These excerpts were copied from an ICR website written by Jerry Bergman
The RLN design is due to developmental constraints. Human-designed devices, such as radios and computers, do not need to function until their assembly is complete. By contrast, living organisms must function to a high degree in order to thrive during every developmental stage from a single-cell zygote to adult. The embryo as a whole must be a fully functioning system in its specific environment during every second of its entire development. For this reason, adult anatomy can be understood only in the light of development.
Developmental Variations
The human body begins as a sphere called a blastocyst and gradually becomes more elongated as it develops. Some structures, such as the carotid duct, are simply obliterated during development, and some are eliminated and replaced. Other structures, including the recurrent laryngeal nerve, move downward as development proceeds. The movement occurs because the neck's formation and the body's elongation during fetal development force the heart to descend from the cervical (neck) location down into the thoracic (chest) cavity.
As a result, various arteries and other structures must be elongated as organs are moved in a way that allows them to remain functional throughout this entire developmental phase. The right RLN is carried downward because it is looped under the arch that develops into the right subclavian artery, and thus moves down with it as development proceeds.
The left laryngeal nerve recurs around the ligamentum arteriosum (a small ligament attached to the top surface of the pulmonary trunk and the bottom surface of the aortic arch) on the left side of the aortic arch. It likewise moves down as the thoracic cavity lengthens. The body must operate as a living, functional unit during this time, requiring ligaments and internal connections to secure various related structures together while also allowing for body and organ movement. For the laryngeal nerve, the ligamentum arteriosum functions like a pulley that lifts a heavy load to allow movement.
As a result of the downward movement of the heart, "the course of the recurrent laryngeal nerves becomes different on the right and left sides." These nerves cannot either be obliterated or replaced because many of them must function during every fetal development stage. No organ could exist that is functionless during its development, an axiom that also applies to the nervous system. This movement appears designed to position the left RLN downward as the body elongates.
In addition, the laryngeal branch splits up into other branches before entering the larynx at different levels. These many RLN branches serve several other organs with both motor and sensory branches, including the upper esophagus, the trachea, the inferior pharynx, and the cricopharyngeus muscle, the lowest horizontal bandlike muscle of the throat just above the esophagus. The fact that the left RLN also gives off some fibers to the cardiac plexus is highly indicative of developmental constraints because the nerve must serve both the larynx (in the neck) and the heart (in the chest).
After looping around the aorta, the RLN travels back up to innervate the larynx. The superior and recurrent laryngeal nerves then innervate an area known as Galen's anastamosis.
It then migrates caudally as the embryo enlarges by differential growth of the head and thorax areas, taking the nerve with it. The diaphragm cannot have evolved step-wise, since a partial diaphragm results in an imperfect chest-abdomen separation. Even a small defect results in herniation of the gut contents into the chest--which either compresses the lungs or results in strangulation of the gut.
Without explaining the nerve structure's design system, function, and ultimate connections, alleging that the RLN is a poor design is a meaningless assertion.
Edited by ICDESIGN, : listed the source of information
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