Wolf-Ekkehard L�nnig: Under Neo-Darwinism, the Recurrent Laryngeal Nerve Must Have a Rational Design
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.
But a new article by pro-ID biologist, recently retired from the Planck Institute for Plant Breeding Research in Germany,Wolf-Ekkehard L�nnig, 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)
L�nnig 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 L�nnig 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.
In the next post and subsequent posts, we'll explore some potential benefits to the current design of the innervations of the larynx.