At the Water’s Edge
Carl Zimmer is a senior editor at Discover magazine and a winner of numerous awards for science journalism. In this, his first book, Zimmer attempts one of the most valuable and challenging goals of science journalism: the presentation of current scientific findings and thinking to general readers. His subject is macroevolution, the development of new species with fundamental variations in physical features, such as fins or arms and legs, over long periods of time. Macroevolution is distinguished from microevolution, the small changes from one generation to the next produced by natural selection. Some of the biggest macroevolutionary transformations have been the emergence of air-breathing, land-traveling tetrapods (four-limbed animals) from gilled water animals and the later return of some descendants of these tetrapods to the water.
Zimmer begins his exploration of the cutting edge of scientific research on shifting life forms at the water’s edge with the story of his own brief return to the sea. While diving along the coast of the Grand Bahama Island to watch how scientists study dolphins, the author was struck by the differences and similarities among a yellowtail snapper, the dolphins, and himself. Life underwater and life on ground surrounded by atmosphere pose radically different challenges. Still, humans, fish, and cetaceans (mammals such as whales and dolphins) are all vertebrates; they all have backbones. This is evidence of kinship. Moreover, even though humans and dolphins are clearly more closely akin to each other than either are to fish, both dolphins and fish are creatures of a watery world utterly foreign to humans.
To answer the question of how these distant relatives took on their current forms, Zimmer looks briefly at the development of biology as a historical science under the influence of Charles Darwin. He describes the attempts of Richard Owen, an influential nineteenth century biologist and opponent of Darwin, to account for creatures that could not easily be classified as fish, reptiles, or mammals. These included the platypus and the Lepidosiren, an apparent fish with lungs. Owen, believing that the different forms of life were variations on archetypes created by God, wanted to maintain boundaries between different classes of animals.
Scientists in the decades following Owen have found that life has crossed boundaries and classifications, and they have begun piecing together this macroevolutionary puzzle. On the basis of extensive research and interviews with contemporary scientists, Zimmer gives us an image of the puzzle’s emerging solution. He presents the reader with two of the earliest tetrapods yet discovered, Ichthyostega and Acanthostega. Ichthyostega was discovered in the 1930’s in Greenland by the Swedish paleontologist Gunnar Save-Soderbergh. It had a tail that resembled that of a fish but characteristics in its limbs of a tetrapod. Acanthostega was discovered by more recent scientists, most of whom Zimmer has interviewed, but it is even older than Ichthyostega. Acanthostega had limbs that would have allowed it to move around on the floors of swamps but were not strong enough to allow it to move around on dry land—clear evidence of an intermediate stage between water- dwelling and land-dwelling animals.
Zimmer pulls together the most recent evidence from a variety of scientific fields to suggest how the evolution from fish to tetrapod took place over the course of ten million years. According to physiologists, lungs may have developed before the movement out of the water in order to enable marine animals to take in oxygen from the air and increase endurance in pursuing prey. The famous claim of late nineteenth and early twentieth century scientist Ernst Haeckel that “ontogeny recapitulates phylogeny” or that the growth of individual embryos repeats the history of species, has been found to be false as a strict rule. Modern embryologists, however, have found enough similarities between growing embryos and evolving species to suggest that embryology can sometimes indicate how evolutionary changes took place. Supported by evidence from paleontology, embryology suggests how hands gradually emerged from fins.
After describing the apparent process of the movement from sea to land, Zimmer turns to look at those who made the return journey. He investigates how certain land mammals became cetaceans. About fifty million years ago, wolf-like mammals began making the macroevolutionary movement back across the water’s edge. One of the first pieces of evidence about this movement turned up in the nineteenth century when the early American paleontologist Dr. Richard...
(The entire section is 1933 words.)