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Attribution: Modified version of the original image by Lars Chittka; Axel Brockmann. As modified by Jmarchn in 2018., CC BY 4.0, via Wikimedia Commons

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Mammals are unique, for a multitude of reasons including that our ears are some of the most complex of any tetrapod species(four limbed vertebrae). We have an outer, middle and inner ear that all work together to transmit sound data to the brain while also performing several other jobs. The evolution and development of the ear is rather fascinating, in that while it’s not necessarily known what factors drove the development of the ear, it is possible to trace its development path.
Image source:

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Our middle ear consists of three specialized bones, the Stapes, Incus, and Malleus, and are responsible for our ability to hear a wide range of frequency's. What makes these small bones so interesting is that most animals don’t have all three, as an example reptiles only have one of these bones in their ear(the equivalent to the stapes) while  fish have none. Yet in these two distinctly different animals we can find traces of our ear evolution through their bones¹¹

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Attribution: Georg Friedrich Schlater, Public domain, via Wikimedia Commons

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In the mid-19th century, a variety of studies were being performed in order to determine and understand the development of the ears of a variety of animals. Karl Reichert was a scientist who studied the skulls of reptiles and mammals to understand how they develop from embryo to adult²,3.

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In 1837 from his dissections of pig embryos, Reichert hypothesized that the incus and the malleus are the same as the quadrate and articular, two bones found in reptile jaws and other early jawed vertebras (Gnathostomata) that  are responsible for the articulation of the lower and upper jaw3,4, ⁸,10

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Attribution :AnonymousUnknown author, Public domain, via Wikimedia Commons

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His work would be expanded upon by Ernst Gaupp, who incorporated evolutionary studies and understanding and traced the development of these bones from the jaw, to the ear by comparing a variety of animals skulls 4,5,8. This along with Reicher’s early work led to ‘Reichert‐Gauppsche Theory’ that through evolution the parts of the lower jaw detach when a second jaw joint forms, these detachments shrink to form smaller bony elements(malleus and incus)  that become parts of the middle ear along with the already present stapes 4,5,8.

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To understand how they came to this conclusion a brief overview of embryo development needs to be quickly covered. In the early stages of vertebrae embryo development structures called pharyngeal arches start to appear. These arches are precursors to adult structures, for example arch I in the diaphragm gives rise to the incus and Malleus¹³.  These arches are in all vertebrates,  and between animals the same structures usually come from the same arches. From this knowledge and understanding Reich saw that bones not present in mammals formed from the same arch as the incus and malleus, and undergoes a similar development¹³.This is what made him draw his original hypothesis which has since been supported¹³

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There is a ton of research that has been done that further supports their original theory. Studies in skulls show a clear transition for the incus and malleus slowly dissociating from the jaw as the skull develops and the jaw develops a double joint.¹⁰ The detachment of the malleus and incus are believed to occur in two primary steps. The first is the partial disassociation from the jaw, where the middle ear is still connected to the jaw by the Meckel’s cartilage, and the second step is the dissociation from this cartilage itself⁹, this process which would eventually lead to the mammalian middle ear is not only supported by fossil records but by biology¹⁰.

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If you were to trace the development of ears in an embryo you would see the forementioned Meckel’s cartilage, an important structure found in embryos of most animals including mammals.   This cartilage is connected to the dentary and is not just a connective structure, but an important developmental build site in the early embryo stages as the ear initially develops, with the malleus and Incus attached to the posterior end in mammals¹⁰, ¹⁴. And in non mammal vertebrae’s the proximal part of cartilage makes up the quadrate and articular¹⁰. This is one of the reasons why the quadrate and articular are considered ancestors or homologs to the malleus and Incus. Overtime as the mammal embryo develops the Meckel’s cartilage will start to be absorbed and the middle ear which was at this point being developed attached to the skull by the cartilage will dissociate, ¹⁰,¹⁴  This is illustrated in the figure, where the growth of the middle ear is shown for several different species until it dissociates.

