The The Fossil Record shows change over time. Living animals hold clues too. Compare the skeletons of the Vertebrate Groups and a pattern jumps out.
Key idea
Homologous structures have the same basic bones in the same arrangement, even when they do different jobs, like the arm of a person, the leg of a cat, the flipper of a whale and the wing of a bat. They are evidence that these animals share a common ancestor.
Why it works
If each limb were designed from scratch for its job, a flipper, a wing and a grasping arm would have no reason to share bones. They do: one upper arm bone, two forearm bones, wrist bones, then finger bones. That is what we expect if they were all inherited from one ancestor with that bone plan, and changed over many generations as the descendants took up swimming, flying or grasping.
Worked example: Homologous or not?
A bird's wing and a butterfly's wing both do the same job. Are they homologous?
- A bird wing has the upper arm, forearm, wrist and finger bones of other vertebrates.
- An insect wing has no bones at all: it is a thin sheet of exoskeleton with veins.
- They do the same job but are built differently.
Answer: No. They are analogous structures: similar jobs that evolved separately.
Worked example: A leftover
Whales have no back legs, yet many have small hip and leg bones buried in their bodies. What does this suggest?
- The bones are reduced and do not work as legs.
- Their land-living ancestors had back legs.
Answer: Whales descend from four-legged ancestors; the bones are vestigial structures.
Common mistake
Students think structures that do the same job, like a bird's wing and a butterfly's wing, show close kinship. Similar jobs can evolve separately. The clue to shared ancestry is shared inner structure, not shared function. Check the bones, not the job.