Books, Naturally
Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom
Sean B. Carroll. 2005. W.W. Norton & Company, New York. 350 pp., hardbound. $25.95.
A walk through the Smithsonian's National Zoo reveals only a tiny fraction of the diversity that exists in animal forms, but that diversity amazes us nonetheless. In a short visit, you might pass a giraffe with its preposterously long neck, a flamingo with shocking pink feathers, and a scaly snake slithering along. In the Invertebrate Exhibit, you can see more unusual animal forms: lobsters and other rather formidable crustaceans, flowery anemones with mesmerizing waving tentacles, and starfish that look more like bathtub toys than "real" animals.
How such a vast array of assorted animal forms evolved has long been well understood, in outline if not in detail. Natural selection over long periods of time produces the diversity of living things, while genes—DNA—are the basis of heredity, with differences in genetic makeup accounting for species diversity. Until recently, however, almost nothing was understood about how individual forms are made or how they evolved. The answer to this involves understanding the development of individuals from eggs to complex animals. The big questions are: How does DNA instruct an embryo to eventually grow an arm here and a leg there, for example, and how does one species grow arms and legs and another wings, fins, or no limbs at all? This is really the stuff of evolution, because "it is through changes in embryos that changes in form arise," according to Carroll, and changes in form are what natural selection acts upon.
Long mysterious, the process of development has finally succumbed to the curiosity of scientists in just the last 20 years, spawning a new field of biology known as evolutionary developmental biology—evo devo for short. And the findings of evo devo have been called nothing short of revolutionary—you could call it the evo devo revo. In Endless Forms Most Beautiful, Sean B. Carroll, a pioneer and leader in this field, has written a lucid, engaging account of the sometimes astonishing and always fascinating findings that have emerged from this new science. Clearly as brilliant a teacher as he is a scientist, Carroll leads us through the complexities of his subject with patience, clarity, and a light touch. (Not every scientist would open a book with a quote from Jimi Hendrix or pen such chapter titles as "Making Babies: 25,000 Genes, Some Assembly Required"!)
If you're wondering why you might want to read about something so seemingly esoteric as evolutionary developmental biology, despite its hip evo devo nickname, I'll let Carroll explain: "[This book is] for anyone who may ponder the question 'Where did I come from?,' this book is also about our history, both the journey we have all made from egg to adult, and the long trek from the origin of animals to the very recent origin of our species." And who isn't curious about this?
In this short space, I can't even begin to summarize the evo devo story revealed in Endless Forms Most Beautiful, which even includes phenomenal color photographs of toolkit genes in action. So let me just offer the following to whet your appetite.
Until evo devo, the assumption was that different genes resulted in different kinds of animal. Think of genes like a box of crayons from which the colors are doled out in threes to budding young artists. Kids who get red, yellow, and blue crayons draw fruit flies; those with purple, green, and orange create Homo sapiens, and so on. The discovery that really sparked the evo devo revo was that this was all wrong. Instead, it's like all of the kids getting the same box of crayons, and it's whether and when they use the various colors that determines what animals they draw.
It turns out all complex animals share a common "toolkit" of master genes that are turned on and off by "switches" at different times and places as an egg develops into an adult. If the box of crayons is the toolkit, the kids' fingers grabbing some colors and not others, and scribbling with them in different orders and places on their paper, might be likened to the switches. What's more, fundamentally the same toolkit genes have been around for more than 500 million years, and are shared by primitive velvet worms, starfish, fruit flies, lobsters, flamingos . . . and you and me. For instance, the same gene that tells a human embryo to grow an arm, tells a lobster to grow a claw, a bird a wing, a fish a fin, and a starfish a ray.
Not surprisingly amid the current evolution versus intelligent design battles, Carroll's last chapter briefly addresses how evo devo discoveries smash (as if more weapons were really needed) the pseudoscientific contentions of the intelligent design camp. Implicitly, the entire book does this as well. Development is no longer the "missing link" between natural selection and genetics, and evo devo has revealed how development explains both the leaps and the bounds, over many millions of years, of the evolution of the diversity of life on Earth.
Finally, Carroll concludes with a plea for conservation. "What a tragic irony," he writes, "that the more we understand of biology, the less we have of it to learn from and enjoy." It's just possible that his book's revelations about the evo devo revolution in our understanding of biology will move us to conserve what remains of life's endless forms most beautiful.
—Susan Lumpkin
ZooGoer 35(1) 2006. Copyright 2006 Friends of the National Zoo. All rights reserved
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