Ears, as we know it, helps us in hearing. In animals, the ear is depicted as having three parts—the outer ear, the middle, and the inner ear. The outer ear comprises an ear canal and the pinna. Since the outer ear is the only noticeable portion of the ear in most animals, the word “ear” often refers to the outer part alone. The middle ear includes the tympanic cavity and the three ossicles. The inner ear sits in the bony labyrinth and contains structures that are essential to several senses.
The evolution of mammalian auditory ossicles was an evolutionary event that followed the development of the bones of the mammalian middle ear.
Let’s see how ears evolved in mammals.
Your gift to hear relies on a formation that got its origin as a gill opening in fish. In other words, ears evolved from gills.
Humans and other mammals have particular bones in their ears that are important in hearing. Ancient fish used identical structures to breathe in water.
Experts had previously thought the evolutionary change transpired after animals had proved themselves on land. However, a new look at an old fossil suggests that ear development was set into motion before any creature crept out of the river.
Scientists studied the ear bones of a close cousin of the first land animals, a fossil fish called Panderichthys. They compared these arrangements to those of another lobe-finned fish and an early land animal and concluded that Panderichthys displays a transitional form.
In another fish, Eusthenopteron, a tiny bone called the hyomandibula, evolved a curve and hindered the gill opening, called a spiracle.
However, in early land animals such as the tetrapod Acanthostega, this bone shrunk, forming a larger cavity in what is now part of the middle ear in humans and other animals.
This is how ears were formed. However, evolution didn’t stop right there. It blessed us with a sensation called ‘hearing.’
Hearing is the sense to recognize sounds by detecting vibrations, variations in the pressure, through an organ such as ears.
The vertebrate hearing evolved as a change to allow animals to sense the acoustic scene. The hearing became possible to allow fish to perform fundamental tasks like detecting prey and predators. Although the middle bone of the ear evolved way before mammals could walk on land, the hearing evolved in the Triassic period, about 100 million years after the transition of the vertebrates from a sea to an earthly habitat in Carboniferous.
Source: Luo Z (2011). “Developmental Patterns in Mesozoic Evolution of Mammal Ears”. Annual Review of Ecology, Evolution, and Systematics, Shubin N (2008). “Chapter 10: Ears”. Your inner fish: a journey into the 3.5-billion-year history of the human body. New York: Pantheon Books.