Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom by Sean B. Carroll. W.W. Norton, $25.95, 350 pages.

The News & Observer

December 25, 2005

How the mindless processes of evolution create life forms brimming with intelligence

By PHILLIP MANNING

Shortly after the publication of "On the Origin of Species," Charles Darwin received a letter from the Reverend Adam Sedgwick, who said he found many of Darwin's conclusions "utterly false and grievously mischievous."

The hostility toward Darwin's theory of evolution continues a century and a half later through the sophisticated descendent of Creationism -- intelligent design or I.D. I.D. rejects the Creationists' belief in the biblical story that God created all the species in their current form in a busy six days about 10,000 years ago. However, it also holds that organisms are too complex and too varied to have been produced by Darwin's mindless process of evolution. Such diverse forms, I.D. advocates claim, must have had a designer, a very intelligent designer.

If polls and headlines are a gauge of public opinion, then anti-evolution sentiment is sweeping the country. Only one-third of the respondents in a recent NBC poll thought evolution could account for the origin of human life, while 44 percent believed that God created the world (and humans) in six days. In another poll, more than 60 percent of the responders favored teaching creationism alongside evolution in public schools.

Of course, there is no scientific evidence for Creationism or I.D. The two movements attempt to cast doubt on evolution -- then suggest that their claims must be true if there are any chinks in Darwin's armor. Yes, Darwinism is a theory -- so is Newton's law of gravity. But both are so thoroughly tested and well-established that they are almost unanimously accepted by scientists.

Oddly enough, all this public questioning of Darwinism is happening just as biologists are making tremendous strides in explaining the mechanics of evolution. The new insights are coming from a marriage between evolutionary and developmental biology. Called evo devo for short, it adds to the mountains of proof of evolution, further undermining I.D. and Creationism.

In his important new book, "Endless Forms Most Beautiful," Sean Carroll tells how evo devo developed and how it is changing evolutionary biology. Carroll is the right man to write this book. A professor of genetics at the University of Wisconsin-Madison, where he manages a cutting-edge evo devo research program, he has an easygoing writing style that helps the reader grasp some of the book's complicated arguments. Even so, it will not be an easy read for those without some background in biology.

The central question addressed by evo devo is the one posed by opponents of evolution: How could random mutations in DNA produce the incredible complexity and diversity of living organisms? I.D.'s answer is, they can't; living organisms are too intricate to have been created by a mindless process. From this assertion, I.D. makes a staggering leap, claiming that a supreme being must have designed life.

Carroll rejects that conclusion as nonsense. The key to understanding biological diversity, he explains, lies in figuring out how organisms develop. According to evo devo, animals are made from sets of the same, or very similar, genes. What we look like -- be it fruit fly, snail or human -- is determined by how and when those genes are expressed in the developing embryo. Although the end products are quite different, the building blocks are the same. Think of it this way: Toy models of the White House and the Washington Monument do not resemble each another from a distance, but close inspection reveals that they are both made of interchangeable parts -- Legos.

Carroll calls these building-block genes "tool kit genes." Their discovery came from scientists' work with mutant fruit flies, called Frankenflies. These are flies, Carroll writes, "with legs coming out of their head, or with extra pairs of wings, or feet in the place of mouthparts." The surprising thing about these weirdies is that they were not created by changing many genes but a single one. How could one gene affect an animal's form so dramatically?

In the early 1980s, scientists determined that a cluster of genes called Hox genes shapes the bodies of fruit flies, specifying to the developing embryo what appendages go where. A Hox gene in the embryonic segment that is to become the fly's head tells that segment to grow an antenna. Another Hox gene causes wings to form here and legs to go there. Fiddling around with a single Hox gene can make flies with legs where antennae should be.
The discovery of Hox genes led to a big question: How could just one gene making one protein have such an enormous effect on a developing embryo? The answer, it turns out, lies in the way an organism expresses its genetic code. Nearly every cell in our bodies has a complete set of genes. In a developing embryo, liver genes are switched on (or expressed) to make a liver, brain genes to make a brain and so on. Tool kit genes, such as Hox genes, produce proteins that toggle the switches of other genes off and on. Thus, disabling or adding a single Hox gene can have major ramifications, such as building a leg instead of an antenna.

Hox genes, scientists soon discovered, are not confined to fruit flies; they are found in almost every animal, from houseflies to house sparrows to humans. In addition to being widespread, they have also been around for a while (or, as biologists say, are deeply conserved). Scientists now believe that Hox genes (and other tool kit genes) were present in organisms 500 million years ago, perhaps even before the Cambrian explosion during which multicellular animals appeared.

The presence of tool kit genes in ancient life forms suggests an elegant mechanism for evolution. Evolutionary change from one form to another does not require a huge number of random mutations to create bucketfuls of new genes and new proteins. Instead, it relies on the simpler process of changing where and when old genes are expressed in the developing embryo. Carroll summarizes this idea and its implication: "[D]ifferences in form arise from changing the way they [tool kit genes] are used. The principle of descent by modification ... is clear."

As cheerleader and spokesman for evo devo, Carroll firmly believes that it refutes the arguments of I.D. advocates. No need to call on a Creator or an intelligent designer, evo devo can account for the complexity of organisms and the vast differences between them. "Developmental genetics," he writes, "has been shedding new light on the making of complexity and the evolution of diversity for twenty years. Creationists just plain refuse to see it."

Nevertheless, the conflict continues. On Tuesday, a Pennsylvania judge ruled that I.D. could not be mentioned in biology classes taught in a public school district. Most scientists are weary of this battle; they believe they won it long ago. Still, they press the point -- in book after book, article after article -- that Creationism and I.D. are not testable theories and therefore are not sciences but matters of faith. Carroll sums up why scientists must continue this seemingly endless struggle with a quote from Henry David Thoreau: "You can hardly convince a man of an error in a life-time, but must content yourself with the reflection that the progress of science is slow. If he is not convinced, his grandchildren may be."
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