The Evolution of Hearing from Amphibians to Mammals

MPF - (Originally written November 2004)

Introduction

The ear is perhaps the most complex of the sensing organs (Lombard, 1988; Bolt, 1988). It is made up of dozens of parts and thousands of pieces that all must work collectively in order for sound to be perceived. The ear allows for: communication over distances, sensing danger and, in some species, determining their surroundings. Not only does the ear give animals a sense of hearing, but it also gives them a sense of balance. With all the complexity and utility ears have, it’s remarkable that they were the last paired organs to develop during evolution (Manley, 1998). This paper will trace the evolution of hearing from early amphibians to modern mammals.

Sound Basics

The most obvious purpose of the ear is to detect sound. Sound is nothing more than a compression wave that travels through a medium. For land vertebrates, the medium of the sound that is perceived is generally air. Even the tiniest disturbance to the air is enough to cause a compression wave that propagates in all directions. Sound waves can also travel through water and the ground. In fact, sound travels better through water and the ground than in air because water and rock have higher densities. For sustained sound the medium must vibrate, that is, the compression wave must constantly change in amplitude. If the speed of this compression change, called frequency, and the amplitude is within an animals hearing threshold, the animal will hear the sound (Sound Reception Article, Encyclopaedia Britannica Website). In the most basic sense, hearing is caused by the vibration in a surrounding medium to resonate some part of an animal’s body. This resonance is converted into electrical signals through some means that can then be interpreted by the animal’s ear. The way in which animals perceive sound has changed a great deal since the ear first appeared.

Here Come The land Vertebrates!

Amniotic land vertebrates, that are: birds, reptiles and mammals, evolved from primitive amphibians that evolved from even more primitive fish (Fay, 1980; Popper, 1980). The beginnings of what would later form the inner ear in the amniotes probably first began in the earliest amphibians around in the Devonian Period (The Geological Time Scale, Back Yard Nature Website). There is much debate as to whether the lateral line system, a system of sensors used to sense changes in pressure in fish and amphibians (Lateral Line System Article, Encyclopaedia Britannica Website), eventually developed into the beginnings of the inner ear in amphibians or if the inner ear developed independently in amphibians (Wever, 1974; Northcutt, 1980; Streit, 2001). In any case, the way in which the early amphibian and fish ear operates was very similar.

FIGURE 1 The basic anatomy of an early amphibian ear.

(Modified from Amphibian Ear Diagram, University of Maryland Life Sciences Website)

The anatomy of the first amphibian ears (Fig. 1) was quite a bit simpler than modern ears. It is believed that early amphibians and even some amphibians living today, such as salamanders, heard by picking up vibrations through the ground and water. These vibrations would be picked up by the creature’s entire body but mostly by the dense parts such as bones. As the vibrations travelled through the body, they would reach the inner ear. This primitive inner ear contained a fluid filled region that was surrounded by dense bone. The vibrations would cause the dense bone to resonate, which in turn would transfer the vibrations to the fluid. Tiny hairs in the fluid region would convert the fluid vibration into electrical signals that would travel through nerves to the brain. The brain would then interpret these signals as sounds. This structure was sufficient in the water because the vibrational energy in water was adequate to cause the dense bones to vibrate. In air, however, the vibrations have much less energy and are therefore more difficult to interpret sounds with this structure. Indeed the first amphibians, like modern salamanders, probably heard the world by picking up vibrations through their skulls from the ground (Ear Article, Encarta Online Encyclopaedia Website). Regardless of these limitations, it is clear that vertebrates had the early stages of an inner ear shortly after they crawled up on to the land (Manley, 1998).

Foundation of the amniotic ear and evolution in early reptiles

Reptiles, and thus amniotes, first evolved from amphibians in the mid-Carboniferous Period (The Geological Time Scale, Back Yard Nature Website). The hearing organ, called the basilar papilla in amniotic land vertebrates, developed from the hearing organ in amphibians (Manley, 1998). By the time that reptiles appeared, these hearing organs were increasing in complexity. However, the first ears were still not well suited for hearing through air. Perhaps now would be a good time to introduce the basic anatomy of a reptile ear.

FIGURE 2 Typical lizard middle ear (Turner, 1980;
Adapted from Weiss, Mulroy, and Altmann 1974).

