Most of this book is a primer on how the study of DNA code from various species sheds light on the evolutionary process. The text is as clear as such a text can be considering how abstract the DNA sequences are to even the well educated reader. There are numerous charts and tables, drawings, black and white photos (and some color plates) and such in this timely, handsome and well-presented book to guide the reader. I only wish that I could have grasped the details in a more concrete manner.
DNA codes for proteins, of which there are vast numbers. These proteins are formed from amino acids of which life uses twenty, and in turn these amino acids are called up by the sequence of letters in the code. Presumably (Carroll does not make this clear) as the zygotic cell divides, working its way to the composition of the complete organism, the DNA code is read in sequence like a tape fed into a bar code. First this protein and then that protein and then still another is made and somehow strung together in an exacting order so that, voila! a massively complex organism is constructed. What is not in this book is an explanation of how these proteins know where to go and when. Presumably that knowledge is part of the very sequence of the code, or perhaps it is implicit in the positions in space of the proteins relative to one another. In others words, the DNA code is only the most obvious and "visible" part of the microscopic reproductive process.
If you are like me and are looking for the same sort of explanation, this book will be of limited value. Prof. Carroll's purpose is not to make transparent the reproductive process at the chemical level. His purpose--and a laudable one it is--is to show how DNA analysis is yet another piece of evidence pointing to the truth of biological evolution. That is why he uses the word "forensic" in the subtitle.
One of the most powerful uses of DNA is in reconstructing the so-called "tree" (or "bush") of life. Carroll shows how it is now clear beyond almost any doubt that our closest relatives are chimpanzees and bonobos followed by the other great apes and then monkeys. He shows how DNA analysis can also (and by the same logic) be used to show the relative age of species. Interesting is the discovery of how exactly similar are some sections of code in diverse species, indicating that such code is very ancient. In fact Carroll points to "immortal" sequences of code that have resisted all attempts at corruption or mutation. He explains that such code is so nearly indispensible to living forms that natural selection is, and has always been, active in keeping it intact.
In this regard (and moving to the latter chapters of the book) we find a particularly delightful refutation of one of the notions of "Intelligent Design." Carroll quotes perhaps the best known of the intelligent designers, Dr. Michael Behe, as writing:
"Suppose that nearly four billion years ago the designer made the first cell, already containing all of the incredibly complex biochemical systems discussed here and many others. (One can postulate that the design for systems that were to be used later, such as blood clotting, were present but not `turned on.' In present-day organisms plenty of genes are turned off for a while, sometimes for generations, to be turned on at a later time)." (p. 244)
How brilliant this sounds! However Carroll writes:
"This is utter nonsense that disregards fundamentals of genetics. Dr. Ken Miller of Brown University has described this scenario as `an absolutely hopeless genetic fantasy of pre-formed genes waiting for the organisms that might need them to gradually appear.' As we saw in chapter 5, the rule of DNA code is use it or lose it. The constant bombardment of mutation will erode the text of genes that are not used, as it has in icefish, yeast, humans, and virtually every other species. There is no mechanism for genes to be preserved while awaiting the need for them to arise." (p. 244)
Indeed, if Behe were correct, there would be in virtually all species "silent pre-formed genes" waiting to be called upon. There aren't.
In the chapter entitled "Seeing and Believing" Carroll recalls Louis Pasteur's struggle to demonstrate to non-seeing and non-believing doctors that childbed fever was caused by their dirty hands. He reprises the horrific and bizarre story of the Soviet head of (political) biology Trofim Denisovich Lysenko who denied genetics, and how Stalin's support of him led to massive failures in agriculture and subsequent starvation. He further recalls how Mao Zedong, using the same unscientific ideas, sponsored massive starvation in China due to crop failures.
What Carroll is getting at is that political corruption of science can be very dangerous. In the United States today under the power of the Bush administration, faith-based (and corporate-sponsored) science is denying global warming and other deleterious effects of rampant pollution. This sort of science denial is likely to lead to human suffering and death, just as did the communist denial of genetics.