Vestigial Structures
It has long been known that humans, and indeed primates such as the orangutan and chimpanzee have ear muscles that are minimally developed and non-functional, yet still large enough to be easily identifiable. These undeveloped muscles are vestigial structures. An ear muscle that cannot move the ear, for whatever reason, can no longer be said to have any biological function. This serves as evidence of homology between related species. In humans there is variability in these muscles, such that some people are able to move their ears in various directions, and it has been said that it may be possible for others to gain such movement by repeated trials. In such primates the inability to move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an ability which is not common to most monkeys—a function once provided by one structure is now replaced by another.
The outer structure of the ear also shows some vestigial features, such as the node or point on the helix of the ear known as Darwin's tubercle which is found in around 10% of the population, this feature is labelled (a) in the accompanying figure.
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