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Did the ear bones of mammals really evolve from the jawbones of reptiles?

Eardrum is an obstacle for evolution of middle ear bones in mammals

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Published: 2 July 2019 (GMT+10)

Introduction

mammal-reptile-skull
Figure 1: Position and names of different bones in the jaw of reptiles and the middle ear of mammals. Credit: University of California, Berkeley

Some evolutionists claim that mammals evolved from reptiles through a group of animals known as ‘mammal-like reptiles’. This is a big step in the evolutionary model. They believe that certain jaw and skull bones in reptiles evolve into the middle ear bones of mammals. Two bones, the articular bone on the very back tip of the jawbone and the quadrate bone at the base of the skull form the jaw joint in reptiles, birds, amphibians, and mammal-like reptiles (see figure 1). In mammalian embryology, these two bones migrate into the middle ear and become the malleus and the incus. These two bones, together with the stapes form the three bones of the middle ear. However, in reptiles, there is only one single bone in the middle ear, namely the columella, which corresponds to the stapes bone in the mammalian ear.1

Evolutionists list several “mammal-like” species where these bones are supposedly in between those of reptiles and mammals. However, is this picture true? Is there evidence that the reptilian skull and jaw bones middle ear evolved into the mammalian ear?

The actual evidence

Contrary to evolutionary expectations, we now know that these bones actually develop from different parts of the embryo and share different genetic regulation in both reptiles and birds.

In developing reptiles and mammals, the bones of the inner ear come from blocks of tissue from the neck area of the embryo called branchial arches (see figure 2).2 There are two competing theories as to which blocks of tissue the malleus and incus in mammals and also the articular and quadrate in reptiles come from.

According to the classical theory, both the malleus and incus in mammals and the articular and quadrate in reptiles originate entirely from the first branchial arch (figure 3, left). In contrast, the so-called dual-arch theory says that the stapes in both mammals and reptiles (sometimes called the columella in reptiles) arise from tissue within the second branchial arch. In reptiles, the articular and quadrate bones arise only from a specific part of the first branchial arch. However, in mammals, part of the malleus and part of the incus come from both the first and second arch (see table 1 and figure 3, right). 3 In other words, these structures develop from different part of the anatomy in both reptiles and mammals. The dual-arch theory is presented in many medical textbooks and is used in clinical diagnoses.4

Based on the latest evidence, it cannot be determined unambiguously which theory is correct. Either way, evolutionists still have a problem. How could these bones have evolved from one to the other? First, how is it possible for parts of the jaw and the skull to get across the eardrum? Second, why are there no species between reptiles and mammals which have only two ear bones and not three? Third, why should bones involved in chewing in reptiles later be involved in transmitting sound to the brain in mammals?

medical-dictionary.thefreedictionary.comBranchial-arches
Figure 2: Position of four branchial arches in a human embryo.

Even if there is ambiguity as to which tissues the middle ear bones come from, the eardrum itself has separate origins in reptiles, birds, and mammals. If all three animal groups did indeed evolve from a common ancestor, then the eardrum and the external ear canal would form in the same way in all three groups. The eardrum is supported by the quadrate in reptiles, whereas it is supported by a lower jaw element called the tympanic ring in mammals.5

Some evolutionists even admit that in early amniotes6 the role of the stapes is to stabilize the skull during biting.5 Other evolutionists also claim that the three middle ear bones evolved separately in three different groups of7 mammals, and which do not even correspond to one another.8

The formation of the jaw joint is regulated differently between reptiles and mammals at the gene level. In fish9 and amphibians,10 a gene called Bapx1 is responsible for the formation of the jaw joint. In the mouse, this gene also affects the formation of tissue in the forelimbs, hindlimbs, spleen, gut and vertebral column.11 It is unlikely that the supposed expression of this gene in at least one of these five new tissues would not cause an abnormality or imbalance of some sort.10

Conclusion

Branchial-arches
Figure 3: Classical vs. dual arch interpretations of mammalian ear bones.