FIGURE 3 Typical lizard inner ear (Turner, 1980;
Adapted from Weiss, Mulroy, and Altmann 1974).

The first amniotic ears were not as complex as the diagrams (Fig. 2 and Fig. 3), but they did show a progression in ear structure and how the amniotic land vertebrates hear. The first change, as aforementioned, was the development of the basilar papilla in the cochlear duct (Fig. 3). This was really just a variation in the structure of the hearing organ of amphibians. It has been suggested that the cochlear duct had either developed out of an outpocketing from the posterior saccular wall of amphibians (Lombard, 1980), or had developed on their own in reptiles (Miller, 1980). Over time, more hair cells would be added to this region allowing for clearer sound interpretation (Manley, 1998). The major breakthrough in ears came in the Triassic period with the development of the tympanic membrane (Fig. 2) or ‘ear drum’ in reptiles, as well as independently in the other amniotic land vertebrates (Manley and Köppl, 1998). This development now gave amniotes a middle ear in addition to their inner ear structure where before they only had an inner ear. This middle ear allowed vibrations in the air to be detected by amniotes with a vast improvement in clarity than was possible before. The tympanic membrane separated the outside world from the ear. When vibrations occurred in the air, the tympanic membrane would also vibrate. This vibration was then transferred to a tiny bone, the stapes (Fig. 3) that had developed which would then cause the dense bone around the fluid filled region to vibrate. The fluid vibration would then be detected by tiny hairs, which would send signals via nerves to the brain.

“My, what big ears you have!” – Evolution of Mammalian Ears

The first mammal-like reptiles evolved from reptiles during the Carboniferous period. These “proto-mammals”, however, had more in common with reptiles than true mammals. True mammals did not develop until the Triassic period. One of the defining characteristics to develop that separated true mammals from mammal-like reptiles was the development of the three-ossicle middle ear. This advancement allowed mammals to hear a vast range of frequencies that would prove tremendously significant for the development of auditory communication (Manley, 1998). It was also during this period that the tympanic membrane or ‘eardrum’ developed independently in each branch of the amniotes (Manley and Köppl, 1998). During the course of evolution, more specialized hair cells developed in mammals allowing for greater sound clarity. Eventually, the cochlear duct coiled in some mammals, the placentals and marsupials. This occurred sometime during the Cretaceous at about the time monotremes (e.g. duckbilled platypuses) branched off from the plecentals and marsupials (Manley and Köppl, 1998). Sometime after the development of the middle ear, the outer ear developed. The outer ear is made up of two basic parts: The ear canal, and the pinna. The pinna is the external protruding part of the ear found in most mammals. It collects the sound and directs it into the ear canal where it then enters the middle ear. The pinna is not only important in collecting sound, but it also allows animals to determine the direction from which a sound emanated more easily (Sound Reception Article, Encyclopaedia Britannica Website).

The Modern Mammalian Ear

Towards the end of the Cretaceous, all the major parts of the amniotic land vertebrate ears had been established. Over the next 65 million years, the evolution of the ear was limited mostly to the specialization of existing parts, rather than the development of new ones. The culmination of all this evolution can be found in the ears of mammals with great hearing ranges in both frequency and amplitude. One such example, the most familiar one to us, is the human ear.

FIGURE 4 (Ear Anatomy Diagram, Enchanted Learning Website) Shows the anatomy of the human ear.

When sound first reaches the ear, it encounters the pinna. The pinna collects the sound and directs it into the outer ear canal. At the end of the ear canal, the sound encounters the tympanic membrane or eardrum. This marks the end of the outer ear and the beginning of the middle ear. When the sound hits the eardrum, it causes the eardrum to vibrate at the same frequency as the sound. This vibration gets transferred through a series of bones, the three-ossicles, which then transfer the sound to the cochlea. The cochlea is coil shaped and contains fluid and thousands of tiny hair cells. As the fluid in the cochlea vibrates, it travels through the cochlea and the hairs move along with the vibration. This movement is then converted into electrical signals that travel through nerves and into the brain where they are processed. This someowhat-complicated process allows for hearing (Sound Reception Article, Encyclopaedia Britannica Website).

