The ear is a physically complex sense organ, especially in mammals. Because some of the ear’s structure involves bones, much of the evolution of the ear can be followed through the fossil record. The evolution of the mammalian ear is well documented and makes a nice evolutionary story.

The ear consists of three parts: inner, middle, outer ear

Some vertebrates lack middle and outer ear (Chondrichthyes, Osteichthyes).

Middle ear appeared in amphibians.

Reptiles, birds, mammals have all three.

Is the ancient part of the ear, originally serving to detect position and later direction of movement. Detection of sound was superimposed afterwards.

Was added in amphibians and serves the primary function of aiding in hearing in air.

Was the final addition of the reptiles and was further elaborated in mammals. Functions to assist in hearing.

Lateral line system, present in fish, larval amphibians, aquatic adult amphibians, is related to inner ear by structure, function & development.

The system consists of thousands of microscopic organs, neuromasts, sensory patches of cells scattered over the surface of the skin and located in shallow pits along canals in the skin, especially along the ‘lateral line canal’.

Sensory hair cells within neuromasts have sensory fibers on the exposed surface, topped by a dome of flexible extracellular material - cupula

Motion of water causes cupula to pivot, tilting the sensory hairs.

Fish can sense motion of water and low frequency sound waves that cause displacement of water.

Embryologically, the inner ear first appears as ectodermal thickening which sink into ectodermal tissue, forming a pit, which then pinches off to become the hollow otic vesicle.

As development progresses, the vesicle assumes the complicated shape of its adult form and is called the labyrinth.

Three general regions of the labyrinth:

vestibule, adjacent to the oval window

three looping semi-circular canals

elaborate coiled cochlea

Labyrinth is filled with fluid endolymph.

Labyrinth is surrounded by perilymph and then by bone or cartilage.

Perilymph cushions labyrinth and assists in transmitting vibrations in tetrapods.

Within the cochlea are patches of sensory cells, the maculae, resembling the neuromasts of the lateral line system.

A calcareous granule, otolith, sits in a patch of sensory cells.

When the head is moved to a new position, the otolith slides over the maculae and the change in position is noted.

Motion of the head causes fluid to slip in the semicircular canals.

The moving fluid stimulates patches of sensory cells, cristae, conveying information about position and movement.

In the evolution of the middle ear, the spiracle gill pouch and the hyomandibular bone play principle roles.

We have already considered the evolution of the hyomandibular, quadrate and articular bones associated with the jaws of fishes.

Extinct amphibians probably rested their large heads on the ground and detected vibrations in the substrate by conducting sound waves through the bones of the jaw and other structures of the skull to the inner ear.

Vibration via the jaw and skull would have stimulated the patches of sensory cells (maculae of inner ear).

This mechanism was improved by incorporating a reduced stubby hyomandibular bone into the line of transmission.

The hyomandibular became an ear ossicle and is known in tetrapods except mammals as the columella.

In early vertebrates, jaws articulated at the articular (lower) and quadrate (upper) bones. In early mammals, or mammal-like reptiles (therapsids), the jaws articulated more anteriorly and a new joint formed, the dentary - squamosal articulation.

The old elements, articular and quadrate, were now superfluous.

The quadrate became the incus and the articular the malleus.

Columella modified a bit and now called the stapes.

One last bone from the lower jaw got incorporated into the ear structure.

Tympanic bone comes from angular bone of old lower jaw

Forms dome shaped auditory bulla at back of ventral side of skull

This dome encloses the inner and middle ears.

In development of middle ear, the spiracular pouch of the embryo forms the middle ear cavity with a connection to the pharynx via the eustachian tube, allowing equalization of pressure between outside and the middle ear.

The other end of the pouch doesn’t break through to the skin but forms a thin membrane, the tympanum or eardrum.

Inpushing from the surface of the embryo occurs during the formation of the ear and forms the external auditory meatus (reptiles and up) or ear canal.

No outer ear existed in amphibians. The tympanum, if present, lies flush with the body surface.

In reptiles, some groups have an outer ear but it consists only of the external auditory meatus.

All birds and mammals (except some aquatic forms) have an external auditory meatus, longer than that of reptiles.

In mammals, a cartilaginous pinna is usually present about the external opening to amplify and channel sound waves.

Evolution of the ear is primarily a result of adaptation to terrestrial environments, brought about by selective pressures to sense noises and vibrations in the environment.

Allowed the detection of predators and prey, eventually led to communication between individuals.

The evolution of the middle ear illustrates the changes in functions that can occur for a single structure.

hyomandibular bone / cartilage ----> columella ----> stapes

feeding ----> respiration ----> feeding ----> hearing