“Unity of organization” revealed

Fig. 1. Human embryo 5 weeks old in left side view showing the position of pharyngeal arches and pharyngeal pouches. Image source: ScienceDirect.

By Jozef Klembara

It was Richard Owen, a famous English anatomist, a founder of the Natural History Museum in London and contemporary of Darwin, who presented a discovery of the common structural plan of all organisms, the so-called archetype, representing a structural “unity” (unity of organization) of all organisms: “The ideal original or fundamental pattern on which a natural group of animals or system of organs has been constructed, and to modifications of which the various forms of such animals or organs may be referred.” (Owen, 1848). He demonstrated it on the skeletons of vertebrates displaying a structural progress – from amphibians to mammals – stating that all bones in the given species of a given group correspond anatomically to those in all other vertebrates, disregarding the systematic level. They differ from one another only by their shape reflecting their different use (e.g., the forelimb of the lizard is used differently than that of a bird). He called this structural similarity – homology (in contrast to analogy depicting the common use of two structures of a different origin). We know today that homology is the basis for our understanding of interrelationships of organisms. Nowadays, homologous structures are those structures that are inherited from the common ancestor.

Fig. 2. Horizontal section of the pharyngeal region in vertebrates showing the position of pharyngeal arches, pouches and clefts. Image source: Memorang.

Therefore, we can reveal homologous structures in different groups of vertebrates, even if they are not externally similar. For example, all fishes have gill arches and clefts (called pharyngeal pouches and clefts or grooves in all other groups of vertebrates having legs – tetrapods) located in the temporal head region in their early stage of development (Fig. 1). With growth, these originally fish structures-tissues forming the internal, endodermal side between the pharyngeal arches, called pouches, start to develop and transform to various structures of the organism (Fig. 2). In such a way, a tissue forming the internal walls of gill arches in fishes is not lost in the evolution of “higher” vertebrates but develops into different cells, tissues and organs in tetrapods. For example, in tetrapods, the hearing tube and a middle ear cavity – and the external auditory tube too – develop from the endoderm of the first gill pouch; thus, the structures connected with hearing in land dwelling vertebrates. The second gill pouch transforms into a shallow cavity where the tonsil starts to develop in mammals. From the third gill pouch arise small parathyroid glands lying close to the thyroid gland, and the organ called thymus, etc. All these, the so-called branchiogenic (Greek, branchial – gills) organs form part of the evolution of the gastropulmonal system in vertebrates, such as the organs of digestion and breathing. This is only one example to show the “hidden” interrelationships of all individual parts composing the bodies of all organisms, which display the incredible and fascinating diversity of the organismal world.

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