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Vestigial Structures


Why do you have a tail bone?

If you look closely at a skeleton, you might notice a triangular bone at the end of the spinal column. This is your tailbone. Why would you have a tailbone when you don’t have a tail? You have a tailbone because your ancient ancestors did have a tail. These sorts of “left-over” structures support the theory of evolution.

Structural Evidence

Even though two different species may not look similar, they may have similar internal structures that suggest they have a common ancestor. That means both evolved from the same ancestor organism a long time ago.


Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution. For example, most birds need their wings to fly. But the wings of an ostrich have lost their original use. Structures that have lost their use through evolution are called vestigial structures. They provide evidence for evolution because they suggest that an organism changed from using the structure to not using the structure, or using it for a different purpose. Penguins also do not use their wings to fly in the air. However, they do use them to move in the water. The theory of evolution suggests that penguins evolved to use their wings for a different purpose. A whale’s pelvic bones, which were once attached to legs, are also vestigial structures. Whales are descended from land-dwelling ancestors that had legs.

Mole rats live under ground where they do not need eyes to find their way around. This mole’s eyes are covered by skin. Body parts that do not serve their original function are vestigial structures.


  • common ancestor: Organism from which two or more different organisms evolved a long time ago.
  • vestigial structure: Structure that has lost its use through evolution.


  • Vestigial structures, or structures that have lost their use through evolution, are important evidence of evolution.


Use the resource below to answer the questions that follow.

  1. Are all cormorants flightless?
  2. What is the vestigial trait most obvious in the flightless cormorant?
  3. If being flightless is an advantage for this cormorant, then why aren’t all cormorants flightless? Think carefully about this question, and be as complete in your answer as you can.

(Information for this blog page sourced verbatim from:


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Fossil Evidence
Vestigial Structures
Homologous Structures

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