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We read with great interest the article by Brodsky1 on the evolution of human stereopsis. Brodsky shows that the oblique muscles in humans seem to have abandoned their previous role in lower organisms in promoting panoramic vision and instead serve to stabilize the eyes to facilitate stereopsis. As Brodsky points out, this may be a very concrete example of evolutionary exaptation. This controversial term was originally coined by Gould and Vrba2 and means that a certain gene or trait that has evolved to fulfill a specific function takes on a new role (ex) and gets a new function (aptus) for which it was not originally designed.
We wish to report that accumulating data from functional analyses of homologous genes in an increasing number of different organisms support the view that exaptation indeed is an important mechanism during evolution. Again, an interesting example can be found in the evolutionary history of the eye. In the zebrafish embryo, the not really finished (nrf) gene is essential for the development of photoreceptors.3 However, its primary role in humans is to facilitate nuclear-mitochondrial interactions in a wide variety of cell types and in chickens to promote erythropoiesis.3 Similarly, the intraflagellar transport genes used by Chlamydomonas to make motile flagella are also used to assemble cilia in chemosensory neurons sensing salt concentration in Caenorhabditis elegans and in mechanosensitive kidney epithelial cells in vertebrates.4 These examples and the article by Brodsky show that the same genes (and muscles) can be used for very different tasks in different organisms and underscore the validity of the exaptation concept for the understanding of evolutionary processes.
Correspondence: Dr H. Zetterberg, Center for Neurologic Diseases, Brigham and Women’s Hospital, Harvard Medical School, Harvard Institutes of Medicine, HIM-610, 77 Avenue Louis Pasteur, Boston, MA 02115 (hzetterberg@rics.bwh.harvard.edu).
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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