Fossils

• Fossils


  Things that have been left behind by long dead organisms, such as skeletons made of rock, impressions in the ground, or entire organisms that haven't decayed yet
• Importance of studying fossils

  • Most organisms that have ever lived are now extinct, so fossils are the only way to learn about them
  • Studying fossils provides evidence for evolution by showing the small incremental changes that took place over millions of years
• Main ways fossils can form

  • Gradual replacement by minerals
  • Casts
  • Impressions
  • Preservation
• Gradual replacement by minerals


  Body parts like bones, teeth, and shells decay slowly, and are gradually replaced by minerals, forming rock-like substances in the exact same shape and size as the original structures
• Casts


  Organisms are buried in a soft material like clay, and as the clay hardens, the organism decays, leaving a gap that's the same size and shape as the organism was
• Impressions


  Organisms leave a mark on the ground, such as footprints, that stays there over time
• Preservation


  Organisms get stuck in amber or tar pits, or are preserved in glaciers or peat bogs, where there is no decay
• The oldest fossils found so far are between 3.5 and 4 billion years old
• We don't have a complete fossil record, with big periods where we don't know what happened
• Many early life forms were soft-bodied, so they would have decayed quickly, making fossil remains unlikely
• Fossils that formed long ago may have been destroyed by geological activity like volcanoes, earthquakes, or tectonic plate movement
• Extinction


  A species has gone extinct when no individuals of that species remain
• Reasons for extinction


  • Environment changes too quickly (habitat destruction, climate change)
  • New predator arrives
  • New disease spreads
  • New species outcompetes for resources
  • Catastrophic event (asteroid impact)