I’ve just been contributing to a BioLogos thread, in which the recurrent laryngeal nerve of the giraffe was flagged as a candidate for “bad design” (the thread was hived off from one on the equally bad design of the human reproductive system).
I’ve written about the giraffe before, citing a book by Craig Holdrege, and an e-book by Wolf-Ekkehard Lönnig has just come out on the same subject.
It’s odd really: the giraffe is a wonderful creature, and its adaptations are wonderful too, and yet it is of interest, for some reason, as an example of the bad design of one nerve in its body – a feature, moreover, that it shares with most land vertebrates. Fortunately, the general consensus on the thread was that the “bad design” argument is a poor one, and that “engineering constraints” or some other genuine reason lies behind the meandering course of the left recurrent laryngeal nerve.
I suggested that, because a direct course for the nerve is on, rare occasions, seen in humans, there may be some quite interesting laws of form, or fundamental genetic constraints, at play as well as, or instead of, the claim that common descent rules out any way of changing a feauture so early in embryonic development. Not only do some embryos do it – yet not show any adaptive advantage, apparently – but the pattern of embryonic development before the differentiation of the relevant structures vary widely between say, fish, frog, chicken and rat. And turtles have managed a similar feat of breaking the boundaries of tissue layers by putting their scapulae inside their ribs (see Turtles as Hopeful Monsters by Rieppel). There seems a good reason for research outside the paradigm of variation and natural selection in such examples.
The giraffe has also been an ikon not only from Darwin’s time, but from Lamarck’s, for other reasons: both assumed their theory explained how giraffes changed to be able to exploit food high in the trees in times of famine, and both turned out to be quite wrong, because there is good evidence that this is not why they are tall. And the alternative, sexual selection, has also been pretty well disproved (quite apart from the fact that what caused a female preference for outlandish height is no less mysterious than why giraffes are tall without it).
And so we are left without any real explanation why giraffes are tall. Yet they are. Very. As to the how, Richard Dawkins’ easy dismissal of any problem once a variation for a longer neck is selected is belied by the long list of associated necessities (listed and referenced by Lönnig):
- Abnormally muscular oesophagus to raise cud 3m for chewing
- Specialised muscular arteries
- Special valves in veins
- Special blood-storing arteries at base of brain
- Adjustments to length, power etc of all skeletal, musle, nerves, arteries
- Pressure sensors in carotid arteries activate arterial muscular layer to control blood flow
- Arterial walls unusually thick
- Tissue fluid maintained at high pressure by thick, tight skin
- High respiratory rate to compensate for increased tracheal dead space
- Large lungs and diaghragm to enable this unusual respiration
And that list just refers to the requirements of the long neck itself – there is much more that needs changing to make a giraffe function as a whole. They tell me all this is nothing to the power of evolution, though changing a recurrent laryngeal nerve proves impossible over the whole course of vertebrate evolution. It seems to me we don’t actually know that either claim is true. So I still suggest that coordinated beneficial adaptive mutations for these changes make this a problem from adaptive evolution – especially given the lack of any clear adaptive requirement, as above. But the currently popular neutral theory is even less explanatory – one can imagine that random changes that produce a long neck without making survival impossible might just make the giraffe an interesting freak – but that shopping list of adaptations isn’t going to happen by chance, and it isn’t going to happen at all unless the giraffe gains the adaptations necessary to survive it in tandem.
And so the usual recourse to gradualism is made, even though evidence for that is lacking. There is a gradual transition waiting to be discoved, or conjured from “intermediate” forms that aren’t, phylogenetically, intermediate. We have to imagine the gradual transition, and we have to imagine an adaptive role for it. Yet thinking outside the scientific box (that is, countenancing teleological explanations) everything about the giraffe might suggest a creature determined to become big (for some overwhelmingly important reason no more speculative than the adaptive reasons proposed by Darwin).
Craig Holdrege, a Goethian, points out in his book how every feature of the giraffe has that same flavour of “tallness”. And it seems to me to make more sense as an animal with “tallness” stamped on every part than as a piecemeal adaptation of an okapi-like creature. Others of course would not see the issue – Steve Mathieson in reply to the post last linked doesn’t see why there’s any mystery at all. Well, ignoramus that I am, I just see mystery everywhere, and very little in the way of plausible answers that stand up to examination.
Darwin, by assuming an adaptive “goal” for the giraffe’s long neck, used as existing evidence in support of the plausibility of natural selection. Steve, it seems, in the absence of any clear remaining evidence of an adaptive goal, instead assumes (a) that natural selection could easily make such a transition and that (b) ergo it did.
I must lack imagination – though I know a supremely beautiful wonder of nature when I see it.