Mammalian Middle Ear Evolution: Story Gets More Wobbly
The Evolution of Mammalian Middle Ear Bones:
New Study Makes a Fantastic Evolutionary Tale Even More Fantastic
by Jerry Bergman, PhD
One of the traits used to identify mammal species, besides mammary glands which produce milk, is the presence of bony ear ossicles — the incus, the malleus, and the stapes (commonly called the anvil, hammer, and stirrup). This auditory trait is a feature shared by all living mammals, including monotremes (egg-laying mammals such as the platypus and the echidna). All known reptiles and birds have only a single middle-ear ossicle, equivalent to the stapes, called the columella. Thus, given the evolutionary theory that reptiles evolved into mammals, the problem for evolutionists is how the single ear-ossicle design became the tri-ossicular design.
When and where the three-ossicle design evolved is speculation, but because the three ossicles are not present in any of the early Triassic “mammal-like reptiles,” the therapsids, no solid clues exist. The importance of this event is explained by an anti-intelligent design website boasting that the evolution is “well documented by fossils, beginning with animals essentially 100% reptilian and resulting in animals essentially 100% mammalian.” The website adds with this confident assertion (which will be refuted in this paper) that mammal
ear ossicles evolved from skull bones present in most tetrapods, including the reptilian lineage. The reptilian quadrate bone, articular bone, and columella evolved into the mammalian [tri-ossicle design], respectively… This is well documented in the fossil record by a massive volume of incontrovertible data that cannot be explained away. Such large-scale, progressive, continuous, gradual, and geochronologically successive morphologic change (Sidor & Hopson, 1998) is descent with modification, and provides compelling evidence for evolution on a grand scale.
The evolution of mammalian auditory ossicles was explained in similar terms by Wikipedia as
an evolutionary event that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution.
These quotes fail to address an important issue: “Both the jaw and the ear had to function at all stages of the transition. If multituberculates had adopted palinal chewing before the separation of the middle-ear bones from the jaw, how would this arrangement have worked?” This is a critical question and is at the heart of the evolution of jaw-bones-to-ossicle-bones theory.
The Jaw-to-Ear-Bones Theory
The eardrum in reptiles is connected to the inner ear by a single bone, the columella, and the upper and lower reptile jaws contain several bones not found in mammalian jaws. In contrast, the mammalian lower jaw consists of only two bones, one on each side. The theory postulates that the two jaw bones in reptile jaws were lost when reptiles evolved into mammals. And those two jaw bones “lost”, the theory claims, were turned into the additional ear ossicles used in mammals. Over the course of mammalian evolution, one bone from the lower rear of the jaw, and one from the upper rear, lost their function, radically changed, and “migrated” to the middle ear where they functioned to greatly improve mammalian hearing.
The Darwinian story goes like this: two reptilian jaw bones (the lower articular and upper quadrate bones) and one auditory ossicle per ear in reptiles evolved in mammals into six ossicles, three in each ear. The three ear ossicles were also joined together to transmit sound from air to the fluid-filled labyrinth (cochlea) of the inner ear. The chain of three ossicles in each ear served to more effectively transmit air-based vibrations and facilitate more acute hearing. In mammals, the columella became the stapes, the articular bone the incus and the quadrate bone the malleus. In contrast, the columella remained the same in reptiles, not changing thereafter for many millions of years.
In short, the theory postulates the loss of some structures and the gain of other structures through a long and convoluted pathway. This story is widely and uncritically repeated as fact. For example, the authoritative Grzimek’s Animal Life Encyclopedia states “Mammals have only one bone in the lower jaw, not several as reptiles do. However, the bones of the former reptilian jaw are now part of the mammalian inner ear structure.”
The Theory is Problematic for Several Reasons
The jaw bones, depending on the reptile, are large, strong bones and the mammal ear ossicles are the smallest bones in the body (“oss-” and “osteo-“ mean “bone”, and “ossicle” means “tiny bone”). The stapes is less than 3 millimeters long, smaller than a pea (see Figure 2). In college anatomy labs, the bones have to be encased in plastic because they are so small and fragile it is the only way to display them. Another problem is that the shape of mammalian ear ossicles is nothing like the reptilian jaw bones from which they supposedly originated (see Figure 1).
