Wednesday, October 13, 2010

Circular, or at Least Recurrent, Reasoning

Casey Luskin has an interesting article on the Evolution News and Views website today.  He first notes the oft-cited principle that the strongest evidences for evolution are not shared instances of excellent design, but shared instances of kludged-up, jerry-rigged, or bizarre design: the inverted retina in vertebrate eyes, the panda's thumb, or (to take today's example) the infamous recurrent largyngeal nerve, which loops around the heart as it connects the brain to the larynx; in giraffes, this takes 20 feet of nerve to accomplish what one would think should take no more than one foot.

Then, citing a paper by "pro-ID biologist" Wolf-Ekkehard Lönnig, "The Laryngeal Nerve of the Giraffe: Does it Prove Evolution?", he notes that the laryngeal nerve is not, in fact, recurrent in all human beings.  In about half a percent of the population, the nerve in fact takes a direct and short route to the larynx.  From this, and based on the principle that if a mutation can occur once, it has probably recurred many times in humanity's time on Earth (and that of other species), Lönnig reasons that on Darwinian grounds, the circuitous path of the nerve must be more beneficial than a more direct course.  He notes that this is sensible since the nerve sends out fibers that in fact innervate several organs before it loops around to get to the larynx, so a direct route might well impair some of the other functions of the nerve.

This, in turn argues against the recurrent laryngeal nerve as evidence for common ancestry: if it is good design, then a competent Designer might well have used it.

I should point out, in passing, that Lönnig offers no actual evidence that variations in the path of the recurrent laryngeal nerve are mutations rather than developmental aberrations: the papers he cites make no mention of hereditability.  If these variations were beneficial but offspring were unlikely to inherit them, they could not spread through the population through natural selection.  But that's just a nit-pick; it seems very likely that some mutations could affect the path of the nerve.

A more telling response, I think, is this: why, given the multiplicity of nerves in the typical vertebrate body, should a single nerve serve the heart and larynx in all tetrapods?   It seems rather bizarre that, given the immense variety of tetrapod anatomies and ecological niches, that the recurrent path of the laryngeal nerve should be optimal in all of them.  Why should there not be some species, even if humans are not one of them (or giraffes another), in which a direct path did not prove best?

Common descent can explain this: the nerve, in an ancient vertebrate ancestor, did indeed follow a direct path to its ultimate destination (with, presumably, other stops along the way), as it still does in sharks today.  Given that many features of anatomy are affected by many different genes each (polygeny), and that a single gene may have multiple functions (pleiotropy), a change to one trait may affect many others, and "editing" a developmental path may change more than, say, the pathway taken by one nerve -- and of course may not change features that have evolved to take advantage of, e.g. the path of that nerve.  There may be no way to reach a particular improvement that does not pass through intermediates that work worse than either the present structure or the possible improved structure (this has been suggested as a reason why we still have the vermiform appendix: reductions in it would make it even more vulnerable to blockage or infection).

But how does design account for such features?  As David Hume noted over two centuries ago, if we found a house many of whose features were awkward and inconvenient, we should hardly accept as an adequate defense of the architect the fact that any single change might leave the house worse off; we would wonder why the architect didn't just scrap the whole plan and come up with a better one whose function didn't depend on such awkward features.  Evolution can't do that because it can't look ahead to see how mutation and selection will change a given structure in the distant future, so new designs are jerry-rigged out of and within the constraints imposed by older ones.

Meanwhile, in yet a further article on his own blog complaining about evolutionist testimony in Kitzmiller v. Dover, Cornelius Gordon addresses a similar point from a different angle.  Shared ERVs and pseudogenes, after all, are evolutionist arguments of precisely the same type (except at the molecular rather than the comparative-anatomical level) as arguments from shared recurrent laryngeal nerves.  Dr. Gordon has been reminded, repeatedly, that such comparisons of functionally-unrequired genetic homologies are, after all, the basis of paternity tests whose results we routinely accept in court cases affecting property and other rights.  Why should we accept the results of such tests within our own species but reject them between species?

Dr. Gordon, of course, focuses on that "between species" aspect.  We know that human beings produce human offspring; the only question is which human being produced this particular human offspring.  He then spends a couple of paragraphs pointing out that we've never seen a cat give birth to a puppy, and reminds us  that a common ancestor can give rise to both humans and gorillas is the very point at dispute.  I don't find this dispositive.  We know that speciation occurs.  We know that both significant changes in bodily proportions and behavior, and in interfertility between populations, can occur in the course of descent.  By this point, it becomes reasonable to demand what is supposed to prevent such changes from accumulating until one achieves descendants as different from one another as humans from rhesus macaques, or indeed humans from sharks?

