Danish researchers reveal how hearing evolved
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Lungfish and
salamanders can hear, despite not having an outer ear or tympanic middle ear.
These early terrestrial vertebrates were probably also able to hear 300 million
years ago, as shown in a new study by Danish researchers.
February 5, 2015, by Christian Bech Christensen and Peter Teglberg
Madsen
Lungfish and salamander ears are good models for different stages of ear
development in these early terrestrial vertebrates. Two new studies published
in the renowned journals Proceedings of the Royal Society B and The
Journal of Experimental Biology show that lungfish and salamanders can
hear, despite not having an outer ear or tympanic middle ear. The study
therefore indicates that the early terrestrial vertebrates were also able to
hear prior to developing the tympanic middle ear. The research findings thus
provide more knowledge about the development of hearing 250–350 million years
ago.
The physical properties of air and tissue are very different, which means
in theory that up to 99.9% of sound energy is reflected when sound waves reach
animals through the air. In humans and many other terrestrial vertebrates, the
ear can be divided into three sections: the outer ear, the middle ear and the
inner ear. The outer ear catches sound waves and directs them into the auditory
canal. In the middle ear, pressure oscillations in the air are transferred via
the tympanic membrane (eardrum) and one or three small bones (ossicles) to
fluid movements in the inner ear, where the conversion of sound waves to nerve
signals takes place. The tympanic middle ear improves the transfer of sound
energy from the surroundings to the sensory cells in the inner ear by up to
1,000 times, and is therefore very important for hearing in terrestrial
vertebrates. This is reflected in the fact that different configurations are
found in the vast majority of present-day terrestrial mammals, birds, reptiles
and amphibians. However, available palaeontological data indicate that the
tympanic middle ear most likely evolved in the Triassic period, approximately
100 million years after the transition of the vertebrates from an aquatic to a
terrestrial habitat during the Early Carboniferous. The vertebrates could
therefore have been deaf for the first 100 million years on land.
It is obviously not possible to study the hearing of the early terrestrial
vertebrates, which became extinct long ago. However, by studying the hearing of
present-day vertebrates with a comparable ear structure, it is possible to
learn about the hearing of the early terrestrial vertebrates and the
development of aerial hearing. A team of Danish researchers from Aarhus University ,
Aarhus University
Hospital and the University of Southern
Denmark
They studied the hearing of lungfish and salamanders by measuring auditory
nerve signals and neural signals in the brainstem as a function of sound
stimulation at different frequencies and at different levels. Surprisingly, the
measurements showed that not only the terrestrial adult salamanders, but also
the fully aquatic juvenile salamanders – and even the lungfish, which are
completely maladapted to aerial hearing – were able to detect airborne sound
despite not having a tympanic middle ear. By studying the animals’ sense of
vibration, the researchers were able to demonstrate that both lungfish and
salamanders detect sound by sensing the vibrations induced by sound waves.
The results show that even vertebrates without outer and middle ears are
capable of detecting airborne sound. This means that adaptation to aerial
hearing following the transition from aquatic to terrestrial lifestyles during
the Early Carboniferous was presumably a gradual process, and that the early terrestrial
vertebrates without tympanic middle ears were not deaf to airborne sound during
the first 100 million years on land. In addition to making us wiser about
hearing in general, the results can provide inspiration in the future to
developing clinical treatments for hearing loss.
The research projects were supported by the Oticon
Foundation.
Link to the scientific articles:
·
Proceedings of the Royal Society B: Better than fish on land? Hearing across metamorphosis in
salamanders
Entire article above is online at:
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Why This Research Is
Important
By the Blog Author
Mammals evolved from “mammal-like reptiles” which were
distinguished by the migration of jaw bones to the middle ear to form an
advanced tool for hearing. Since earlier
methods of hearing still survive, it is important to develop a clearer picture
of how and when the changes occurred for the mammal-like reptiles.
The more sophisticated the reptilian ears became, the more
auditory information was made available to the brain; thus, it became
imperative for the brain to increase in size.
A large brain was inconsistent with remaining cold-blooded and reptilian. As warm blooded creatures, the brain
increased even more in size. Mammals
heard better, thought more, and put themselves on the path to dominating as
land animals.
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