Monday, 27 April 2015

"Of what use is half an eye?" Plenty, actually. It's better than no eye at all

One of the classic one-liners employed by evolution denialists is "what use is half an eye?" Closely related to this is the claim that "there are no half-formed organs" in nature. Evidently, the evolution denialists have never encountered the nautilus. Here it is:

"Nautilus pompilius (head)" by © Hans Hillewaert. Licensed under CC BY-SA 4.0 via Wikimedia Commons.

Look at its eye. Unlike its fellow cephalopods, its eye does not have a lens. Instead, its eye has to make do with a pinhole lens:

An eye that lacks a lens definitely counts as a half-formed eye, but it is one that definitely works, and an eye with a pinhole lens is definitely much better than no eye at all.

The eye of the nautilus poses a few troubling questions for special creationists. Given that the nautilus' cousins such as the squid, octopus, and cuttlefish have complex eyes with lenses, why is the nautilus saddled with a suboptimal eye design compared with its cousins? The problem becomes more acute for the special creationist when we realise that the nautilus retina is comparable to that of its cousins as Muntz and Raj (1984) point out:
The retina, however, appears much more finely organized than the optical quality of the image would warrant: the size and packing of the receptors in Nautilus is, for example, comparable to that of animal with a minimum separable visual acuity of 17' or better.
The best analogy is that of a camera with a 10 megapixel sensor but a cheap plastic lens, or to bring the analogy closer, a pinhole lens. Muntz and Raj consider various possibilities such as response to bioluminescence (which they point out is unlikely) and conclude:
The presence of a well-differentiated retina, a mobile pupil, statocyst-mediated eye stabilisation, and a well-developed optomotor response, all suggest that vision is important for Nautilus, and under the circumstances it is surprising that the optics of the eye are so simple. 
Given that evolution does not look ahead, but simply selects what works, such subtoptimal design is not unexpected. Conversely, the special creationist is faced with the fact that the nautilus was created with simple optics and a well-differentiated retina, while its cousins have a much superior optical system:

One awaits a credible, peer-reviewed special creationist explanation for this curious example of suboptimal design.


Muntz W.R.A., Raj "On the Visual System of Nautilis pompilius" J. Exp. Biol. (1984) 109:253-263