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The Recurrent Laryngeal Nerve Does Not Refute Intelligent Design

Casey Luskin

Part 1: Evolution Dictates a Function for the Design of the Laryngeal Nerve

In his book Why Evolution is True, evolutionary biologist Jerry Coyne claims that "Imperfect design is the mark of evolution; in fact it's precisely what we expect from evolution." (p. 81) He makes this prediction because "[n]ew parts don't evolve from old ones, and we have to work well with the parts that have already evolved. Because of this, we should expect compromises: some features that work pretty well, but some not as well as they might, or some features--like the kiwi wing--that don't work at all, but are evolutionary leftovers." (p. 81)

Thus according to Coyne, evolution predicts that some features will work well, some will work not-so-well, and some will work not at all. This is not exactly a useful set of predictions, but when he couples the argument with the dubious assertion that intelligent design (ID) requires "perfect design," then Coyne places evolution in a unique position to explain such examples of allegedly "imperfect design."

Coyne is by no means the first person to make this argument. In the largely anti-ID volume Intelligent Design Creationism and its Critics, Kelly C. Smith states that "if a design in nature is clearly inferior to what a human engineer could produce, then we are entitled to [reject ID]." (p. 724) (Smith also purposefully mislabels ID as a form of creationism, but we'll ignore that for the moment.)

What both Coyne and Smith share in common is their granting of favored-argument-status to the recurrent laryngeal nerve (RLN) as an alleged example of this "imperfect" or "inferior" design. As the argument goes, the RLN innervates the larynx. However, instead of running directly from the brain to the voice box, as one might expect, the nerve path travels down past the larynx, with the RLN branching off the vagus nerve down near the heart, then looping around the aorta and traveling back up the neck to innervate the larynx from below.

Coyne calls this extra distance the RLN takes to get to the larynx "[o]ne of nature's worst designs," (p. 82) and Smith calls it "a very poor design for its intended purpose." (p. 725) Likewise, Richard Dawkins says in a National Geographic video, "Obviously a ridiculous detour! No engineer would ever make a mistake like that." They all go further and argue that the seemingly circuitous route of the nerve is easily explained if one understands that our body evolved from a fish body and must operate under those constraints.

An article by pro-ID biologist at the Planck Institute for Plant Breeding Research in Germany,Wolf-Ekkehard Lönnig, titled "The Laryngeal Nerve of the Giraffe: Does it Prove Evolution?," explains that observed variation within humans implies that such so-called evolutionary constraints do not exist: The fact is that even in humans in 0.3 to 1% of the population the right recurrent laryngeal nerve is indeed shortened and the route abbreviated in connection with a retromorphosis of the forth aortic arch. ("An unusual anomaly ... is the so-called 'non-recurrent' laryngeal nerve. In this condition, which has a frequency of between 0.3 - 1%, only the right side is affected and it is always associated with an abnormal growth of the right subclavian artery from the aortic arch on the left side" - Gray's Anatomy 2005, p. 644.; see also Uludag et al. 2009; the extremely rare cases (0.004% to 0.04%) on the left side appear to be always associated with situs inversus, thus still "the right side"). Nevertheless, even in this condition its branches still innervate the upper esophagus and trachea (but to a limited extent?). Although this variation generally seems to be without severe health problems, it can have catastrophic consequences for the persons so affected: problems in deglutition (difficulties in swallowing) and respiratory difficulties (troubles in breathing) (see Rammerstorfer 2004; moreover "dysphagia (if the pharyngeal and oesophageal branches of nonrecurrent or recurrent inferior laryngeal nerve are injured)" - Yang et al, 2009) Lönnig thus makes the following observations:If mutations for such a short cut are possible and regularly appearing even in humans (not to mention some other non-shorter-route variations), - according to the law of recurrent variation (see Lönnig 2005, 2006), they must have occurred already millions of times in all mammal species and other vertebrates taken together from the Silurian (or Jurassic respectively) onwards. And this must also be true for any other (at least residually) functionally possible shorter variations of the right as well as of the left recurrent laryngeal nerve. Inference: All these 'short-cut mutations' were regularly counter-selected due to at least some disadvantageous and unfavourable effects on the phenotype of the so affected individuals (including any such mutants in the giraffes). Hence, they never had a chance to permeate and dominate a population except for the above mentioned very small minority of the (right) 'non-recurrent' laryngeal nerve, which is perhaps already accounted for by the genetic load ("The embryological nature of such a nervous anatomical variation results originally from a vascular disorder, named arteria lusoria in which the fourth right aortic arch is abnormally absorbed, being therefore unable to drag the right recurrent laryngeal nerve down when the heart descends and the neck elongates during embryonic development." Defechereux et al. 2000). Thus, even from a neo-Darwinian point of view, important additional functions of the Nervus laryngeus recurrens should be postulated and looked for, not to mention the topic of embryological functions and constraints.It seems quite likely that there are mutational pathways to a more efficient route for the RLN. Under neo-Darwinian thinking, this implies this pathway should have evolved. At the very least, it shows that there are no in-principle constraints based upon our alleged fish-ancestry which prevent this route from evolving. The fact that the pathway remains--under evolutionary logic--that there's some benefit to the current design, which implies that the current design isn't so imperfect after all.

