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 that stays there over time, such as footprints
• Preservation

  Organisms get stuck in amber or tar pits, or in glaciers or peat bogs, and because there's no oxygen or moisture, no decay can take place, and the organism remains completely intact
• The oldest fossils found so far are between three and a half to four billion years old
• We don't have a complete fossil record, and there are currently big periods where we don't really know what happened
• Many early life forms were soft-bodied, so they would have decayed quickly, making any fossil remains unlikely
• Fossils that formed so long ago may have been destroyed by volcanoes, earthquakes, or the movement of tectonic plates
• Extinction

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

  • Environment changes too quickly (e.g. habitat destruction, climate change)
  • New predator arrives and kills them
  • New disease spreads through the population
  • New species outcompetes them for resources
• The asteroid that hit Mexico 66 million years ago wiped out the dinosaurs