One might as well point out that in fact not all men can sire children, but enough shared distinctive alleles between a man and his putative child put a strong burden of proof on the apparent father to show that he is one of those men.  By the same token, pile up the shared pseudogenes, endogenous retroviruses, and anatomical quirks like the recurrent laryngeal nerve, and the presumption ought to be that those ancient tetrapods really were capable, given time and opportunity, of evolving into ourselves.


  1. To your question: "Why should there not be some species, even if humans are not one of them (or giraffes another), in which a direct path did not prove best? Common descent can explain this:..."
    Well, there are some species. See Lönnig's Notes added in Proof: "b) According to Dietrich Starck – one of the leading German evolutionary anatomists of the 20th century – the recurrent laryngeal nerves are missing in the suborder Tylopoda (family Camelidae with camels, lamas and vicugnas), see Starck 1978, p. 237. However, Hans Joachim Müller, who published the results of his careful dissections on Camelus bactrianus and Lama huanacus [guanicoe] in 1962 , found that – although in fact, the innerveration of the larynx by the Nervus laryngeus inferior [the part of the RLN proximal to the Larynx] is exceptional  in these animals – there still is a ramus recurrens sinister, which arises from the vagus nerve near the heart and ‘curves around the arch of aorta’ in order to ascend at the latero-dorsal (and during further development at the more dorsal) part of the trachea, but does not innervate the larynx. Müller writes (p. 161):

    “Beim Überkreuzen der Aorta verlassen mehrere Äste den Nervus vagus und ziehen zum Herzen und zum Lungenhilus. Einer der Äste („Ramus recurrens sinister“) umschlingt den Aortenbogen und steigt rückläufig am latero-dorsalen Rand der Trachea auf. Im weiteren Verlauf liegt er mehr auf der Dorsalseite der Trachea, verbindet sich mit entsprechend rückläufigen Ästen des rechten Nervus vagus zu einem Nervenkomplex und anastomosiert schließlich mit dem absteigenden Ramus descendens n. vagi.”
       The fact that the ramus recurrens sinister does not innervate the larynx in the Camelidae, but still takes the ascendent course of the normal recurrent laryngeal nerve of all the other mammal families (so much so that J. J. Willemse thought he had even found a normal Nervus recurrens in a young camel ), yet to eventually anastomose with corresponding recurrent branches of the right vagus to take part in the formation of a special network of nerves, also implies important and indispensible functions of that route. As for similar observations on the ramus recurrens dexter, see footnote below . To discover or deepen our understanding of these necessary and probably further vital functions will be a task of future research."  

  2. „Lönnig offers no actual evidence that variations in the path of the recurrent laryngeal nerve are mutations rather than developmental aberrations“

    Doing my own research on the RLN I indeed think that it is most likely a developmental aberration, not heritable (at least in humans the direct route can also cause problems so we don't want to inherit it....), not (directly?) caused by any mutation. That of course does not refute Lönnig's point or as you put it: „it seems very likely that some mutations could affect the path of the nerve“ - therefore evolution *could* have changed the path of the nerve as those variations illustrate.

    The variations of the RLN also give a hint towards something more important for the discussion here: The route of certain nerves do not follow an exactly specified blueprint. In this regard it's not comparable to the wiring human constructions. Nerve growth follows a certain logic, it seems to be a very flexible and adaptive process. Consider possible requirements in development as well: Could a organ like the larynx develop properly if not innervated by a nerve at the right time? Unless there is a good understanding on such questions – ultimatly on the ontogenetic development of complex animals - how could we ever judge on the quality of the route that a specific nerve or vessel takes?

    And is the similar route of the RLN in so many diverse animals really a surprise considering that they share the same basic vertebrate body plan? Should the route of the RLN be interpeted as a special case of an evolutionary relic (suboptimal on top) that could not be changed over the course of millions of years (save some possible exceptions)? Or isn't it just a consequence of the same basic body plan, the same logic of nerve growth and a few developmental constraints in complex animals ('an organ must be innervated by it's nerve early in development in order to develop successfully')? In this case it would just be another feature showing similarities between species.

    Granted: If someone thinks that the exact route of each and every nerve and vessel in our body is specified in a blueprint and executed in a manner that is similar to wiring a car than the standard route of the RLN (and certain vessels) must seem pretty odd (unless he finds a certain functional advantage of the route). But thats not what we are dealing with here and that seems to me the real message of those variations of the RLN (as well as of many other variations in the anatomy of organisms). Calling certain features 'odd' or 'stupid' might serve the need for quick ammunition against creation but it does not inspire biological research.

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