Part 2: The Route of the Laryngeal Nerve Gives It Unique and Specific Functions

Let's assume, for the sake of argument, that common ancestry between mammals and fish is the best explanation for the nerve's path. Would that refute intelligent design? Evolutionary biologist Jerry Coyne assumes that ID is incompatible with common ancestry, which it isn't. As one pro-ID biologist wrote me on this topic, "this is only a problem for design if one assumes design means designed from scratch for each taxon, and if one believes that the designer would necessarily use the shortest distance between two points (in other words, that the designer thinks like we do), and that there are not other design considerations at play."

But if we set aside the question of whether evolutionary history explains the RLN's path, it's also never been clear to me why "imperfect design" should refute design. I've complained before about the breakdowns and flaws I've had with computers, but obviously computers are designed. In fact, every piece of technology that has ever had a flaw shows that imperfect designs are was still designed! "Imperfect design"--a term used by Coyne--is still design.

Regardless of whether evidence for common ancestry or evidence of imperfect design actually do refute design, both Jerry Coyne and Kelly C. Smith admit that if we find some rational function or a reasonable purpose for a design, then the "imperfect" or "inferior" design objection falls apart. Coyne tells us that imperfect design doesn't refute evolution because evolution predicts that both highly functional or poorly-functional parts may exist. Therefore it seems to actually really predict nothing whatsoever on this matter.

While Coyne's argument may not be coherent, one way to refute it is to to find rational function for the RLN's pathway in normal humans.

Indeed, hints of important functions for the RLN nerve can be seen in the old authority, Gray's Anatomy, which states regarding the normal human design:As the recurrent nerve hooks around the subclavian artery or aorta, it gives off several cardiac filaments to the deep part of the cardiac plexus. As it ascends in the neck it gives off branches, more numerous on the left than on the right side, to the mucous membrane and muscular coat of the esophagus; branches to the mucous membrane and muscular fibers of the trachea; and some pharyngeal filaments to the Constrictor pharyngis inferior.So it seems that the RLN is innervating a lot more than just the larynx. Pro-ID biologist Wolf-Ekkehard Lönnig, in his article "The Laryngeal Nerve of the Giraffe: Does it Prove Evolution?," quotes a passage from a much more recent 1980 edition of Gray's Anatomy stating much the same thing:As the recurrent laryngeal nerve curves around the subclavian artery or the arch of aorta, it gives several cardiac filaments to the deep part of the cardiac plexus. As it ascends in the neck it gives off branches, more numerous on the left than on the right side, to the mucous membrane and muscular coat of the oesophagus; branches to the mucous membrane and muscular fibers of the trachea and some filaments to the inferior constrictor [Constrictor pharyngis inferior].

(Gray's Anatomy, 1980, p. 1081, similarly also in the 40th edition of 2008, pp. 459, 588/589)
Lönnig further states: "I have also checked several other detailed textbooks on human anatomy like Sobotta - Atlas der Anatomie des Menschen: they are all in agreement. Some also show clear figures on the topic. Pschyrembel - Germany's most widely circulated and consulted medical dictionary (262 editions) - additionally mentions 'Rr. ... bronchiales'."

So the RLN's sole purpose, or as ID-critic Kelly Smith put it, its "intended function," is not simply to innervate the larynx, as it provides innervations for the heart and even for the esophagus. And for those organs it takes a direct, or as Coyne might put it, "rational" route from the brain.

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 (See Smith's article in the volume "Intelligent Design Creationism and Its Critcis," p. 725). So what is Smith complaining about?

