A recent study makes a compelling evolutionary argument that the vestibular system, specifically the otolith organs, are the primary component of the labyrinth and that the cochlea, and in some respects, the semicircular canals take second place.
Ramos de Miguel et al (2019) ascertain that detection of gravity is essential for complex organisms, plants, and animals alike. They report that even flowering plants have “statocytes,” which provide gravitational information to the plant.
Similar statoliths have also been found in jelly fish specimens dating back to the Ediacaren Period (635 million years ago). Fast forward to 400 million years in the past and the first evidence of a macula and a single semicircular canal are found in a jawless fish. It is not until 200 million years ago that we see the appearance of mammals and the cochlea-vestibular system we are familiar with.
The study also highlights the remarkable gravitational-magnetometer, the lagena, a specialized portion of vestibular system present in many fish and birds, which is responsible for assisting in migratory behaviors of these species.
The authors conclude that in addition to evolutionary evidence, the rarity of otolith specific disorders, and their ability to detect linear, and angular acceleration, further strengthens the superiority of the otolith organs in the modern inner labyrinth system. Perhaps we should be calling ourselves “otolithologists?”
Ramos de Miguel A, Zarowski A, Sluydts M, Ramos Macias A, Wuyts FL. (2020) The Superiority of the Otolith System. Audiol Neurootol 25(1-2):35-41. doi:10.1159/000504595.
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