Why are the auditory and vestibular systems evolutionarily built into one vestibulocochlear organ?

Answer

Dear Levin,

it simply has to do with the fact that both systems develop within the same organ during embryogenesis. In other words, they have always been present together in the course of evolution. During development, a paired ear vesicle is formed (small vesicle filled with fluid, left and right in the head), which will further develop into a complex structure in which one specific type of sensory cells will be formed: hair cells. These are then placed in three places in that ear vesicle, and will all be responsible for detecting a displacement. Only the way in which displacements are measured will differ between the three sets of hair cells, so that different types of stimuli can be distinguished with this one system. The first set in the vestibular portion (which is the upper part of what will develop from that ear vesicle) has hair cells that detect movement of the fluid in that vesicle when the head will turn (these are called rotation receptors). The second set is also located in the vestibular part (but slightly deeper) and has similar hair cells, but with the help of crystals, they are able to detect accelerations and decelerations, as well as tilts of the head in relation to the gravitational field (these are the so-called statoreceptors). The third set is then located in the lower part of the ear vesicle, and this is the auditory part (or cochlear part). This is so called because here the hair cells are responsible for detecting movements that are translated into sound perception (but in essence they do almost the same as the other sets) (these are the so-called phonoreceptors).

In summary: vestibular organ and auditory organ are actually one and the same organ that contains one type of sensory organs (hair cells), but can detect different types of movements. That is why in this one vestibulocochlear organ those different functions were always combined during evolution (and therefore they did not originate from two different organs that then came together or something).