Part 3: Embryological Considerations for the RLN's Design

Lönnig also hints that there may be embryological reasons for the design of the RLN:However, just to refer to one possible substantial function of the Nervus laryngeus recurrens sinister during embryogenesis: "The vagus nerve in the stage 16 embryo is very large in relation to the aortic arch system. The recurrent laryngeal nerve has a greater proportion of connective tissue than other nerves, making it more resistant to stretch. It has been suggested that tension applied by the left recurrent laryngeal nerve as it wraps around the ductus arteriosus could provide a means of support that would permit the ductus to develop as a muscular artery, rather than an elastic artery" - Gray's Anatomy, 39th edition 2005, p. 1053.Another pro-ID thinker commented to me privately about this matter as follows:So according to this description, nerve filaments emanate along its length from the cardiac plexus to the esophagus, which if accurate might well dictate its positioning. Whether due to a requisite embryogenic sequence, for optimal nerve routing, or even an evolutionary carryover, it is a workable routing in all mammals, and therefore lacks substance as an argument against design based upon "poor design."Dr. Lönnig thus concludes:To innervate the esophagus and trachea of the giraffe and also reach its heart, the recurrent laryngeal nerve needs to be, indeed, very long. So, today's evolutionary explanations (as is also true for many other so-called rudimentary routes and organs) are not only often in contradiction to their own premises but also tend to stop looking for (and thus hinder scientific research concerning) further important morphological and physiological functions yet to be discovered. In contrast, the theory of intelligent design regularly predicts further functions (also) in these cases and thus is scientifically much more fruitful and fertile than the neo-Darwinian exegesis (i.e. the interpretations by the synthetic theory).

To sum up: The Nervus laryngeus recurrens innervates not only the larynx, but also the esophagus and the trachea and moreover "gives several cardiac filaments to the deep part of the cardiac plexus" etc. (the latter not shown below, but see quotations above). It need not be stressed here that all mammals - in spite of substantial synorganized genera-specific differences - basically share the same Bauplan ("this infinite diversity in unity" - Agassiz) proving the same ingenious mind behind it all.
Lönnig goes on to make a devastating critique of Dawkins and others who use the alleged poor design of the laryngeal nerve to refute ID. See Lönnig's article, "The Laryngeal Nerve of the Giraffe: Does it Prove Evolution?," for details.

The next section will show that the exact form of direct innervations of the larynx from the brain--demanded by ID-critics--in fact exists.

Part 4: Direct Innervation of the Larynx Demanded by Intelligent Design Critics Does Exist

As noted, ID critics Jerry Coyne, Kelly Smith, and Richard Dawkins have all argued that the allegedly circuitous innervations of the larynx from the brain by the RLN is an "imperfect design" that refutes ID. What they rarely disclose, however, is that there are in fact nerves that innervate the larynx directly from the brain through the superior laryngeal nerve (SLN), without taking the longer path of the RLN--exactly as they demand. Thus, the larynx is in fact innervated from both above and below, by both the RLN and the SLN. This is clearly seen in the diagram below, from Elsevier's Atlas of Regional Anesthesia, 3rd ed., hotlinked from here:

Additionally, the next diagram, hotlinked from a course website at the University of Pittsburgh's Nurse Anesthesia Program, also shows how the larynx is innervated directly from the brain:

As noted here, damage to the SLN can in fact affect the ability of people to swallow, yell, sing, or properly control voice pitch.

Thus, the preferred design of Coyne, Dawkins and Smith actually DOES exist in our bodies, as the larynx is innervated from above, directly from the brain.

Given the different medical conditions encountered when the laryngeal nerves (such as the SLN and RLN) are damaged, it seems that the two nerves are performing distinct functions. The SLN--which innervates from above--has something to do with producing higher-pitched sounds, and the RLN--which innervates from below--has something to do with producing lower-pitched sounds.

Rather than being an "imperfect design," perhaps the dual innervation of the larynx from both above and below is a good design principle -- a form of redundancy, or complementation to help minimize the impact upon function if one of these two nerves is damaged. Thus we see hints of function and design optimization in the innervation of the larynx. Again, this is evidence of optimization for a "global function," which ID-critic Kelley C. Smith admits is a good design principle.

Part 5: Medical Considerations for the Intelligent Design of the Recurrent Laryngeal Nerve

Thus far, 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:
  • (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.
  • 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: 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. Conclusion

    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).