Medical Considerations for the Intelligent Design of the Recurrent Laryngeal Nerve
In the previous three posts (see part 1, part 2, and part 3), we've seen that the arguments of intelligent design (ID) critics based that the recurrent laryngeal nerve (RLN) is an "imperfect design" fail for a variety of reasons. These include:
To add another reason, pro-ID professor of neurosurgery Michael Egnor has suggested that the RLN may have a medical function which gives the organism a warning that it is sick, and needs to heal from an internal infection or disease originating in the chest area. Dr. Egnor offered me some insightful comments about function of the design of the RLN pathway from his vantage as a doctor and professor of medicine:
(1) There is evidence that supposed fundamental evolutionary constraints which would prevent loss of the circuitous route of the RLN do not exist. This implies that there is some beneficial function for the circuitous route.
(2) The path of the RLN allows it to give off filaments to the heart, to the mucous membranes and to the muscles of the trachea along the way to the larynx.
(3) There is dual-innervation of the larynx from the SLN and RLN, and in fact the SLN innervates the larynx directly from the brain. The direct innervation of the larynx via the superior laryngeal SLN shows the laryngeal innervations in fact follows the very design demanded by ID critics like Jerry Coyne and Richard Dawkins. Various medical conditions encountered when either the SLN or RLN are damaged point to special functions for each nerve, indicating that the RLN has a specific laryngeal function when everything is functioning properly. This segregation may be necessary to achieve this function, and the redundancy seems to preserve some level of functionality if one nerve gets damaged. This dual-innervation seems like rational design principle.
There is actually a design advantage to the course of the recurrent nerves, if one wishes to pursue this line of argumentation. The course of the nerves brings them through the mediastinum, where the heart and lungs meet. There are many lymph nodes there, and enlargement of these lymph nodes from processes such as cancer or infection (e.g. tuberculosis) often irritates these nerves and causes hoarseness or coughing. The course of the nerves reveals disease in an otherwise hidden part of the body (deep in the chest) by interfering with a process (speech) that is readily evident. It serves as an early warning to get medical care (or, with infectious diseases, as a warning to others that this person is ill), and this early warning has saved many more lives than the redundant course of the nerves has cost lives. The risk/benefit ratio needs to be examined comprehensively before one claims that the course of the nerves is biologically disadvantageous.While it's impossible to get into the mind of the designer and there's a lot we still don't understand about development, it's equally clear that the RLN's pathway allows it to serve a variety of functions. We do have solid positive evidence for multiple important functions of the RLN. Indeed, Michael Egnor wrote an elegant explanation of why it is the evolutionists who are basing their arguments upon our ignorance, not facts, of developmental biology:
Of course ID advocates have never claimed perfect design. But the argument that the anatomy of the recurrent laryngeal nerve is evidence for 'bad design' fails on many levels. The descent of the recurrent nerves below the aortic arch and subclavian artery is the result of patterns of coalescence and movements of components of the aortic arch during embryogenesis. It appears that proximity of various layers and structures in the embryo serve to guide embryogenesis (it's called induction). The details of this process are only beginning to be understood, and the Darwinist argument that the relationship between the recurrent nerves and the aortic arch is evidence of bad design fails to take into account the enormous complexities of embryonic development. It's analogous to a 3 year-old taking apart a computer and asserting that it was designed badly because some of the circuit board patterns were 'curvy' instead of straight. The design wisdom of the anatomy of the recurrent nerves can only be judged by someone who knows all of the design specifications necessary for that region of the human body. Even the best embryologists are pre-schoolers when it comes to that.
Clearly, the RLN is performing many jobs, not just one. Its "intended function" is much more than simply innervating the larynx; and the larynx is in fact innervated directly, exactly as ID-critics say it should be.
All of this this would seem to satisfy what Kelly Smith called evidence of a "global" function, which Smith admits makes "an instance of local imperfection" (i.e. the circuitous route of the RLN) more acceptable in an argument for design ("Intelligent Design Creationism and Its Critics," p. 725). The argument against intelligent design of the RLN has collapsed.
If anything is clear here, it's that we don't know enough to presume that the RLN is "[o]ne of nature's worst designs" (Coyne's words), but that we do know enough to see that it's definitely not "a very poor design for its intended purpose" (Smith's words).