Science: 4: Explaining evolution 4 – What is a fossil?

Here is the 4th in our series of science blogs explaining evolution.

Explaining evolution 4 – What is a fossil?

The blooming of a flower is a relatively short event in the life of a plant. The appearance of the colourful bloom can be extended by pressing the flower between dry pieces of paper that absorb water from the flower which halts decay due to decomposition of plant materials and the effect of external microorganisms. However, drying only slows down decomposition and eventually the flower turns to dust. All living organisms eventually die and their bodies decay into matter that can be recycled as raw material for new life.

A fossil is different because the dead organism has lasted long enough to form an imprint in an inert material (e.g. sand, clay, tar, etc) that is compressed and hardened over millions of years to become rock. Splitting the rock reveals an imprint of a flattened plant or animal whose harder parts have formed a pattern in the rock layer. The imprint is often coloured due to organic materials causing crystallisation in the inert material. Some larger organisms (e.g. dinosaurs) had large, strong skeletons that decayed very slowly turning to large crystal rocks, slightly different from the surrounding rock, which are accurate copies of the original skeleton shape. Volcanic ash has a similar effect as can be seen in the stone human victims of the AD 79 eruption of Vesuvius at Pompeii.

Fossil formation requires optimum conditions over millions of years, so it is amazing that there are so many fossils in layers of rock that can be accurately dated. This fossil record is evidence of the development and demise of living species and it supports the concept of evolution. For example, there is fossil evidence that shows the evolution of the horse as a distinct species over the past 50 million years. However, fossil evidence is incomplete and there are gaps – but it is true to say that no fossil has been discovered in a rock layer which can be dated before its initial development. For example, no fossils of the primitive horse have been discovered in rocks that are more than 50 million years. Furthermore, the fossil record is evidence of how species have changed in the course of their development. Nineteenth century scientists, who were given initial fossil evidence of species development, were now faced with the question: ‘How do new species of plants and animals develop?’ (see the next blog, no. 5: Explaining evolution – What is a mutation?)

John Roach


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