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Of course a lot of other changes must occur for the middle ear to go from simply a jaw joint to a functional ear. This development  is a complex yet important evolution in mammals, and at first you may think that such a complex development would have only occurred once in evolution. Yet evidence of convergent evolution (the same evolution change occurring in different groups without a common ancestor) has been suggested. By mapping changes in complex structures in skull fossils and different groups of mammals, it is believed that the detachment of the middle ear from the skull and thus its development may have occurred multiple times, along with possible reversels¹⁴.

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Although the true reasons and mechanism behind this detachment of these bones is not known two main driving forces was hypothesized.  That A: this change was due to the functional benefits that it provided the animals, and B: that due to changes in gene factors and signal pathways pushed this evolution¹⁴.  These hypotheses can be seen in the figure which shows history and relationships of the development of the middle ear in order to show possible points where the mammalian features of the middle ear arose and possibly converged.

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For an example of functionality being a driving force for evolution look no further then  a small fossil found in 2019. A team of researchers found a well-preserved cretaceous fossil of a new species named Jeholbaatar kielanae, an early mammal from around 120 million years ago. In the fossil was a well persevered middle ear that showed what appears to be remnants of the jaw still attached to it¹.  From their findings they hypothesized that this mammal evolved to have the middle ear detached from the jaw to allow for greater jaw movement improving its ability to chew  rather then its hearing ability¹.  This is further explained in the video here.

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This concept that the detachment of the middle ear from the jaw is an evolutionary step that did not immediately improve hearing is not a new concept, there have been other hypothesis in why the incus and malleus initially detach. One idea was that as species evolved to have larger brains, they also had to develop larger skulls, to accommodate the growth of these larger bone structures the mallues and incus had to detach to make room². Although this idea has been heavily debated as evidence points towards hearing and eating being the main driving force¹⁵.

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Attribution: Original artist unknown:Modified artist: Payan et al [16].  https://www.researchgate.net/publication/248

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Backtracking from the final stages of middle ear development to the much earlier initial stages,  Reichert was also one of the first people to see the correlation between the ear bone stapes and bones in fish³. His findings would eventually lead to the discovery of how bones originally  from fish would evolve into the initial middle ear seen in present day reptiles,  and eventually form the stapes in our own mammal ears. This concept at first may seem strange  since fish don't have bones in their ears, they only have the inner ear which is seen in the figure.

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But fish do have bones in their skulls, One series of bones called the Hyomandibula are found in most fish and other early jawed vertebrates (gnathstones)8, 11 but is generally not found in present day land animals. These bones play a important role in anchoring the upper jaw by connecting the jaw joint to the skull⁸ and is also associated in respiration by helping reduce stress of pumping water through the gills¹²  . From his research, Reichert Hypothesized that these bones are equivalent to the stapes found in reptiles and mammals, as their morphology(development) are almost identical3, ¹¹.

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The evolutionary drive that caused the hyomandibula to evolve into the stapes is easy to see in the first animals to walk the land. Tiktaalik roseae, is believed to be an intermediate between fish, and early land animals¹². By studying the skull, we can see that its much thicker and flatter than that of a fish, with a reduced hyomandibula¹².  It should be noted, that like the detachment of the incus and malleus, this development was believed to be not driven by the need to improve the creatures hearing. Instead this  reduction is hypothesised to be partly due to the hyomandibula no longer playing a major role in respiration as the animal moves from breathing water, to air¹². There’s also evidence that this reduction played a role in increasing the articulation of the skull, as it is attached to fewer different parts of the skeleton¹².  Another possible purpose is seen in early fossils of amniotes, where the stapes appears to be repurposed in stabilization of the jaw as it bites¹⁷

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The evolution of the ear is a fascinating topic to discuss, and sadly there is only so much that can be covered in a single setting. While this padlet did briefly cover several main topics of the middle ear development such as the transformation of jaw bones into their respective ear bones; many theories, early research findings, insights, and topics have been not been included in this padlet. The evolutionary history of the inner, and outer ear also has not been discussed here, along with many of the ears other functions including its ability to maintain balance. However, hopefully you were able to gain interest in this topic, and for further information please check any of the referenced articles.

