Recently, new genes and their proteins that revealed striking new insights into the early evolution of multicellular animals have been identified and characterized from members of the lowest metazoan phylum, the porifera (sponges). The unexpected result was that the sequences obtained from sponge displayed high similarity to those found in higher metazoa; in consequence, it was concluded that during the transition from protozoa to metazoa the major structural and regulatory proteins evolved only once. The data gathered are now powerful arguments to establish monophyly of metazoa; in addition, new insights on the evolutionary diversification of metazoa were obtained.

This volume concentrates on the origin of multicellular animals, Metazoa. Until now, no unequivocal phylogeny has been produced. Therefore, the questions remain: Did Metazoa evolve from the Protozoa only once, or several times? Is the origin of animals monophyletic or polyphyletic? Especially the relationships between the existing lower metazoan phyla, particularly the Porifera (sponges) are uncertain. Based on sequence data of genes typical for multicellularity it is demonstrated that all Metazoa, including Porifera, should be placed into the kingdom Animalia together with the Eumetazoa. Therefore it is most likely that all animals are of monophyletic origin.

Through an integration of systematics, genetics, and related disciplines, the Modern Synthesis of Evolutionary Biology came into being over fifty years ago. Knowledge of evolution has since been transformed by several revolutions: the way we interpret the fossil record has been radically affected by theories of continental drift and asteroid impacts; the way we classify organisms has been influenced by the development of cladistics. Perhaps the most dramatic revolution has been the explosion in molecular biology of information about the genome. Aiming to capture the excitement of modern evolutionary biology, six prominent scientists here explore important issues and problems in their areas of specialization and identify the most promising directions of future research. The scope of this volume ranges from macroevolutionary patterns in the Precambrian to molecular evolution of the genome. Major themes include the origin and maintenance of variation and the causes of evolutionary change. Chapters on paleontology, ecology, behavior, development, and cell and molecular biology are contributed by Jim Valentine, Graham Bell, Mary Jane West Eberhard, Leo Buss, Marc Kirschner, and Marty Kreitman. The book contains an introductory chapter by John Bonner, whose seminal work is honored here. Originally published in 1992. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.

The evolution of animal diversity is strongly affected by the origin of novel cell and tissue types and their interactions with each other. Understanding the evolution of cell types will shed light on the evolution of novel structures, and in turn highlight how animals diversified. Several cell types may also have been lost as animals simplified – for example did sponges have nerves and lose them? This book reveals the interplay between gains and losses and provides readers with a better grasp of the evolutionary history of cell types. In addition, the book illustrates how new cell types allow a better understanding permitting the discrimination between convergence and homology.