When changes in the environment occur, plants and animals may not survive and die out. Scientists can use fossil records to show how organisms are able to thrive, flourish then, eventually, become extinct. These records can also be used to show how new species are formed. However, the records are incomplete.

By the end of this section you should understand:

• the reasons why scientists are unable to explain exactly how life began on earth

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Old and New Species

We’re able to find evidence for the earliest forms of life by using fossils. Fossils can be found in rocks and the preserved remains of dead organisms.

There are a number of ways in which fossils can be formed:

• they could be formed from the hard parts of animals which don’t decay very easily like bones or shells
• it’s possible for parts of an organism to be preserved intact in special conditions, for example in ice, a peat bog or amber
• as an organism decays these parts could be replaced by another material
• it could be in the form of a preserved trace like a footprint or a burrow

Not all forms of animal have been preserved in this way. Many forms of very early life were soft-bodied and so there are hardly any traces of the organisms left. Most traces have been lost due to geological activity.

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The Fossil Record

Using the fossil record we can find out how different species have changed, or not, over time. It provides us with an interesting insight into the evolution of life on Earth. It’s very much in support of Darwin’s theory that life began with very simple organisms. This is because these simple organisms have been found in the oldest rocks while more complex ones have been discovered in the newer rocks.

There are no eyewitnesses to evolution or how life began on Earth and so scientists can’t be completely certain of either: they only make educated guesses on the evidence available to them and, to date, Darwin’s theory is the best available so far.

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HIGHER TIER:

• Three other ways in which speciation can occur are:
  • Genetic variation: each of the populations contains a wide range of alleles which control the characteristics of each.
  • Natural selection: the alleles which help the organism to survive in its environment are the ones which survive.
  • Speciation: interbreeding is no longer possible because each population becomes so different from the other.