Evidence for Evolution

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  • evidence for evolution
    • a fossil is any trace of an animal or plant that lived and died long ago
    • fossils can be formed in three ways:
      • from gradual replacement by minerals
        • things like teeth, shells and bones don't decay easily so they can last for ages when buried.
          • they are then eventually replaced by minerals, forming a rock-like substance shaped like the original hard part.
            • the fossil stays distinct inside rock until it is dug up
      • from casts and impressions
        • fossils can form when an organisms buried in soft material like clay.
          • the clay then hardens around it and the organism decays, leaving a cast of itself
            • things like footprints can be pressed into these soft materials, leaving an impression when it hardens
      • from preservation in places where no decay happens
        • this is because the conditions arent suitable for microbes to work
          • in glaciers it is too cold
          • in peat bogs it is too acidic
          • in amber there is no oxygen or moisture
    • fossils found in rock layers can tell us three things:
      • what the creatures and plants used to look like
      • how long ago they existed
        • generally the deeper the rock, the older the fossil
      • how they have evolved
        • from studying the similarities and differences between fossils in differently aged rocks, we can see how species have changed and developed over billions of years
    • the fossil record is incomplete, this is because:
      • very few dead plants or animals actually turn into fossils, most just decay away.
      • some body parts, like soft tissue, tend to decay away completely
      • there are fossils yet to be discovered that might help complete the overall picture
    • the pentadactyl limb is evidence for evolution
      • a limb with five digits
      • in many species, mammals, reptiles, amphibians
        • in these species the pentadactyl limb has a similar bone structure but has a different function
      • the similarity in bone structure provides evidence that species with a pentadactyl limb all derived from a common ancestor.


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