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Homology: From jaws to ears — an unusual example of a homology

You (and all living mammals) have one bone in your jaw and three bones inside your ear. Lizards, on the other hand, have many bones in their jaw and one bone inside their ears. What's the relationship between these bones? Which ones are homologous, if any? To figure out the answer to these questions, scientists studied both living organisms and fossils.

lizard and mammal skulls, with ear bones

Biologists found that as a baby mammal grows in its mother's womb, two bones in the skull become ear bones. However, as a baby lizard develops in its egg, the same two bones become part of the jaw! This is strong evidence that these mammal ear bones are homologous to jaw bones in lizards! Compare the colors of the bones in the diagrams below to find out which mammalian and lizard bones develop from the same structures.

lizard and mammal skulls, with ear bones

Furthermore, paleontologists have found fossils that show how the jaw bones of a lizard-like ancestor evolved into the ear bones of modern mammals. This is even more evidence that these bones are homologous. Even though they have different shapes today, mammalian ear bones and lizard jaw bones both evolved from the same jaw bones in their lizard-like ancestors.