As a paleontologist, Jenny Clack wants to find the transitional form between fish and land vertebrates or tetrapods. She spent years teasing a fossil from its rock and studying the fish-like animal that shows evolution in mid-process during the Devonian. It was an animal with both gills and lungs. Out in the field, Dr. Clack tries to understand the lives of transitional forms by studying fossilized footsteps, “an action replay” from deep time.
The complementary nature of structure and function is shown in the presence of both gills and lungs in tetrapods, which could breathe both in water and in air.