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 (single-celled) 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