From Nature:
Our ears could have started evolutionary life as a tube for breathing, say scientists, after examining the ancestral structure in a 370-million-year-old fossil fish. Evolutionary biologists are intrigued by how complicated sensory organs evolved from structures that may have had completely different uses in ancestral creatures. The ear is a relatively easy organ to study. Its evolving bones have been preserved as fossils, whereas the soft tissues of other specialized features, such as eyes and noses, have long decayed.
So Martin Brazeau and Per Ahlberg of Uppsala University in Sweden decided to take a close look at the ear-like features of an ancient, metre-long monster from the Latvian Natural History Museum in Riga. Panderichthys was a fish, but is thought to be closely related to the earliest four-limbed tetrapods that eventually climbed on to land and gave rise to modern vertebrates. The researchers examined Panderichthys and found that the bony structures in its head combine features of fish and tetrapods, capturing a snapshot of evolution in action.
More here.