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4.5: Structural Evidence for Evolution

Difficulty Level: At Grade Created by: CK-12
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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. Common ancestry can also be determined by looking at the structure of the organism as it first develops.

Vestigial Structures

Some of the most interesting kinds of evidence for evolution are body parts that have lost their use through evolution (Figure below). 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.

Similar Embryos

Some of the oldest evidence of evolution comes from embryology, the study of how organisms develop. An embryo is an animal or plant in its earliest stages of development. This means looking at a plant or animal before it is born or hatched. Centuries ago, people recognized that the embryos of many different species have similar appearances. The embryos of some species are even difficult to tell apart. Many of these animals do not differ much in appearance until they develop further.

Some unexpected traits can appear in animal embryos. For example, human embryos have gill slits just like fish! In fish they develop into gills, but in humans they disappear before birth. The presence of the gill slits suggests that a long time ago humans and fish shared a common ancestor.

The similarities between embryos suggests that these animals are related and have common ancestors. For example, humans did not evolve from chimpanzees. But the similarities between the embryos of both species suggest that we have an ancestor in common with chimpanzees. As our common ancestor evolved, humans and chimpanzees went down different evolutionary paths and developed different traits.


  • common ancestor: An organism from which two or more different organisms evolved a long time ago.
  • embryology: The study of how organisms develop from a fertilized egg.
  • 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.
  • Studying the embryos of organisms also provides evidence that two very different animals could have descended from a common ancestor.


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.


  1. What is a vestigial trait?
  2. How does embryology provide evidence for evolution?

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common ancestor Organism from which two or more different organisms evolved a long time ago.
embryology Study of how organisms develop from a fertilized egg.
vestigial structure Structure that has lost its use through evolution.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Nov 29, 2012
Last Modified:
Aug 30, 2016
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