Vital Dust: Life as a Cosmic Imperative

MUSICIANS have discovered that if they have a good theme, they can create a rich work by repeating the theme many times in different variations. Composers of serial music have exploited this formula to the utmost, by moving from one variation to another in almost imperceptible steps. Something similar happened in animal evolution. After the basic blueprint was completed, further progress was made by duplication and variation of the blueprint.

The first step in the new direction was accomplished by a remarkable genetic modification that led to the formation of a multisegmented animal looking for all the world like a string of primitive worms joined end to end by minimal connections. Each segment of this strange creature was, in itself, an essentially complete organism, with an alimentary canal, two nephridia, a circulatory network linked to a pair of gills situated in lateral outgrowths of the body, male and female sex organs, a rudimentary innervation radiating from a centralized group of nerve cells (a ganglion), a set of circular and longitudinal muscles, and a surrounding reinforced skin, or cuticle. Separated by incomplete partitions, the segments were linked to each other mainly by the skin; by the alimentary canal, which was continuous; by two large blood vessels, one running along the back, the other along the belly of the organism; and by a nerve cord joining the ganglia. In this early form of the organism, the head and tail were constructed like the other segments.

This organism clearly originated from a number of copies of the same individual, linked end to end. Yet a molecular biologist sent to Earth at that time would have found that most genes were present in the organism's genome in only single copies. Only a small number of genes, all grouped together, were present in as

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