Suppose that I told you that I have an appointment in Atlanta in two days, and that therefore I had to leave from Athens right now, because one cannot get to Atlanta from Athens in less than two days. Suppose that I further claimed that the idea that people actually could live in Athens and commute to work in Atlanta on weekdays was preposterous, because you can't get there in less than two days.
Probably you'd quietly take me aside and explain how we have cars, bikes, trains, and busses, all of which can get a person from Athens to Atlanta in less than a day (although it would be a long day on the bike!).
The Intelligent Design community is quite enthused about the publication in 2004 of an article in a peer-reviewed, mainstream journal, The Proceedings of the Biological Society of Washington, by Dr. Stephen Meyer of the Discovery Institute. In this article, Meyer attempts to establish that conventional evolutionary mechanisms would not have been able to produce the sudden diversity of organisms and body forms that burst on the scene in the Cambrian 'explosion'. The diagram below, from a publication by Meyer in 2003, illustrates that the majority of known animal phyla had their origin in the Cambrian period, and that very few phyla had their first appearance before or since. On the face of it, this appears to be a problem for mainstream evolutionary biology, since such a variety of phyla appear in a geologically brief period of 10-15 million years. If we assume that these phyla did not exist during the pre-cambrian period, then, according to Meyer, there is a big problem in how so much morphological diversity could have been generated in such a short time window to produce all of the numerous phyla that suddenly appeared in the Cambrian 'explosion'. This criticism, though, is an example of walking to Atlanta.
|To understand the main points of Meyer's article, one needs to know a little
about DNA and genetics. DNA is the molecule of heredity in the nucleus of each
of the cells of an animal (or a plant!). It is shaped roughly like a twisted ladder,
and is extremely long -- for humans, the number of "rungs" of this twisted ladder
is > 3 billion! Each rung is called a base. The instructions for building an organism
are encoded in the sequence of many, many bases that make up the rungs of the DNA ladder.
In parts of the DNA that code for proteins, each protein subunit (called an amino acid)
is specified by a three-base piece of DNA called a codon. Genes are long stretches
(usually a few hundred to a few thousand bases) of DNA, each of which is the instructions
for making one or more proteins. The diagram to the right is a schematic of part of a
DNA molecule. Two codons are outlined by the boxes; reading from the top down, and
reading only on the left side, the upper codon is TGA, and the lower codon is TGT (note
that there are others in between these two). Each codon specifies some amino acid,
but since there are 64 possible codons and only 20 amino acids used in making proteins,
there is a lot of redundancy; in other words, several codons may specify the same
amino acid. Finally, there are several codons that signal "stop", to terminate a
Proteins are the building blocks of organisms, but proteins are not the only things specified in the DNA code. There are large stretches of DNA that have no known function, both within genes, and between genes. Also, many sections of DNA are regulatory, in that they turn genes on or off at certain times or places.
The genetic basis of evolution centers on the fact that the huge DNA molecule contains all the genetic instructions for making an organism. DNA is passed on from parents to offspring, so there is genetic continuity across generations. Mutations are changes in the DNA that affect the proteins that are made, or the way genes are regulated. A simple type of mutation is called a 'point mutation', and it might change one base from a G to C, with the result that a codon changes from CGA to CCA. That codon change may cause a different amino acid to be built into a protein, and possibly change that protein's structure or function.
|The diagram at the right is a different representation of the Cambrian 'explosion'. It shows that most phyla of animals have no fossil record before the Cambrian period, but that many appear quite suddenly in the early Cambrian; the open circles represent the hypothesized divergence from a common ancestor, as predicted on the basis of neo-Darwinian theory. To reiterate the major point of Meyer's paper: the Cambrian 'explosion' represents a lot of morphological diversity appearing in a geologically brief moment, so there is no way that neo-Darwinian mechanisms of mutation and selection could produce so much morphological change in so short a time. Meyer points out that new organisms require new cells, new tissues, new organs, and new organ systems, in addition to the regulatory genes to control the building of these things. Even if we imagine some primitive unknown Pre-Cambrian ancestor of such a diversification, Meyer claims that most mutations are harmful, and that given known rates of point mutations, natural mutation & selection would be able to accomplish only a tiny fraction of the morphological diversification that appears in the fossil record in 10-15 million years at the beginning of the Cambrian period. Thus, Meyer concludes that the clear alternative, and superior explanation, is Intelligent Design.|
|To emphasize the needed amount of genetic change, Meyer points to one possible measure of the morphological complexity of organisms -- the number of different cell types. This is illustrated in the diagram at right. New protists and sponges are not so hard to imagine evolving in 10-15 million years, because they are simple, and have only a few cell types. On the other hand, chordates (illustrated by the fish) have 60 different cell types, in addition to the complex organs and organ systems not found in simpler organisms.|
|The presumed difficulty with evolving the morphological changes shown by the fossil record of the Cambrian explosion is illustrated at right. Meyer points out that proteins are extremely complex molecules, and that some studies have demonstrated that even slight changes in the amino acid sequence that makes up a protein makes it non-functional. This, he says, is why it would be EXCEEDINGLY difficult to evolve proteins with a new function: any change in the amino acid sequence of the original protein would result, in all probability, in a non-functional intermediate that could not serve as a bridge to a protein with a new function. A language analogy is the illustration here -- the starting sentence at the bottom is analogous to the original protein (each letter in the sentence corresponding to an amino acid in the protein), and the ending sentence at the top represents the putative new protein. If we randomly replace letters, as shown in 'the abyss' of the central part of the diagram, Meyer contends that the sequence quickly becomes gibberish that cannot evolve into the ending sentence. Similarly, he says, a few random amino acid substitutions in a protein will most likely destroy its function long before a new protein function could appear. This problem must then be multiplied out across many proteins, many cell types, etc.|