Evolution of sound localisation in land vertebrates.

The story of the evolution of hearing in land vertebrates is fascinating but complex. The water-to-land transition changed the physical environment in which hearing happens so dramatically that both the peripheral receptor structures and the central auditory circuits underwent a revolution, leading to the sensitive hearing of higher-frequency airborne sound. This (r)evolution took a very long time indeed. Most of it happened after the early divergence of the major clades of land vertebrates. Hearing, at least hearing as we commonly understand it today, is the youngest of the major senses and much of its evolutionary history is not shared between amphibians, lepidosauromorphs (lizards and snakes), archosauromorphs (birds and crocodilians) and mammals. There was no linear evolution of complexity from 'lower' to 'higher' vertebrates. We are only just beginning to appreciate the implications of this for central auditory processing. There is no consensus, yet, on the evolution of sound localisation. The multitude of physical cues involved in sound localisation means that different selective pressures interact and need to be considered. The use and neural processing of interaural time differences is just one example. It has taught us that long-standing assumptions, such as the homology of the mammalian medial superior olive and the avian nucleus laminaris, need to be questioned and that important insights may arise from unexpected directions, such as the paleontology of middle-ear ossicles. There is still much to discover.