Evolution, the Logic of Biology by John S. Torday & Virender K. Rehan

Author:John S. Torday & Virender K. Rehan
Language: eng
Format: epub
ISBN: 9781118729281
Publisher: Wiley
Published: 2017-07-31T00:00:00+00:00

Evidence from Developmental Biology

Meckel’s cartilage is composed of two rods of cartilage that overarch the sides of the mandible; the proximal portion forms the jaw bone in all but mammalian vertebrates. In avian embryos, portions of Meckel’s and the quadrate cartilage derive from the first pharyngeal arch, whereas the retroarticular process that develops proximal to the articular and the columella derive from the second pharyngeal arch. Separated by the jaw joint, cartilage generates the two skeletal elements, giving rise to the quadrate and articular bones of the jaw.

The malleus and incus are formed from a single cartilage that subdivides, whereas the stapes derives from a separate cartilage that extends toward the incus to form a joint. The malleus and incus derive from the posterior of Meckel’s cartilage like the other two ear ossicles; the malleus remains attached to Meckel’s cartilage throughout most of embryonic development, forming a conduit between the jaw bone and the middle ear.

In mice, the cartilaginous connection between the jaw and ear breaks down after birth, starting on or about day 2 of life, with the transformation of Meckel’s cartilage next to the malleus into the sphenomandibular ligament. The dissolution of Meckel’s cartilage functionally separates the ear from the jaw. Meckel’s cartilage supports the bones that ossify along its length.

Genetic data are also consistent with Reichert’s theory. The homeodomain transcription factor Bapx1 is found in the jaw joints of birds, fish, and reptiles, whereas it localizes to the middle ear in developing mammals, pointing to the common origin and homology of the ear ossicles.

The homologies between the ear ossicles suggested by comparative anatomy nearly two centuries ago have been confirmed by molecular and developmental biology. Additional fossil evidence has facilitated further documentation of the transition from the fish jaw to the mammalian middle ear bones.

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