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The wing of a bird, the anterior fin of a dolphin and the arm of a man, although very different, have very similar bone and muscle structure.
The similarity can be explained by assuming that these beings had common ancestors from which they inherited a basic plane of body structure.
The evolutionary kinship between birds and mammals, for example, also explains the similarities between the internal organs of these animals. The heart and the circulatory and nervous system, among others, are made up of the same basic parts.
The similarities between the embryos of certain groups of animals are even greater than the similarities found in adult forms. For example, it is difficult to distinguish young embryos from fish, frogs, turtles, birds and humans, all belonging to the vertebrate group. This similarity can be explained by taking into account that during the embryonic process the basic structural plane of the body, which they all inherited from a common ancestor, is outlined.
Homologous organs or structures
Certain organs or structures develop very similarly in the embryos of all vertebrates. They are the homologous organs. Although they have the same embryonic origin, homologous organs may have different functions, such as the human arm and wing of a bird, for example.
Similar organs or structures
If two organs or structures perform the same function but have a different embryonic origin, they are called analogs. The wings of birds and insects, for example, are similar structures: both serve to fly, but their embryonic origins are totally distinct.