Why are vestigial structures considered critical evidence of evolution?

Vestigial structures are considered evidence since scientists believe they are structures that were once used by a species ancestors but are no longer needed. They do not impair the organisms in any way, so there is no need for evolution to get rid of them. Vestigial structures are anatomical features that no longer seems to have a function or purpose in the current form of an organism of the given species. Examples are wisdom teeth, the muscles of the ear, the appendix, the tail bone among others in human. 

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