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A Good Question from Michael Denton About the Fixity of Animal Body Plans

Biochemist Michael Denton (Evolution: A Theory in Crisis, Nature’s Destiny: How the Laws of Biology Reveal Purpose in the Universe) was in our offices this week and he casually posed a question that I, for one, had never considered. Hundreds of millions of years ago, all these animal body plans became fixed. They stayed as they were and still are so today.
Before that — I’m putting this my way, so if I get anything wrong blame me — of course they had been, under Darwinian assumptions, morphing step-by-step, with painful gradualness. Then they just stopped and froze in their tracks.
The class Insecta with its distinctive segmentation, for example, goes back more than 400 million years to the Silurian period. It gives the impression of a creative personality at work in a lab. He hits on a design he likes and sticks with it. It does not keep morphing.
This is exactly the way I am about recipes. I experiment with dinner plans, discover something I like, and then repeat it endlessly with minor variations from there onward.
Why does the designer or the cook like it that way? Well, he just does. There’s no reason that can be expressed in traditional Darwinian adaptive terms. There is no adaptive advantage in this fixity of body plans. Why not keep experimenting and morphing as an unguided, purposeless process would be expected to do? But nature doesn’t work that way. It finds a good plan and holds on to it fast, for dear life. This suggests purpose, intelligence, thought, design. Or is there something I’m missing?