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1.       Wang, H., Meng, J. & Wang, Y. Cretaceous fossil reveals a new pattern in mammalian middle ear evolution. Nature 576, 102–105 (2019). https://rdcu-be.prxy.lib.unbc.ca/cLdkS

2.       Rowe T. Coevolution of the mammalian middle ear and neocortex. Science. 1996 Aug 2;273(5275):651-4. doi: 10.1126/science.273.5275.651. PMID: 8662557.

3.    Reichert, C. (1837). Über die Visceralbögen der Wirbeltiere im Allgemeinen und deren Metamorphosen bei den Vögeln und Säugetieren. Archiv für Anatomie, Physiologie und Wissenschaftliche Medizin, 1837, 120–222.

4.       Pfaff C, Schultz JA, Schellhorn R. The vertebrate middle and inner ear: A short overview. J Morphol. 2019;280(8):1098-1105. https://doi.org/10.1002/jmor.20880

5.  Gaupp, E. (1911a). Beiträge zur Kenntnis des Unterkiefers der Wirbeltiere. I. Der Processus anterior (Folli) des Hammers der Säuger und das Goniale der Nichtsäuger. Anatomischer Anzeiger, 39, 97–135.

6.  Bogislaus Reichert.jpg. (2020, October 28). Wikimedia Commons, the free media repository. Retrieved 23:49, April 2, 2022 from https://commons.wikimedia.org/w/index.php?title=File:Carl_Bogislaus_Reichert.jpg&oldid=504968931.

7.  File:Gaupp.JPG. (2020, August 25). Wikimedia Commons, the free media repository. Retrieved 02:14, April 3, 2022 from https://commons.wikimedia.org/w/index.php?title=File:Gaupp.JPG&oldid=443055732.

8.  Takechi M, Kuratani S. History of studies on mammalian middle ear evolution: a comparative morphological and developmental biology perspective. J Exp Zool B Mol Dev Evol. 2010 Sep 15;314(6):417-33. doi: 10.1002/jez.b.21347. PMID: 20700887.

9.       Anthwal N, Urban DJ, Luo ZX, Sears KE, Tucker AS. Meckel's cartilage breakdown offers clues to mammalian middle ear evolution. Nat Ecol Evol. 2017 Mar 6;1(4):93. doi: 10.1038/s41559-017-0093. PMID: 28459103; PMCID: PMC5405799.

10.   Anthwal N, Joshi L, Tucker AS. Evolution of the mammalian middle ear and jaw: adaptations and novel structures. J Anat. 2013;222(1):147-160. doi:10.1111/j.1469-7580.2012.01526.x

11.    Eaton, Theodore H. “The Crossopterygian Hyomandibular and the Tetrapod Stapes.” Journal of the Washington Academy of Sciences, vol. 29, no. 3, 1939, pp. 109–17, http://www.jstor.org/stable/24530372. Accessed 12 Apr. 2022.

12.   Downs, J., Daeschler, E., Jenkins, F. et al. The cranial endoskeleton of Tiktaalik roseae. Nature 455, 925–929 (2008). https://doi.org/10.1038/nature07189

13.   Burford CM, Mason MJ. Early development of the malleus and incus in humans. J Anat. 2016;229(6):857-870. doi:10.1111/joa.12520

14.  Luo, Zhe-Xi. (2011). Developmental Patterns in Mesozoic Evolution of Mammal Ears. Annual Review of Ecology Evolution and Systematics. 42. 355-380. https://doi.org/10.1146/annurev-ecolsys-032511-142302.

15.  Wang, Yuanqing, et al. “An Ossified Meckel’s Cartilage in Two Cretaceous Mammals and Origin of the Mammalian Middle Ear.” Science, vol. 294, no. 5541, 2001, pp. 357–61, http://www.jstor.org/stable/3084841. Accessed 9 Apr. 2022.\

16.   Payan, Patrick & De Pontual, Hélène & Gilles, Boeuf & Mayer-Gostan, Nicole. (2004). Endolymph chemistry and otolith growth in fish. Comptes Rendus Palevol - C R PALEVOL. 3. 535-547. 10.1016/j.crpv.2004.07.013.

17.  Tucker AS. Major evolutionary transitions and innovations: the tympanic middle ear. Philos Trans R Soc Lond B Biol Sci. 2017;372(1713):20150483. doi:10.1098/rstb.2015.0483