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Fossils and evolution

Fossils and evolution

  • Fossils are the preserved traces of organisms that lived millions of years ago.
  • Fossils are formed in 3 ways:
    • A buried organism in soft material such as clay decays and leaves a cast/imprint of the organism. Footprints can be preserved this way.
    • When no decay happens due to preservation such as in peat bogs (too acidic), glaciers (too cold) and sticky tree resin which traps organisms (no oxygen and water). Bacteria cannot live in these conditions and therefore no decomposition.
    • A gradual replacement by minerals in bones, teeth and shells, forming rock like substance. Fossil remains in the rock.
  • Fossils in rock layers tell us three things:
    • What the creatures/plants looked like
    • How long ago they existed: deeper the rock, the older the fossil
    • How the creature has evolved. From studying the similarities and differences in differently aged rocks we can see how species have changed and developed over billions of years.

Fossil record is incomplete

  • There are many gaps in the fossil record due to a number of reasons:
    • Soft tissues decay so soft-bodied organisms leave little fossil evidence behind.
    • Other organisms did not form fossils because the hard parts were destroyed.
    • Not all fossils have been found.
  • This means scientists have to interpret how organisms have changed over time using incomplete data.
  • An example of the effect of this is the Megalosaurus fossils. In 1954 scientists believed it to have a huge head and walked on four legs. This is because not all of the bones were found.
  • In our modern interpretation scientists have concluded that it had a small head and walked on two legs. This was due to better fossil evidence.

The pentadactyl limb

  • A pentadactyl limb has five digits. Interestingly this bone structure has been found in many species.
  • The limb on the outside may look different (and have a different function) but on the inside the bone structure is remarkably similar.
  • These bones are called homologous structures.
  • This similarity suggests a common ancestor, many millions of years ago. If these animals had evolved from different ancestors, it would be highly unlikely they would share the same bone structure.