I'm busy with this book at the moment, by Matt Ridley, and am otherwise deep in personal study of embryology, how the body plan starts the unfoldment of its eventual self (once the proctodeum/stomodeum is in place) from the neck down and neck up at once, not in any linear manner but three- dimensionally.
This comment in his book (p. 33) has taken hold at the moment, made in the context of remarks about how few genes we have (30,000) compared to how many we had guessed we had (100,000):
The beauty of the system is that the same gene can be reused in different places and at different times simply by putting a set of different promoters beside it... To make grand changes in the body plan of animals, there is no need to invent new genes, just as there is no need to invent new words to write an original novel... All you need to do is switch the same ones on and off in different patterns. Suddenly, here is a mechanism for creating large and small evolutionary changes from small genetic differences. Merely by adjusting the sequence of a promoter, or adding a new one, you could alter the expression of a gene. And if that gene is itself the code for a transcription factor, then its expression will alter the expression of other genes. Just a tiny change in one promoter will produce a cascade of differences for the organism. These changes might be sufficient to create a wholly new species without changing the genes themselves at all. (Carroll, S.B. 2000. Endless forms: The evolution of gene regulation and morphological diversity. Cell 101:577-80)
I very much like the idea that timing and context are just as important as actual matter or code in this regard, just as much as they are in any regard.