The eye is the original instance of “irreducible complexity,” a biological structure supposedly too complicated to have evolved by undirected mutation and natural selection. Darwin made a point to deal with the evolution of the eye in The Origin of Species. He argued that, in spite of appearances, a surprisingly complete gradation of eye complexity is seen in nature, and it’s not too hard to connect the dots. Brace for Victorian prose:
In the Articulata [Arthropods] 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 … can be shown to exist, until we reach a moderately high stage of perfection. In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. With these facts … I can see no very great difficulty (not more than in the case of many other structures) 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.
Since Darwin’s day, biologists have developed much more detailed descriptions of how eyes might have evolved [$-a] from simple light-sensitive “eyespots” all the way up to the complex structure of mammalian eyes. But we haven’t had a good description of how those original, hyper-simple eyes actually work. Light hits them, and their owner responds to it – but what’s the connection between stimulus and response?
As part of an early kickoff for Darwin’s 200th birthday celebration next year, this week’s issue of Nature has a paper that provides the answer: eyespots directly control how their owners move [$-a]. The authors, Jékely et al., use a variety of molecular biology methods to dissect the connection between eyespots and movement in the larvae of a marine flatworm, Platynereis dumerilii. Most of the experimentation wasn’t too kind to the larvae.
Platynereis larvae are tiny spheres with belts of cilia, their only means of propulsion, and an eyespot on either side of one hemisphere. The eyespots consist of only two cells each, a pigment cell and a photoreceptor, and they seem to be useful in helping the larva move toward light sources (i.e., further up in the water column). This tendency to move toward light is called “phototaxis.”
First, the authors burned off one eyespot or the other using a laser, and showed that larvae missing both eyespots were unable to move toward light, but those missing only one were mostly able to do so. Then they cut larvae in cross sections and, under an electron microscope, traced the body of an eyespot’s photoreceptor cell – which turned out to extend all the way to the cells in the equatorial cilia. It’s as if a human’s eyes were directly connected to her legs. The authors further show, in fact, that the Platynereis larvae swim in a manner perfectly adjusted for steering by eyespots; when one spot receives light, it makes the cilia on its side beat harder, and the larva banks toward the light source.
Like evolution itself, science proceeds slowly, step by tiny, hopefully useful step. This paper is one more piece in the enormous puzzle of life on Earth – the kind of work that has moved biology as far beyond Darwin’s first conjectures as the human eye is from a flatworm’s.
References
G. Jékely, J. Colombelli, H. Hausen, K. Guy, E. Stelzer, F. Nédélec, D. Arendt (2008). Mechanism of phototaxis in marine zooplankton. Nature, 456 (7220), 395-9 DOI: 10.1038/nature07590
T. Lincoln (2008). Cell biology: Why little swimmers take turns. Nature, 456 (7220) DOI: 10.1038/456334b