The Usefulness of Hearing

The development of ears allowed animals to receive information at a distance. Unlike eyes, ears can detect phenomenon in any direction without having to be focused on it. Not only are ears used by animals to observe their surroundings, they are also vital, for communication.

The coiled cochlea and the three-ossicle system in mammals gives them a great range in hearing ability and thus allows for more complex communication. Some mammals, such as bats, also have the ability to use sound to locate objects. These animals developed the ability to transmit high frequency sounds that are then reflected off of objects and picked up by the ear (Sound Reception Article, Encyclopaedia Britannica Website). This echolocation is an advanced trait that could only have been possible with the development of the ear.

Conclusion

The evolution of the ear from the first amphibians to modern mammals is a fascinating one that stretches over 300 million years. The ears are an important feature that many animals depend on for survival. While this paper did not cover every detail in the evolution of ears, it does illustrate that ear development was a fairly complex and significant process in the evolution of mammals.

References

Lombard, R.E., Bolt, J.R., 1988, The Evolution of the Stapes in Paleozoic Tetrapods, Chapter 3 in: The Evolution of the Amphibian Auditory System. Wiley-Interscience. Toronto. 37 p.

Manley, G.A., 1998, Design Plasticity in the Evolution of the Amniote Hearing Organ, Chaper 1 in: Deutsche Forschungsgemeinschaft Auditory Worlds: Sensory Analysis and Perception in Animals. Wiley-VCH. Bonn 7-17 p.

Sound Reception Article, Encyclopaedia Britannica Website http://www.britannica.com/eb/print?tocId=9109525&fullArticle=true

Fay, R.R., Popper, A.N., 1980, Structures and Functions in Teleost Auditory Systems, Chapter 1 in: Comparative Studies of Hearing in Vertebrates. Springer-Verlag. New York. 3-35 p.

The Geological Time Scale, Back Yard Nature Website

http://www.backyardnature.net/g/geo-time.htm

Lateral Line System Article, Encyclopaedia Britannica Website

http://www.britannica.com/eb/article?tocId=9047277

Wever, E.G., 1974, The Evolution of Vertebrate Hearing in: the Handbook of Sensory Physiology Vol. V/1. Springer. Berlin. 423-454 p.

Northcutt, R.G, 1980, Central Auditory Pathways in Anamniotic Vertebrates, Chater 3 in: Comparative Studies of Hearing in Vertebrates. Springer-Verlag. New York. 79-90 p.

Streit, A., 2001, Origin of the Vertebrate Inner Ear: Evolution and Induction of the Otic Placode. Journal of Anatomy, Volume 199 Issue 1-2, 99-103 p.

Wilkinson , G. S, Amphibian Ear Diagram, University of Maryland Life Sciences Website

http://www.life.umd.edu/faculty/wilkinson/BSCI338/L6reception/sld018.htm

"Ear," Microsoft® Encarta® Online Encyclopedia 2004
http://encarta.msn.com © 1997-2004 Microsoft Corporation. All Rights Reserved.

Turner, R.G., 1980, Physiology and Bioacoustics in Reptiles, Chapter 7 in: Comparative Studies of Hearing in Vertebrates. Springer-Verlag. New York. 206,210 p.

Manley, G.A. and Köppl, C., 1998, Phylogenetic development of the cochlea and ist innervention. in: Current Opinion in Neurology, Vol. 8. Elsevier Science Ltd. London. 468-474 p.

Miller, M.R., 1980, The Reptilian Cochlear Duct, Chapter 6 in: Comparative Studies of Hearing in Vertebrates. Springer-Verlag. New York. 170 p.

Lombard, R.E., 1980, The Structure of the Amphibian Auditory Periphery: A Unique Experiment in Terrestrial Hearing, Chapter 4 in: Comparative Studies of Hearing in Vertebrates. Springer-Verlag. New York. 121 p.

Ear Anatomy Diagram, Enchanted Learning Website

http://www.enchantedlearning.com/subjects/anatomy/ear/

Summary: Evolution of Hearing, Evolution, Sound, Mammals, Amphibians, Reptiles, Fish, Birds, Vertebrates, Ears, Evolution of Ears, Amniotic, Vibration, Cochlea, Communication, Darwin, Modern, Frequency, Amplitude