ENHS Nature Trails Apr. 98 Article 2

President's column: The Eyes Have It
by Nathan Tublitz

Ever since Biology, and its predecessor Nature History, has been a scientific discipline, its practitioners have wondered about the underlying reasons for the structural similarities between disparate organisms. A comparison of the structural similarities of a particular organ found in different but related organisms oftentimes reveals a basic similarity of form. For example, the wing of a bird, the wing of a bat, the flipper of a whale, and the arm and hand of a human have strikingly resemblances in terms of bones, nerves, and muscles. Each has a single bone in the part of the appendage closest to the body, two bones in the next part, multiple wrist bones, and a variable set of digits. What makes this even more interesting is that there appears to be no inherent functional reason for these similarities since each is used in different ways for different functions. Darwin noticed this nearly 150 years ago and hypothesized that such similarities in organs of different species were expected if they all descended from a common ancestor. He suggested that once the basic structure was established in an ancestral species, various descendants would be able to modify the basic plan through evolutionary forces. Such structures in different species with a common evolutionary origin are said to be homologous structures.

However not all similar-looking structures arise from a common ancestor. Those that are not homologous but have similar functions in various species are said to be analogous structures. For example, the mammalian lungs and the air tubes (tracheae) of insects have each evolved independently as a solution to the common problem of gas exchange. Insect and bird wings are yet another example of very different analogous structures that arose independently to meet the challenges of flight. Both analogous and homologous structures are of great interest to biologists because they offer critical clues about evolutionary relationships. Analogous structures are of particular interest since they demonstrate that species with very different origins adapt in similar ways to similar environmental forces, a type of evolution known as convergent evolution.

One of the most often cited examples of convergent evolution has been the similarities in the structure of the eyes of mammals and cephalopods, that amazing class of molluscs that includes octopus, squid, and my personal culinary and scientific favorite, cuttlefish. Both mammalian and cephalopod eyes have an outer protective cornea, an adjustable iris diaphragm, a semi-elastic lens for varying the focal length, and a highly light-sensitive retina. Although both are remarkably similar in many respects, conventional biological wisdom has always believed this to be a superb example of convergent evolution since molluscs and humans are thought not to have had a common ancestor since the advent of multicellular organisms in the Cambrian.

Recent molecular evidence however has sent this long-held exemplar of convergent evolution careening to that portion of the intellectual scrapheap containing outdated biological theories. A Swiss developmental biologist has surprised the biological community by identifying a gene called Pax-6 which is expressed in the eye of mammals and octopus. These data raise the intriguing question about the likelihood of the same gene being expressed in the eyes of both organisms if each evolved independently. The same lab has shown that several other genes are also expressed in both mammalian and cephalopod eyes, giving rise to the idea that the eyes of both groups must be built from the same set of genetic building blocks. Same building blocks giving rise to a similar structure? Sounds suspiciously like they have a common evolutionary origin and that they are homologous rather than analogous structures.

But this story doesn't end here. The most recent data from the same Swiss lab indicates that the compound eyes of insects also express the product of the Pax-6 gene. Although involved in vision, multi-faceted compound eyes have little anatomical resemblance to the camera-type eye of cephalopods and mammals. Yet these data suggest that the eyes of insects are probably built from the same protein "building blocks" as the eyes of mammals and cephalopods. A good analogy is that of 3 children asked to build a house out of the same 20 Lego pieces. Although each child will produce a different looking structure, each will fundamentally be a serviceable house with at least one door and multiple windows. In the case of eyes, evolution may have taken the same approach, generating many different solutions to the problem of light detection using the same set of proteins. The data from insects, cephalopods, and mammals suggest that a set of eye-related proteins arose very early in evolution, probably back in the early Cambrian when multicellular organisms first arose, and that these proteins were used combinatorally to build different light-detecting eyes in different organisms. The evolution of eyes is just one of many examples emerging at all levels of Biology that i uminate the conservative nature of evolution, which appears to have used the same or closely related proteins in new combinations to produce new solutions to changing environmental conditions. This unlikely marriage of molecular biology with classical morphological and physiological analyses will undoubtably produce more exciting surprises in the years to come.

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