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The Human Coccyx: A Study in Vestigiality

Following from the discussion in Detecting Design, here is a more in-depth look at two particular structures, the human coccyx and human appendix, preceded by some principles.


Creationists often claim that certain features are not vestigial because they perform a function. 

Here is dictionary.com's definition of 'vestigial':

1. Of, relating to, or constituting a vestige.

2. Biology. Occurring or persisting as a rudimentary or degenerate structure.


Of or pertaining to a vestige or remnant; like a vestige.


(biology) not fully developed in mature animals; "rudimentary wings" [syn. rudimentary]

Other dictionaries give similar results. 

So vestigial does not mean 'without function'. It means a leftover, a historical remnant. 'Vestigial' says nothing at all about how functional something is; it say nothing about a 'vestigial feature being useless, unimportant or worthless. 

Indeed, as far as this author is concerned, bird wings can be thought of as vestigial forelimbs. 

What the term does assume, however, is a history for the structure. Since it is whether the structure has a history that is in question, I prefer to avoid the term vestigial (and advise others to do so too), and instead concentrate on how, or if, the structure relates to the function (if any).

Testing competing hypotheses

There are two competing hypotheses here: that things have evolved by descent with modification, and that they have been created, designed. We donít need to let those two battle it out; instead we can look at each on its own merits, and see how each fits the facts.

In science, we test hypotheses by trying out predictions that can be drawn from them. If the predictions do not match reality, then the hypothesis is cast into doubt, or even completely refuted. If that happens, we can safely ditch the explanation as being wrong, even if we have no substitute, even if it leaves the problem explanationless.

An analogy might help here.

It is claimed that a well-known 'psychic' can bend spoons with the power of his mind. The hypothesis is that itís his super-natural powers that make said phenomena happen.

But what happens when we take the proposed hypothesis -- the power of an individual's mind over matter -- and test it? What are we to make of the powers when they show unexpected limitations? Are there any elements of the phenomenon that are not predicted by the hypothesis?

On investigation, oddities emerge. The mind-over-matter hypothesis does not predict that the 'psychic' could only bend the spoons after having had sole control over them, however briefly; that he can bend spoons, but not pound coins; that he can bend the spoons by touching them with his fingers, but not with, say, his nose; that magicians can replicate the phenomenon (perhaps not with the finesse of the 'psychic', but in all its particulars nevertheless); that his powers would evaporate when a knowledgeable magician (rather than a scientist) has set up the controls for the test.

If the hypothesis were correct, none of these other facts would be likely; nothing about the hypothesis leads us to expect these limitations on the guy's 'mind-over-matter' abilities. There are too many irrelevant (from the hypothesis) factors and features involved. Since there is no reason why, if the hypothesis were correct, he should be tied to handling things and replicating what magicians can do non-supernaturally, we can reject it.

We do not have to know how he does do it to know that it isnít by the power of his mind.

So one way to tell if a hypothesis is valid is to look for unexpected limitations, for places where the explanation has too much design of the wrong kind, for irrelevant features not directly explained. After all, the point of an explanation is to explain something.

Let's try this out with the human coccyx and appendix.

The Human Coccyx

Is the coccyx something that was designed as it is, or is it a reduced tail?

Neither creation nor evolution require the coccyx to be utterly functionless. But creation goes a little further, and denies that the structure is vestigial because it has a function. Therefore the structure must sensibly fit that function; the claim, in effect, is that it was designed that way for a purpose. So it should not contain unexpected elements or features: things the hypothesis does not predict, and / or things that hinder the claimed function.

What could the function of this interpelvic nubbin be, and does its structure fit that function? 

The creationist page Do Humans really have a Vestigial Tailbone? If not then what is it? is at great pains to explain all its multifarious functions, and quotes from Bergman and Howe's 'Vestigial Organs' are Fully Functional. (This does not bode well, given that, as we've seen, a vestigial organ does not have to be functionless...) In short, several muscles and ligaments insert on the coccyx. 

At this point I should direct the reader to the page Citing Scadding (1981) and Misunderstanding Vestigiality, since that is what the creationist page does. It also has a much fuller discussion of definitions than is required here. See you shortly!

Welcome back! 

