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Cellular Zip Codes: Where's the Postmaster?

In 1970, Nobel laureate Jacques Monod called DNA the "secret of life" and said that the discovery of its structure and function -- especially "the understanding of the random physical basis of mutation" -- means that "the mechanism of Darwinism is at last securely founded" and that humans are "a mere accident."[ 1]

According to neo-Darwinism, all living things are descended from a common ancestor, modified by natural selection acting on random variations that are generated by DNA mutations. But only if an embryo's development were programmed by its DNA could mutations in DNA provide the raw materials for large-scale evolution. So neo-Darwinism assumes that embryo development is controlled by a genetic program.

But there is a serious problem with this assumption.

The many different kinds of cells in an animal or plant develop from a single fertilized egg cell. Humans, for example, consist of cells that form bone, skin, muscle, digestive organs, nerves and many other tissues. Such cells are so different from each other in form and function that an untrained observer might conclude that they represent different species.

Yet all of these cells contain the same DNA, a fact long known to embryologists as "genomic equivalence." As the fertilized egg divides, it bequeaths a complete set of DNA (its "genome") to all of its descendants -- with a few minor exceptions, such as red blood cells, which have no DNA at all. But if bone, skin, muscle, digestive and nerve cells all have the same DNA, why are they so different? Why don't nerve cells secrete juices that digest the brain? Part of the answer is that although brain cells have the genes for digestive juices, those genes are turned off in nerves. As an embryo develops, its cells go through a phase called "differentiation" that turns some genes on and leaves others turned off.

But this does not solve the problem, since it begs the question of why two cells with the same DNA would differentiate in two distinct ways.

Another part of the answer is that cells somehow know where they are in the body and differentiate appropriately. In July 2006, a scientific article reported that certain cells have "zip codes" in their DNA that correspond to their locations. According to the article:

A major question in developmental biology is, How do cells know where they are in the body? For example, skin cells on the scalp know to produce hair, and the skin cells on the palms of the hand know not to make hair... In this study, the authors present a model that explains how cells know where they are in the body. By comparing cells from 43 unique positions that finely map the entire human body, the authors discovered that cells utilize a ZIP-code system to identify the cell's position in the human body. The ZIP code for Stanford is 94305, and each digit hones in on the location of a place in the United States; similarly, cells know their location by using a code of genes. For example, a cell on the hand expresses a set of genes that locate the cell on the top half of the body (anterior) and another set of genes that locates the cell as being far away from the body or distal and a third set of genes that identifies the cell on the outside of the body (not internal). Thus, each set of genes narrows in on the cell's location, just like a ZIP code. [2]
Yet the existence of "cellular zip codes" still doesn't solve the problem either (and the authors of the article don't claim that it does). If the human body were the United States and cells were postal envelopes, each would start out bearing every zip code in the country on its face. Only after the postmaster had stuck each envelope into one of many slots on the wall to direct it to its final destination would a particular zip code be highlighted. Obviously, the postmaster and the array of slots play a major role in determining where each letter goes.

If the DNA corresponds to zip codes that are originally the same on every envelope, where in the embryo are the postmaster and the slots? What is it that highlights one zip code but not others? Where is the all-important developmental information that directs cells to different parts of the body and tells them where they are and how to differentiate?

By focusing attention on DNA as the supposed source of raw materials for evolution, neo-Darwinism has systematically downplayed the nature and location of developmental information elsewhere in the embryo. Obviously, there is more to embryo development than is dreamt of in neo-Darwinian philosophy.

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NOTES

[1] Quoted in Horace Freeland Judson, The Eighth Day of Creation: The Makers of the Revolution in Biology (New York: Simon and Schuster, 1979), pp. 216-217.

[2] Rinn JL, Bondre C, Gladstone HB, Brown PO, Chang HY (2006) Anatomic Demarcation by Positional Variation in Fibroblast Gene Expression Programs. PLoS Genet 2(7): e119 DOI: 10.1371/journal.pgen.0020119.