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History of Life on Earth�AP Topic 7.13 – part 2

  • Learning Objectives
    • Describe the history of life on Earth.

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Fossils Tell a Story

  • A fossil is the remains or traces of past life, usually preserved in sedimentary rock.
    • Most dead organisms are consumed by scavengers or decompose
    • Paleontology is the study of fossils and the history of life, ancient climates, and environments
    • Sedimentation has been going on since the Earth was formed; it is an accumulation of particles forming a stratum, a recognizable layer in a stratigraphic sequence laid down on land or in water

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Sequence in rocks�indicates the age of fossils

  • relative dating
    • rock strata is older than the one above it and younger than the one below it
    • same age in England and Russia may have different sediments
      • geologists discovered that strata of the same age contain the same fossils - index fossils
    • does not establish the absolute age of fossils in years

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Absolute Dating of Fossils

  • relies on radioactive dating, or radiometric, to determine the actual age of fossils
    • radioactive isotopes have a half-life
      • time it takes for half of a radioactive isotope to change into a stable element
  • carbon 14 (14C) is a radioactive isotope contained within organic matter
    • half of the carbon 14 (14C) will change to nitrogen 14 (14N) every 5,730 years
      • comparing 14C radioactivity of a fossil to modern organic matter calculates the age of the fossil
      • after 50,000 years, the 14C radioactivity is so low it cannot be used to measure age accurately
  • it is possible to determine the ratio of potassium 40 (40K) and argon 40 to date rocks and infer the age of a fossil

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Geological Timescale

  • Geologists have devised the geological timescale
    • divides the history of Earth into eras, and then periods and epochs
  • Life arose in the Precambrian Era
    • 87% of the geologic time scale
    • early bacteria

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Early Milestones in �Early Earth

  • First prokaryotes 3.8 BYA
  • First photosynthetic prokaryotes 3.46 BYA
  • Anaerobic prokaryotes declines due to oxygen-rich atmosphere 2 BYA
  • First eukaryotes 2.1 BYA
    • endosymbiotic theory

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Prokaryote Fossils of the Precambrian

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Eras in �Geologic Timescale

  • Paleozoic
    • starts with the Cambrian Period (first fossil forming animals)
    • contained three mass extinction events
  • Mesozoic
    • marked with advanced land plants and reptiles
  • Cenozoic
    • marked with the rise of the mammals

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Summary�(Topic 7.13 – part 2)

  • Fossil provide evidence of the great age of the Earth
    • relative dating uses the position of fossils in the rock strata
  • Radioactive decay helps to determine the absolute date of fossils
  • The geologic time scale helps to determine the speciation and extinction of groups of species

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