Another dizzying difficulty is how a single articular bone that is at the rear of the reptilian skull’s lower jaw became two incus bones located on each side of the head, and one quadrate bone, located at the base of the upper skull, became two malleus bones located on each side of the head. The reptile’s articular bone is part of a hinge on the posterior side that articulates with the quadrate bone located above it.
In contrast to reptiles, mammals have a single bone in the posterior of the lower jaw, which still has to articulate with the skull, although it uses a very different design – namely two hinges on each side of the ramus of the jaw. The mammalian mandible consists of two sides called dentaries that articulate directly with the cranium located above it, “a key criterion that defines mammals.” For humans, the top of the ramus, called the condylar process, serves as a loose hinge.
Embryological Recapitulation Again?
In mammalian embryology, the two jawbone precursors migrate into the middle ear area and become the malleus and the incus bones. As the zygote is spherical in shape, development for many structures begins in the early clump of cells. Then, as the elongated body begins to develop, the precursors of many structures, including the ear bones, migrate toward their eventual locations. In the embryo, the middle ear bones (ossicles) are derived from separate origins in the first and second arch mesenchyme. There, two muscles (the tensor tympani and stapedius) are formed from arch mesenchyme. Eventually they form the three ossicles in each middle ear. This process is tightly controlled by genetics and other designed regulatory-control systems.
An additional problem is that these bones develop from different parts of the embryo and use different genetic regulation systems in both reptiles and birds. The “evolution of the mammalian middle ear is thought to provide an example of ‘recapitulation’—the theory that the present embryological development of a species reflects its evolutionary history.” Differences in embryological development and discoveries from new fossils, however, indicate the following: “data from both developmental biology and paleontology have suggested that the transformation of post-dentary jaw elements into cranial ear bones occurred several times in mammals.” Thus an event that was extremely unlikely before is now even more mathematically improbable if it is postulated to occur by chance several times. (See Figure 3.)
Because “questions remain concerning middle-ear evolution, such as how and why the post-dentary unit became completely detached from the dentary bone in different clades of mammaliaforms,” new theories have been proposed to answer these questions. One researcher concluded in 2019: “questions remain concerning middle-ear evolution, such as how and why the post-dentary unit became completely detached from the dentary bone in different clades of mammaliaforms.”
Stories in the media and textbooks that purport to show the evolution of reptilian jaw parts into mammalian ear ossicles are produced by selective evidence. Writers select from a large number of fossils and attempt to fit them into a progressive evolutionary sequence. Now, however, newly discovered fossil evidence announced in January 2021 has forced a “re-evaluation of one of the fundamental transitions in mammalian evolution: the transformation of bones of the lower jaw into those of the middle ear.
One attempt to harmonize the new findings had been to
suggest that the co-evolution of the primary and secondary jaw joints in allotherians was an evolutionary adaptation allowing feeding with unique palinal (longitudinal and backwards) chewing. Thus, the evolution of the allotherian auditory apparatus was probably triggered by the functional requirements of the feeding apparatus.”
The new study reported in Nature, however, “may push the origin of mammals back millions of years earlier than previously thought” – back to the Mesozoic era, the age of the dinosaurs. The time squeeze came from finding new evidence of ear ossicles in much older fossils. This was a critical find because ear ossicles are evidence the animal was a mammal. Consequently, that pushes the origin of mammals several million Darwin years earlier.
