An organ that is fully developed and functional in an ancestral organism that is no longer pertinent in the recently evolved organism is called a vestigial organ.
Vestigial organs in an organism prove that it is descended from the organism that had a fully functional organ that has become vestigial.
There are 180 vestigial organs in humans.
Examples of vestigial organs in humans include wisdom teeth, vermiform appendix, ear pinna etc.
The wisdom teeth have become vestigial as the human jaw started to become narrower.
Hence, an example of a vestigial organ is wisdom teeth.