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The History of Life Chapter 17 Pgs. 417-420; 435-440 17-1 The Fossil Record • Palentologists: Scientists who study fossils and infer what past life was like • The Fossil Record: Shows that: – Most organisms that ever lived on earth are now extinct – Modern organisms have unicellular (singlecelled) ancestors – Fossils occur in a particular order with older fossils in older rocks 17-1The Fossil Record • Most fossils form in sedimentary rock • Examples of fossils include: – Eggs – Footprints – Body Parts • When rock erodes from wind, rain and shifting mountain ranges, older fossils can be exposed which can then be studied 17-1 Fossils- Relative Dating • Paleontologists use two techniques to determine the age of fossils: – Relative Dating – Radioactive Dating • Relative Dating: – The age of a fossil is determined by comparing its placement with fossils in other layers of rock – Scientists can use index fossils to compare the relative ages of fossils – Allows paleontologists to estimate fossil’s age compared to other fossils 17-1 Fossil Dating • Radioactive Dating: Uses radioactive decay to assign time ranges to rocks – Uses half-life toe determine the length of time required for ½ of the radioactive atoms in a sample to decay – Scientists calculate the age of a fossil by the amount of remaining radioactive isotopes it contains 17-1 Geologic Time Scale • Geologic Time Scale: Scientists divisions of the geologic time scale to represent evolutionary time – Studied rock layers and index fossils worldwide and put them in order according to relative age – Precambrian Time covers about 88% of Earth’s history – After Precambrian Time the basic division are eras and Periods 17-1 Geologic Time Scale • Eras: There are 3 eras between the Precambrian Era and the present: – Paleozoic Era (500-250 million years ago) – Mesozoic Era (250-65 million years ago) – Cenozoic Era ( Earth’s most recent period: 65 million years ago-present) • Periods: Each of the 3 eras are subdivided into periods (pg.421 fig. 17-5) – Earths most recent period is the Quaternary 17-2 Earth’s Early History • The Formation of the Earth occurred nearly 4.6 billion years ago: – – – – – Early atmosphere contained poisonous gasses Earth was struck by objects that generated heat and melted it Earth cooled enough for rocks to form 4 billion years ago Volcanic Activity began and made the Earth very hot Comets and asteroids bombarded its surface • Oceans did not exist until 3.8 billion years ago, because the Earth was too hot and water remained a gas 17-2 The Puzzle of Life’s Origin • One necessary condition for the presence of the first life on Earth was the presence of liquid water – Microspheres: 1st molecules that had some characteristics of living systems • Had permeable membranes, simple energy systems • Were similar to modern-day bacteria 17-2 Origin of Eukaryotic Cells • Eukaryotic Cells: Cells that have a nucleus – Original prokaryotic cells began developing cell membranes – Other prokaryotic cells invaded and began living in the original prokaryotic cells – These cells developed a symbiotic relationship with one another • Endosymbiotic Theory: Eukaryotic cells developed from a symbiosis of several different prokaryotic organisms 17-2 Origin of Eukaryotic Cells • The Endosymbiotic Theory gained support in the 1960’s by looking at chloroplasts and mitochondira. Lynn Margulis of Boston University made the following observations: – Mitochondrial DNA and chloroplast DNA resemble bacterial (prokaryotic) RNA – Their ribosomes resemble the ribosomes of bacteria – Like bacteria, they reproduce by mitosis 17-4 Patterns of Evolution • Macroevolution:Large-scale evolutionary patterns and processes that occur over long periods of time. There are six of these: – – – – – – Extinction Adaptive Radiation Convergent Evolution Coevolution Punctuated Equilibrium Changes in Developmental Genes 17-4 Patterns of Evolution • Extinction: More than 99% of all species that have ever lived are now extinct. – There are several hypotheses to explain this • EX: large asteroids hitting earth wiped out the dinosaurs, errupting volcanoes and the continents moving – Each time one species became extinct it opened up habitats for new species to develop which cleared the way for evolution to continue 17-4 Patterns of Evolution • Adaptive Radiation: A single species that has evolved into several different forms that live in different ways – EX: Darwin’s finches evolved into more than a dozen different species from one original species, Dinosaurs evolved at the same time as 1st mammals, but eventually ruled the earth until extinction 17-4 Patterns of Evolution • Convergent Evolution: The process by which unrelated organisms come to resemble one another because they live in similar environments with similar environmental demands – EX: Aquatic birds & swimming mammals like Sharks, dolphins, seals and penguins all live in water environments and have streamlined bodies 17-4 Patterns of Evolution • Coevolution: The process by which two species evolve in response to one another over time. – An evolutionary change in one organism may be followed by a change in another organism • EX: A flower can reproduce only if it attracts a specific type of pollinator 17-4 Patterns of Evolution • Punctuated Equilibrium: Long stable periods of evolutionary equilibrium interrupted by brief periods of more rapid change or evolution – Can occur because: • Small population becomes isolated from the main part of the population • Small group of organisms migrates to a new environment- EX: Darwin’s finches