Movement of heated material due to differences in density which is caused by differences in temperature b. Example: Boiling water in a pot 2. Earth s Crust a. Energy from Earth s core & radioactivity within mantle heat mantle material b. Heated material rises c. Cooler denser material flows away from hot and sinks into mantle to replace rising material d. As mantle material moves drags overlying plates with it 11
Video IV. Effects of Continental Change A. Modern climates result of past movements of plates B. Continent s location (equator/poles) affects overall climate C. Continent climate affected by location in relation to oceans, mountains, other continents D. Mountain Ranges affect air flow and wind patterns around globe E. Mountains affect amount of moisture that reaches certain parts of continent F. When continents move flow of air and moisture around globe changes & causes climate to change G. Changes in Climate 1. Geologic evidence shows ice covered most of Earth surface 2. Ice sheet formed when all continents were close together & located near South Pole 3. As continents began to drift apart temperatures changed and ice melted H. Orbital Changes 1. Changes in the shape of Earth s Orbit & tilt can also cause climate changes 2. Variation in orbit affects distance from sun & temperature 3. Decreasing tilt decreases temperature differences between seasons I. Differential heating 1. Creates mountain and valley breezes 2. On warm, sunny days, the floor and slopes of a valley are heated 3. Heated air rises up the sides of the valley, creating a valley breeze 4. At night, the air in close proximity to the sides of the valley cools 5. Cooler air drains down slope in the valley, creating a mountain breeze 12
6. Water warms up faster or slower depending on time of year 7. Sand gets hotter than water, air temperature and amount of sunlight same, water temperature and soil different 13