According to Genesis 1:25, “ … God made the beasts of the earth according to their kinds and the livestock according to their kinds, and everything that creeps on the ground according to its kind. And God saw that it was good.” This means that reptiles and mammals are fundamentally different in their anatomy, because they are different kinds of animals. This includes differences in the structure of their jaw, skull and middle ear.

The evolutionary narrative of the transformation of bones from the jaw and the skull into the ear bones in mammals is problematic. In order to make their story work, evolutionists merely shift the position and re-size specific bones which they think changed between reptiles and mammals. They never explain the actual origin of bones such as the articular, quadrate and stapes. Nor can they explain the different functions of the Bapx1 gene in reptiles and mammals. The idea that the bones that form the reptile jaw are related to the middle ear bones of mammals was first proposed way back in 1837 by the German anatomist Karl Reichert, long before the field of genetics even came into existence.12 This theory however, is not consistent with what we now know from genetics and developmental biology.

Proof of evolution of the mammalian middle ear is therefore shallow. There is no reason to mistrust the Bible.

Table 1: Origin of bones of the middle ear in reptiles and mammals according to the dual-arch theory.

Bone (mammal/reptile) reptiles mammals
Malleus/Articular 1st branchial arch 1st and 2nd branchial arch
Incus/Quadrate 1st branchial arch 1st and 2nd branchial arch
Stapes/Columella 2nd branchial arch 2nd branchial arch

References and notes

  1. Sodera, V. One small speck to man, the evolution myth 2. Bekaam Printers Pte Ltd, Malaysia, 2009. Return to text.
  2. There are usually five branchial arches in different classes of vertebrates, and they are arch-like in structure. Among other things, they form structures in the jaw, middle ear, and pharynx. Return to text.
  3. Orozco, J.W., et al., Ontogenic peculiarities of the human tympanic ossicular chain. Acta Otorrinolaringol Esp. 54(1):1–10, 2003. Return to text.
  4. Burford, C.M., and Mason M.J., Early development of the malleus and incus in humans, J. Anat. 229(6):857–870, 2016. | doi: 10.1111/joa.12520. Return to text.
  5. Tucker, A.S., Major evolutionary transitions and innovations: the tympanic middle ear, Philos Trans R Soc Lond B Biol Sci. 372(1713): 20150483, 2017. Return to text.
  6. Amniotes are a group of animals including reptiles, birds and mammals. Return to text.
  7. These three groups of mammals include monotremes (egg-laying mammals), marsupials (mammals whose young develop outside the mother’s body, usually in a pouch) and placentals (mammals whose young are attached to their mother by a placenta). Return to text.
  8. Rich, T.H., Hopson, J.A., Musser, A.M., Flannery, T.F., Vickers-Rich, P., Independent origins of middle ear bones in monotremes and therians. Science. 307(5711):910–914, 2005. Return to text.
  9. Tucker, A.S., Watson, R.P., Lettice, L.A., Yamada, G., Hill, R.E., Bapx1 regulates patterning in the middle ear: altered regulatory role in the transition from the proximal jaw during vertebrate evolution. Development. 131(6):1235-45, 2004. Return to text.
  10. Lukas, P. and Olsson, L., Bapx1 is required for jaw joint development in amphibians, Evol Dev. 20(6):192–206, 2018. | doi: 10.1111/ede.12267. Return to text.
  11. Chatterjee, S. et al., Gene expression profiles of Bapx1 expressing FACS sorted cells from wildtype and Bapx1-EGFP null mouse embryos, Genom Data. 5:103–105, 2015. Return to text.
  12. Anthwal, N., Joshi, L. and Tucker, A.S., Evolution of the mammalian middle ear and jaw: adaptations and novel structures, J Anat. 222(1):147–60, 2013. | doi:10.1111/j.1469-7580.2012.01526.x. Return to text.