Fossils

• Fossils


  The remains of organisms from many thousands of years ago, which are found in rocks
• Fossils


  • They provide the evidence that organisms lived ages ago
  • They can tell us a lot about how much or how little organisms have changed (evolved) over time
• How fossils form


  1. Gradual replacement by minerals

  2. Casts and impressions

  3. Preservation in places where no decay happens
• Gradual replacement by minerals

  • Things like teeth, shells, bones etc., which don't decay easily, can last a long time when buried
  • They're eventually replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part
  • The surrounding sediments also turn to rock, but the fossil stays distinct inside the rock and eventually someone digs it up
• Casts and impressions


  • An organism is buried in a soft material like clay, the clay later hardens around it and the organism decays, leaving a cast of itself
  • Animal's burrow or a plant's roots (rootlet traces) can be preserved as casts
  • Footprints can also be pressed into these materials when soft, leaving an impression when it hardens
• Preservation in places where no decay happens


  • In amber (a clear yellow 'stone' made from fossilised resin) and tar pits there's no oxygen or moisture so decay microbes can't survive
  • In glaciers it's too cold for the decay microbes to work
  • Peat bogs are too acidic for decay microbes
• No one knows how life began
• Hypotheses on how life first came into being

  • First life forms came into existence in a primordial swamp (or under the sea) here on Earth
  • Simple organic molecules were brought to Earth on comets-these could have then become more complex organic molecules, and eventually very simple life forms
• These hypotheses can't be supported or disproved because there's a lack of good, valid evidence
• Many early forms of life were soft-bodied, and soft tissue tends to decay away completely-so the fossil record is incomplete
• Fossils that did form millions of years ago may have been destroyed by geological activity, e.g. the movement of tectonic plates may have crushed fossils already formed in the rock