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Theory of Evolution

How scientists developed explanations for evolution

Origins of life

Living organisms on Earth today, and the chemicals which work together to keep them alive, are extremely complex, even in single-celled micro-organisms. Some people find it hard to accept that such complexity could have evolved through natural selection. Some religious people believe that all living things on Earth were made by God, or that life was begun by God but then evolved through natural selection. We will probably never be absolutely certain about how life began, as no one was there to observe it. But scientists must base their theories on evidence.


Darwin was not the only person to develop a theory of evolution. Jean-Baptiste Lamarck was a French scientist who developed an alternative theory at the beginning of the 19th century. His theory involved two ideas. These are:

  • the law of use and disuse
  • the law of inheritance of acquired characteristics

His theory stated that a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring.

This table summarises the two different explanations for giraffes having long necks.

Summary of explanations for giraffes having long necks

A giraffe stretches its neck to reach food high upA giraffe with a longer neck can reach food high up
The giraffe's neck gets longer because it is used a lotThe giraffe is more likely to get enough food to survive and to reproduce
The giraffe's offspring inherit its long neckThe giraffe's offspring inherit its long neck

Lamarck's theory cannot account for all the observations made about life on Earth. For instance, his theory would predict that all organisms gradually become complex, and simple organisms disappear.

Darwin's theory can account for the continued presence of simple organisms.

Back to Life on Earth index

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