Where do they all come from?

A new theory traces animal forms to an 'ancient tool kit'

For those who want to enhance their sense of kinship with butterflies, zebras, apes, and even ancient dinosaurs, Sean B. Carroll offers a treasure trove in "Endless Forms Most Beautiful: The New Science of Evo Devo."

Evo Devo, evolutionary developmental biology, intertwines Earth's family of animals in a way not done in the past: Its most surprising finding is that all animals, including those with arms, wings, or fins, originated from a small number of primitive "master" genes.

Over long spans of time, that "ancient tool kit" of genes evolved animals and created the enormous diversity around us: stripes in zebras, spectacularly colored butterfly wings, and intricate human hands. One ancient gene led to the creation of eyes across the entire animal kingdom, writes Carroll, a genetics professor at the University of Wisconsin-Madison.

Evo Devo is the third revolution in the field known as evolutionary biology or how animals were made and evolved.

The first revolution came when Charles Darwin published his seminal book on evolution, "The Origin of Species."

Darwin explained how, over eons, living organisms became diverse through a process called natural selection, meaning that nature decided which species had best adapted to their environment, and thus would thrive.

The second revolution came with the merging of Darwin's theories and the science of genetics.

But neither of those approaches revealed how individual animal forms were made or how they evolved. That's where Evo Devo comes in, attempting to explain a process through which a single- celled egg develops into a multibillion-celled animal, and why there are such deep connections among animals.

And while this third revolution may seem complex, it's based on the ancient tool kit with the small number of common ingredients. Carroll contends that Evo Devo simply has stunned biologists, reshaping their picture of how evolution works. Not a single biologist, he writes, ever anticipated that the genes controlling how a tiny fruit fly's body and organs are made also control the making of most animals, including man. By studying fossils, Evo Devo shows that animals evolved with a pervasive use of repeated, modular parts.

So why are there such great differences in the way a butterfly and a human appear? Carroll goes to great lengths to explain how differences in form arise from changes during evolution in terms of where and when certain genes are used; for instance, the joints in the flippers of a sea turtle versus the paws of a dog. In addition, DNA that regulates or determines when, where, and how many of the parts of a gene are made contributes to the diversity of form we see around us. He says part of learning how animal forms developed correctly is studying animals that are malformed; for example, animals born with six rather than five digits.

Carroll's title, "Endless Forms Most Beautiful," was inspired by the ending of Darwin's book: "...whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

An acknowledged expert in the field, Carroll takes us through the steps of evolution with the inquisitiveness and thought process of a scientist and helps us appreciate evolution and the relationships of all forms of life. His language is simple and rich, and 116 detailed illustrations make the book accessible to novice and scientist alike. For those who'd like to know more, he provides an extensive reading list.

Carroll deals briefly with the controversy surrounding how evolution is taught in schools, and he cites statistics showing that, even when it is taught, there is widespread ignorance about the topic. For example, a National Science Board survey found that 52 percent of Americans believed that the earliest humans lived at the same time as the dinosaurs. He also briefly tackles the struggle between teaching evolution and teaching creationism, pointing out that even Darwin added the words "by the Creator" to "The Origin of Species" to appease critics of the concept of evolution. But he believes Evo Devo may help deepen the case for evolution and suggests that theology should evolve or face the possibility of becoming irrelevant.

Carroll concludes by saying the stakes for broader adoption of an evolutionary perspective are critical not only to understanding our planet's history but also to its preservation in the future.

• Lori Valigra is a freelance writer in Cambridge, Mass.