Fossils of the lobe-finned fish and amphibians of the Devonian period show many similar bones in the limbs. However, these limbs also have more than just bones in common. They also show a gradual change in the structure of the whole limb over geological time. Each different species seemed to have small changes in its general structure, such as bone shapes and the number and position of toes.

This apparent change in species over time is called evolution. Evolution is defined as a genetic change in the characteristics of a species over many generations, resulting in the formation of new species. A generation is the time between the birth of an individual and when that individual produces their own offspring.

The fossil history of the horse is a good example of changes occurring over many generations. Fossil skeletons have been found of a horse-like animal that was about the size of a small dog. The scientific name of the genus of this animal is Hyracotherium (Figure 3.1.2 on page 70). It is not classified as a horse, but is similar enough that biologists consider it to be a likely ancestor of horses. Radioactive dating methods show that Hyracotherium lived about 52 million years ago.

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