So, are there any features of the human coccyx that the designed-as-it-is hypothesis does not predict? Please examine the pictures below.


Now some points about coccyx anatomy.

  1. The coccyx is made up of small, separate bones, apparently an extension of the vertebral column. These fuse during development to form a single piece. The hypothesis does not predict this: to 'serve as a point of attachment for ligaments and several important muscles', it need not be small separate-then-fused pieces. One has to wonder why it is not a single bone to start with.

  2. The small, separate-then-fused bones of the coccyx bear an unanticipated similarity in shape to the tail-bones of creatures with tails, especially to creatures with very small tails, such as hamsters, guinea pigs and mandrills.


This does not mean that it is a result of evolution; we're setting that aside for now. It is simply something the designed-as-it-is hypothesis does not predict, and something that is not required for the coccyx to perform its function. Why should a muscle-support structure resemble a tail?

  1. The human coccyx often, though not always, has another muscle attached to it, the extensor coccygis. As Grayís Anatomy puts it:

The Extensor coccygis is a slender muscular fasciculus, which is not always present; it extends over the lower part of the posterior surface of the sacrum and coccyx. It arises by tendinous fibers from the last segment of the sacrum, or first piece of the coccyx, and passes downward to be inserted into the lower part of the coccyx.

This muscle would, if it contracted, flex the coccyx. Unfortunately for the muscle, the coccyx is a fused single piece, and so it cannot do that. This is another feature of the coccyx that the hypothesis does not predict. (I'll note in passing that in animals that do have tails, the equivalent muscle to the human extensor coccygis has an obvious function.)

  1. It seems from genetic research that the same genes that form the tails of mice are also responsible for the formation of the human coccyx. Again, there is no reason, from the designed-as-it-is hypothesis, why this should be. We know from experiments that genes are often interchangeable even between radically different organisms (eg mice and chicks, mammals and insects). On the other hand, the necessary muscle-attachment structure might reasonably be formed as part of the pelvis, and / or as a single bone, and be shaped by the genes that do that elsewhere. It is odd at the least that tail-making genes should be responsible for something designed to "serve as a point of attachment for ligaments and several important muscles".

  2. Human embryos at around 28 days clearly have tails.

This structure is later reabsorbed by apoptosis (programmed cell death). It is made, only to be mostly destroyed. The made-as-it-is hypothesis does not predict this.

  1. Furthermore, there is a separate mechanism controlling the embryonic tail's apoptosis, so that the occasional human born with a tail isn't like that because of the reactivation of old genes, but rather because the genes to remove it have malfunctioned. The x-ray below shows that such 'tails' really are tails, complete with internal vertebrae. 

Special genes to reduce a substantial feature into a tiny one, just to be a 'point of attachment for ligaments and several important muscles', is not predicted by the hypothesis.

Coccyx summary

The coccyx does have some functionality. But nothing about what it does requires it to be made of fused, vertebrae-like bones; to have a muscle to move it, since the fused parts cannot move; nor to be made by the particular genes that it is. In short, nothing about what it does requires it to have the form of a tiny tail.

Its structure does not match with its claimed function. It is therefore overdesigned, containing features that are irrelevant.

Thus we can reject the made-as-it-is hypothesis as an adequate explanation. Even if we have no other to offer.

As it happens, we do. Descent with modification explains how one structure can, over the course of generations, become enlarged, reduced, added to or subtracted from, and even change function or lose function. And intriguingly, other creatures have coccyxes. It is just that they are often longer, unfused, and made of more bones. But when we encounter such longer, unfused coccyxes, we call them tails.

Evolution predicts that a tail-less creature whose ancestors had a tail might have a thing just like a coccyx, just as it predicts that a lineage that lost its hind limbs on returning to the sea might still have bits of bone shaped like parts of a pelvis and femur inside it. As many whales do.

Evolution is therefore a satisfactory explanation, while creation can be rejected as inadequate.

Creationists: Just for a moment, perhaps you could imagine that evolution is true. Please could you tell us what a reduced tail ought to look like?

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© Oolon Colluphid 2007. The contents of this site may be freely used for educational purposes provided they are attributed - only so that Oolon himself is not accused of plagiarism!