The new fossil is an animal that looks very much like a modern “sugar glider” — a small, omnivorous, nocturnal, arboreal, gliding marsupial. The fossil was crushed almost flat, making it difficult to determine what it looked like originally. A part that appeared to be ear ossicles was examined by computerized tomography (CT scan). The results convinced some that the section contained the ear ossicles, but other scientists remained unconvinced. Instead of solving the debate, and filling in an evolutionary gap, they engendered more debate and created a few more evolutionary gaps:
The relationship between euharamiyidans and multituberculates on the evolutionary tree is a matter of lively debate, with some studies, including that of Wang and colleagues, showing them to be closely related within mammals, whereas others place euharamiyidans on a lineage that branched off before the common ancestor of living mammals evolved. If the latter scenario is the case, then euharamiyidans would represent a fourth instance of the independent evolution of a fully detached middle ear.
The major problem with the ear ossicles evolution theory is that all known mammals have the three auditory bones, and without all three properly designed and correctly assembled and working in the middle ear, hearing would not be possible. For hearing to work, the complete set must exist as a unit, along with the many ligaments required to connect the bones together. Specifically, the ligaments are in the walls of the tympanic cavity, including three ligaments to support the malleus, and one each for the incus and the stapes.
In addition, two synovial joints are specifically designed to “filter potentially damaging impulsive stimuli by transforming both the peak amplitude and width of these impulses before they reach the cochlea.” Furthermore, two tiny muscles in the middle ear alter the ear bone tension, thus regulating the sound intensity. The tensor tympani is attached to the malleus and draws the malleus inward, increasing drum membrane tension. Additionally, the stapedius muscle pulls the stapes footplate outward from the oval window, thereby functioning as a method of lowering loud sounds that could damage the ear. As one source details, the ear
ossicles are a complex system of levers whose functions include: reducing the amplitude of the vibrations; increasing the mechanical force of vibrations; and thus improving the efficient transmission of sound energy from the eardrum to the inner ear structures. The ossicles act as the mechanical analog of an electrical transformer, matching the mechanical impedance of vibrations in air to vibrations in the liquid of the cochlea. The net effect of this impedance matching is to greatly increase the overall sensitivity and upper frequency limits of mammalian hearing, as compared to reptilian hearing.
In short, the system is irreducibly complex, requiring all parts to exist and be assembled properly in order to function at all.
The picture of evolutionary progress from reptilian skull bones to mammalian auditory ossicles is decidedly less than convincing: fragments break off of jaws, act as hinges and become pea sized highly-tuned hearing amplifiers? Evolutionists themselves recognize the jaw-to-ear-ossicles theory is hard to believe. For example, Hoffmann wrote: “Oddly, these bones evolved from remnants of jaw bones, and how they migrated to form the ear has fascinated biologists for 200 years.” More fossils have only exacerbated the problems. The “striking increase in fossil discoveries, in particular new specimens from China, has fueled routine revision of the mammalian tree of life during the past few years” which affects the jaw to ear ossicles theory
Darwinian ossicle-origin frustration is one more example of the bankruptcy of molecules-to-mankind evolutionary speculations. What they proclaim loudly in the media falls apart when the specifics are investigated.
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 Wang, Haibing, Jin Meng, and Yuanqing Wang. 2019. Cretaceous fossil reveals a new pattern in mammalian middle ear evolution. Nature 576:102–105, November. https://www.nature.com/articles/s41586-019-1792-0
 Wang, et al., 2019, p. 102. Emphasis added
 Wang, et al., 2019, p. 102.
 Hoffmann, Simone. 2021. Lend an ear to a classic tale of mammalian evolution. Nature–News and Views, January 27. https://doi.org/10.1038/d41586-021-00064-5.
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 Baker, Noah. 2021. Fossilized glider takes the origin of mammals back to the Triassic:
New fossil suggests mammals evolved earlier than previously thought, Nature—Nature Video, January 27.
 Baker, 2021.
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 “Ligaments of auditory ossicles.” IMAIOS. https://www.imaios.com/en/e-Anatomy/Anatomical-Parts/Ligaments-of-auditory-ossicles.
 Gottlieb, Peter, et al. 2018. Human ossicular-joint flexibility transforms the peak amplitude and width of impulsive acoustic stimuli. Journal of the Acoustical Society of America 143(6):3418–3433 June, p. 3418.
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 Hoffmann, 2021.
Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.