Over the last few years several of New York City's most notable cultural institutions have announced plans for renovation and expansion, presumably prodded by the approaching millennium and the impetus it provides for a fresh statement of mission and direction.

Witness the venerable American Museum of Natural History, where for months now passersby on West Eighty-first Street in Manhattan have been watching as the construction of a new Earth and Space Center fills up the empty spot that had been occupied by the beloved, copper-domed Hayden Planetarium for much of this century.

Inside, visitors to the museum have been enjoying newly renovated galleries in which the latest achievements in understanding the history of life have been presented in narrative form, drawing on findings in paleontology, physiology, systematics, and molecular biology. The lovely, old, dark galleries are still a major attraction, of course, but the clean white paint and natural lighting of the newer exhibits suggest a conscious effort on the part of the museum's staff to design a backdrop reflective of a new approach to their educational mission.

In At the Water's Edge: Macroevolution and the Transformation of Life, Carl Zimmer takes the reader on a similar journey through science's attempt in the last 150 years to put some meat on the bones of the Darwinian revolution. He introduces the contributions of the great nineteenth century scientists Richard Owen, Georges Cuvier, and Etienne Geoffroy Saint-Hilaire, followed by the contribution of prominent twentieth century paleontologists such as Alfred Romer. Romer's career was inspired, Zimmer tells us, by childhood visits to the aforementioned Museum of Natural History in between a series of rabies shots. Finally, he brings us to the present by describing the work of a number of contemporary paleontologists, physiologists, ecologists, embryologists, and molecular biologists. Their research has not only filled in the gaps in two of the most important evolutionary transitions but has also pointed the way toward an interdisciplinary research program that promises to make great strides in evolutionary biology.

Zimmer, a senior editor at Discover, has taken as his subject the evolutionary transitions from fish to tetrapod and from land mammal to cetacean; that is, "the line that divides water and air." His introductory chapter is a masterful exposition of the hurdles that a vertebrate faces in making a home in a new medium:

Zimmer goes on to describe the different effects of air and water on the speed of sound, on electrical conductivity, on the speed of light, and on the ability to move, each time raising the bar for understanding this evolutionary transition and, by extension, raising the bar for understanding how it might have happened.

Despite the considerable challenge of this approach, Zimmer satisfactorily assembles explanations that become the heart of his book. In describing work on the evolution of tetrapods, for example, Zimmer introduces us to Acanthostega and Ichthyostega, the two earliest-known tetrapods. Reconstructions of Ichthyostega, first discovered in the 1930s in Greenland by the Swedish paleontologist Gunnar Save-Soderbergh, show it to have the elbows, knees, ankles, and wrists of a tetrapod with a tail like that of a lobe-finned fish. Acanthostega, unearthed and described by Erik Jarvik, John Nicholson, Jenny Clack, and Michael Coates, is an even older tetrapod, whose arm had a conventional tetrapod humerus attached to a radius that was twice as long as its ulna - characteristic of a lobe fin. Here was a tetrapod that might well have been adapted to the business of mucking around at the bottom of a swamp, but was not ready to support its full weight on land.

And the trip back? Zimmer's writing is equally compelling on the evolution of cetaceans from land mammals. Again, he refers to the work of "a loose coalition of paleontologists, geneticists, functional morphologists, comparative psychologists, and embryologists" that has assembled a phylogeny of whales going back to the mesonychids - meat-eating, hoofed mammals that lived from 65 to 34 million years ago. By analogy to the development of primitive lungs and limbs in fish that were later to prove essential to life on land, Zimmer presents speculations that the massive head and ears of land-loving Ambulocetus ("walking whale") may have allowed it to detect sound waves along the ground, an advantage that would later evolve into a complex suite of structures used for underwater echolocation.

There is much more here, from a fascinating chapter on the anatomy and physiology of bottlenose dolphins, to a discussion of the relative merits of cladistics as an approach to determining the phylogeny of a particular lineage, to an explanation of the power of "correlated progression" in evolution, through which a change in one part of the body may make other changes more beneficial.

In addition, the book is nicely illustrated by Carl Buell. In fact, a few more illustrations might have been warranted in light of the complexity of some of the evolutionary transitions discussed, although it must be said that Zimmer's clear and frequently eloquent prose is usually up to the task.

Scott Gilbert's Developmental Biology carries what he calls a "disciplinary road map of the evolutionary side of biology" that outlines the history of the various subdisciplines of the biological sciences and projects a new developmental synthesis that is "under construction." The work that Zimmer has documented so well in At the Water's Edge is part of this ongoing effort to reunify biology, and it promises to test the ingenuity of museum curators and educators everywhere as they invent new ways to present to the public what Darwin called "mankind's pleasant genealogy."