In his current book, “The Greatest Show On Earth”, Richard Dawkins includes a fascinating look at embryology. His intention, indeed his accomplishment, is to show that assembly of complex structure can work on a very short time scale. This is valuable, since the concept of geological time is one of the greatest stumbling blocks to understanding evolution. It’s a non-starter for young-earth (6,000 year) creationists, but it’s also a big problem for everyone else because it’s well beyond staggering – it’s actually incomprehensible.
Dawkins discusses several analogies between natural construction and human activities. One of these is the art of paper folding known as origami. In origami, small steps are taken to achieve intermediate ‘larval’ stages. Emphasis is on the simple folding task at hand, not on the finished design. This is similar to the local rules obeyed by cells during embryonic development, by proteins as they fold into 3-D molecules, and viruses as they ‘self-assemble’. No overall blueprint is ever used, or indeed is ever necessary. Each embryonic cell may have a complete copy of the organism’s DNA, but each cell type will have a slightly different subset of these genes turned on for use. This is how successive generations of cells gradually differentiate.
Thus, local, simple rules govern local changes that ripple, augment, and feed back within the embryo. The origami of each small part participates in the eventual production of the finished, complex organism. This complexity is emergent and is very much unlike top-down architecture. Dawkins doesn’t like the commonly used DNA-blueprint analogy. Emergent complexity is not reversible. One could draw a blueprint from a finished building by taking measurements, but there is no way to reconstruct DNA from a finished organism. (Extraction is not reconstruction.)
To his credit, Dawkins honestly and clearly states that embryology is not evolution. Embryology merely utilizes a pre-existing genetic code while evolution is the process that assembled that code over hundreds of millions of years.