Losing My Wings Main Menu Yagharek Longs to Fly Yagharek, from China Mieville's, _Perdido Street Station_, gives up the dream of flight Diptera: Insects with two wings Flies and humans Fallen Angels: Loss as Transformation David Bowie explores themes of space existence in his songs from the 1970s through 1980s From Sensory Bristles to the Spots on a Butterfly's Wing Evolution through co-option Gothic Biology Limb Development in the Human Embryo A description of early human limb development Popular Culture and Extraordinary Bodies

Variations upon variations

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Most thinking about molecular development begins with a paradox. How could the same materials that created a specific stage of an organism’s life create other stages as well? Insects, such as fruit flies, not only grow  larger, they also dramatically change their forms as they grow. The untrained eye can find little similarity between the soft grub of a juvenile fly and the well-articulated, chitin armored appearance of the adult. Even humans go through subtle chemical metamorphoses, as some lactose intolerant
adults lose the ability to metabolize the sugar lactose, a metabolic skill they relied upon as an infant. Why did they lose this ability (or better yet, what happened to allow others to keep this juvenile state?) Or, in more biologically precise terms, how could a limited nucleic acid code account for all the changes organisms exhibit during their lives? Clearly, something was happening to allow some parts of the code to be expressed at one time and not at another. This implied that genes not only needed to be turned on, they had to be turned on at the right time and in the right place.

In his XXXX book, Endless Forms Most Beautiful, Sean Carroll summarizes the conceptual innovations that needed to occur to explain this type of site specific changes. Key for Carroll was the recognition that development occurred as a “logic of making a series of initially similar modules and then making them different from one another.”34 Key to this realization is that bodies varied twice in the evolutionary and developmental logic. The first insight on variation was that animals were composed as a series of body segments that varied from head to tail. The key idea here was the idea of the segment, that could be modified depending on where it was found in the developing embryo. Thus each segment provided the raw stuff to make wings in one section of the body and halters in another. The crucial innovation was that each of these segments could vary as well. This would explain our four winged fly. Depending on some form of mutation the segment that was supposed to be an abodominal segment could be transformed into a thoracic segment. 

This remarkable view of animal development also explained differences and simlarities between species and not just within.