Fossils (AQA GCSE Biology): Revision Note

How are fossils formed?

• Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks

• Fossils may be formed in several ways:

1. From parts of organisms that have not decayed because one or more of the conditions needed for decay are absent.

   • In amber (fossilised tree resin) and tar pits a lack of oxygen and moisture means that decomposers cannot survive there to break down dead organisms.

   • In glaciers and permafrost regions (areas that continuously remain frozen) it is too cold for decomposers to survive.

   • Peat bogs are too acidic for decomposers to survive.

2. When parts of the organism are replaced by minerals as they decay.

   • Body parts such as teeth, shells and bones don’t decay easily and can last a long time after the organism has died.

   • They are slowly replaced by minerals that form a rock-like substance in the same shape as the original body part.

   • They can often be seen distinctly within the surrounding rock.

3. As preserved traces of organisms, such as footprints, burrows and rootlet traces.

   • An impression (e.g. from footprints) can be left in a soft material like clay.

   • The impression becomes fixed as the clay hardens.

   • Sometimes an organism may be buried in clay but decays after the clay has hardened, leaving a cast of the organism.

Gaps in the fossil record

• Many early forms of life were soft-bodied, which means that they have left few traces behind – soft tissues often decay fully, leaving no trace in the fossil record

• What traces there were have been mainly destroyed by geological activity (e.g. tectonic plate movements may have crushed fossils that had already formed)

• This is why scientists cannot be certain about how life began on Earth

Changing planet

• Evidence for early life forms on Earth can be found in the fossil record

• Fossils show how much or how little different organisms have changed as life developed on Earth

Evolutionary trees

• Evolutionary trees are diagrams that show the relationship between species over evolutionary time

• A new branch in the tree shows where speciation has occurred (when a new species has evolved)

• In the evolutionary tree below, for example:

  • Chimpanzees and bonobos share a recent common ancestor. Chimpanzees are therefore most similar to bonobos (more similar than they are to any other primate species)

  • Humans share a more recent common ancestor with gorillas than they do with orangutans – this means we are more closely related to gorillas than we are to orangutans

  • All five primate species shown here share a common ancestor (from the distant past)

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