How can we explain the course of the recurrent laryngeal nerve?

First of all it is essential that the actual use and course taken by this nerve is outlined.

The recurrent laryngeal nerve is just one of many branches of the vagus nerve, the 10^th cranial nerve. This particular branch is important, as it is the sole supplier of motor and sensory innervation to the larynx, commonly referred to as the voice box. The co-ordination of these muscles allows many animals to produce a wide range of sounds. This ability is indispensable in some species, such as humans who use their voice box to produce sounds in the form of words in order to communicate. Communication is an essential asset. Those animals that in the past had this ability had a distinct advantage over those who could not communicate. It provides protect when hunted and stealth when hunting.

In humans and mammals, the recurrent laryngeal nerves take a rather absurd route. As I have already stated, the target of the nerves is the larynx which is located anteriorly in the neck. The nerves originate in the brainstem, specifically the medulla oblongata, which is a portion of the central nervous system between the brain and the spinal cord. The nerves leave the medulla whilst still in the neurocranium (skull), but quickly exit via the right and left jugular foramen. After leaving the skull, the nerves descend down the neck, one on each side, and into the thorax, before looping back on themselves and into the neck once again before reaching their final destination. The route is slightly different on both sides of the body. On the right side, the nerve loops under the right subclavian artery before its ascent to the larynx whereas the left recurrent laryngeal nerve loops beneath the aorta.

Although this route seems to be strange in humans, the extremity of this illogical route is epitomised in a giraffe. The indirect route means that at least six extra feet of nerve is required when compared to a direct alternative. Personally, this seems tremendously inappropriate, as it is an extreme waste of resources, and also poses a larger potential for damage especially for the left recurrent laryngeal nerve, as not only can problems in the neck cause nerve damage but also problems in the left side of the thorax. For example, a tumour in the left lung could cause compression on the left recurrent laryngeal nerve leading to paralysis of the muscles on the left side of the larynx, possibly resulting in a decrease in the ability to communicate and the subsequent loss of an evolutionary advantage.

Consequently, I believe that it is fair to state that this situation lacks an intelligent design, as there is obviously a much more ideal design that would not waste resources or pose an increased risk of problems occurring. If a mere mortal such as myself can easily outline the flaws of this situation, it is acceptable to assume that no omniscient, creator would have designed the recurrent laryngeal nerve as it stands today. Therefore, I shall now look to see if Charles Darwin’s work can help answer my query and provide the reasoning beyond this strange phenomenon.

The presence of this nerve can be traced back down the branching tree of life to fish-like vertebrates, which appear to share the presence of a similar nerve. However, the nerve in fish does not have the same route, and therefore does not share the same name but is still the fourth branch of the vagus nerve.

Instead, the nerve in fish-like vertebrates travels straight to its target, which is the 6^th gill as it appears to follow the route of the 6^th arterial arch. This same general principle, of the nerve following the arterial branch, is also shown in mammals and humans.

During evolution, some fish-like vertebrates appear to have migrated onto land and evolved into land creatures. This was only recently proven by evidence in the fossil record. Before the discovery of a certain fossil, it was only assumed that some of the water living animals evolved into land living creatures, but since the first report in 2006 of the Tiktaalik roseae, evolutionists believe that they have found the missing link; the bridge between aquatic life in the oceans to life on land. These fish-like vertebrates are our ancestors.

This statement is further confirmed by the fact that humans and mammals follow a similar developmental principle as their fish-like ancestors, as during the embryological development of land animals gill like remnants can be seen. However, during the evolutionary step from fish to land animals, the gills of the fish disappeared in the adult form of land animals.

The pharyngeal arches, located on an embryos neck appear extremely like the gills of a fish. However, their function is no longer to supply oxygen by becoming gills as they did in previous aquatic species, but they are now responsible for giving rise to the development of internal bodily structures.

As already stated, we have a particular interest in the 6^th arch. In both fish and humans, this arch gives rise to the 6^th arterial arch. However, the location which this arch migrates to is different. In fish, the arch runs next to the 6^th gill and therefore this means the following nerve has a direct route to its target.

However, in humans, the 6^th arterial arch gives rise to the right and left pulmonary arteries and the ductus arteriosus which are vessels located around the heart. Hence the nerve follows this path and ends up travelling from the brainstem into the thorax. However, the 6^th pharyngeal arch also contributes to the formation of the larynx, including its cartilaginous skeleton and its intrinsic musculature. Therefore, the nerve then takes a detour from its position in the thorax, after the development of the arterial system is complete, back up to its target in the neck to help in the formation of the larynx.

The answer to why this happens in this particular order is all hidden in the past. Once a species develops, the developmental process cannot be undone as there are essential elements that must be maintained in order for an organism to survive i.e. the development of the arterial system. This is often portrayed in words as ‘the knot evolution cannot untangle.’ The need for some developmental elements to be maintained, places evolutionary constraints upon future species that are to evolve. This is evoked strongly in this example.

The constraint of the development of the arterial system means that the fourth branch of the vagus nerve has to follow the migration of the 6^th arterial arch first as this is a fundamental step in the development of an organism. This cornerstone means that any evolutionary advances have to occur after this step has taken place. Therefore, when the larynx evolved, the laryngeal nerve had to evolve too, but in accordance with the constraints put upon it. It therefore had to travel from the thorax to the larynx, and as a result became known as the recurrent laryngeal nerve due to its reoccurring route through the neck.

I believe I have revealed that this nerves route is an example of unintelligent design and that its route can be explained clearly and precisely through the work of Charles Darwin and the process of evolution, that can only move forwards, resulting in some weird and absurd but often wonderful creations.

Sources:

The Rough Guide to Evolution – Mark Pollen

http://www.sciencedaily.com/releases/2008/10/081015144123.htm

Evolution – Mark Ridley

Evolution - Monroe W. Strickberger

Clinically Orientated Anatomy – Keith Moore and Arthur Daley