Friday, November 23, 2007
Many More Than Twice-Told Tales
Here is a bit of reporting from Oz that is interesting but typically off-kilter, as science reporting all-too-often seems to be. However, the Oznians must be congratulated on their ability to broadcast such reports when, in the United States, discussing evolution is a place where even public broadcasting stations fear to tread.
The subject is hagfish and, after a few moments of audience-capturing "eeewh!" descriptions of the hagfish's ability to produce slime, they get down to the business of interviewing Trevor Lamb, head of the ARC Centre of Excellence in Vision Sciences at the Australian National University.
The reporter sets the scene with a brief discussion of Professor Lamb's work on photoreceptors and his realization that hagfish may reveal something about how photoreceptors evolved. Hagfish have organs on either side of the head that are called eyes, but when Professor Lamb's group started looking into the details, they realized that the organs are not really eyes, but something much simpler. Apparently they are similar to the Pineal gland in humans that controls circadian rhythms.
According to the story, the lamprey is a cousin of the hagfish, also eel-like and with no jaw. Supposedly, both the lamprey and the hagfish evolved during the Cambrian Explosion. (Okay, we know they mean "have identifiable ancestors dating back to the Cambrian.") But the lamprey has well developed eyes.
Professor Lamb: It's pretty clear from that, that you can say that the last common ancestor that we share with lampreys and that's 500 million years ago, already possessed an eye that was remarkably similar to our own.That's all very nice and the example of the hagfish (which I assume no one had noticed before) is a nice addition to the argument, but there was this fellow a while back who addressed the problem well enough:
Reporter: And that's why the discovery of Dr. Lamb and his associates is so important.
One of the main questions about Charles Darwin's theory of natural selection is how to explain the existence of an organ as specialised as the eye, unless a series of gradual changes can be shown.
Professor Lamb:You may not realise it but we are related to sea squirts and they just basically have a little eye spot. But, if you look at the characteristics of say their photoreceptors, and you compare them with those of hagfish and then lampreys and then fish and any land animals, you see that there's a smooth transition.
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree. Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real. ...This has not been 'a main question' about Darwin's theory of natural selection for a very long time, if it ever was, except in the fevered imaginings of creationists. And, given how long they've held out already against a mountain of daily increasing evidence, I doubt the hagfish will convince them, even if the hagfish has the ability to match them in slime production.
In the Articulata we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines, can be shown to exist, until we reach a moderately high stage of perfection. ... With these facts, here far too briefly and imperfectly given, which show that there is much graduated diversity in the eyes of living crustaceans, and bearing in mind how small the number of living animals is in proportion to those which have become extinct, I can see no very great difficulty ... in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.
He who will go thus far, if he find on finishing this treatise that large bodies of facts, otherwise inexplicable, can be explained by the theory of descent, ought not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection, although in this case he does not know any of the transitional grades. His reason ought to conquer his imagination; though I have felt the difficulty far too keenly to be surprised at any degree of hesitation in extending the principle of natural selection to such startling lengths.