THE SHAPE OF LIFE

THE UNIVERSITY OF CHICAGO PRESS

Contents

Preface......................................................................xiiiAcknowledgments..............................................................xxi1 Over the Ice for Ontogeny and Phylogeny....................................12 Metazoan Phyla and Body Plans..............................................303 Deep Time and Metazoan Origins.............................................634 Molecular Phylogeny: Dissecting the Metazoan Radiation.....................1035 Recovering Data from the Past..............................................1426 The Developmental Basis of Body Plans......................................1737 Building Similar Animals in Different Ways.................................2118 It's Not All Heterochrony..................................................2559 Developmental Constraints..................................................29210 Modularity, Dissociation, and Co-option...................................32111 Opportunistic Genomes.....................................................36212 Evolving New Body Plans...................................................397References...................................................................435Index........................................................................493

Chapter One

In the strait of Magellan, looking due southward from Port Famine, the distant channels between the mountains appeared from their gloominess to lead beyond the confines of the world.

Charles Darwin, Voyage of the Beagle

THE WORST JOURNEY IN THE WORLD

As a young man, Apsley Cherry-Garrard spent two and one-half years as an unpaid assistant zoologist on Robert Falcon Scott's fatal 1911 expedition to Antarctica and the South Pole, then returned to England in time to see the start of World War I. He survived military service in Flanders, and found the time to publish in 1922 one of the most extraordinary accounts of polar exploration ever written, The Worst Journey in the World. There he vividly and movingly tells of the tragic fates of Scott and his polar party, but within that grander tragedy he records the staggering difficulties of one of the strangest trips ever undertaken, his own appalling winter journey with ornithologist Edward A. Wilson and Royal Navy lieutenant H. R. Bowers to Cape Crozier to collect the eggs of the emperor penguin. The same zeal for scientific discovery that lured Darwin and so many other great scientific voyagers drew Cherry-Garrard and the other members of the party on a journey of scientific exploration that they barely survived. Cherry-Garrard's idealism comes through in the words he wrote many years later: "We traveled for Science. These three small embryos from Cape Crozier, that weight of fossils from Buckley Island, and that mass of material, less spectacular, but gathered just as carefully hour by hour in wind and drift, darkness, and cold, were striven for in order that the world may have a little more knowledge, that it may build on what it knows instead of on what it thinks." Bowers and Wilson were later to die with Scott on their return from the South Pole. Their passion for science may have contributed to their deaths. They had collected several kilograms of fossil plants from some of the few exposed rocks of the Antarctic continent at Beardmore Glacier in the Transantarctic Mountains. Despite their exhaustion, Scott's South Pole party attempted to return with the specimens the three hundred-odd miles from Beardmore Glacier to their base camp on Ross Island. The fossils were found on their sled at their final camp where they died in a blizzard. The fossils were recovered by the rescue party, and much later in the century were to lead to the discovery of fossils of Triassic mammal-like reptiles in Antarctica. The remains of these animals, so akin to the contemporary mammal-like reptiles of Africa, established that about 250 million years ago Antarctica, along with Africa and Australia, made up the southern part of the supercontinent of Pangaea.

The main goal of Scott's expedition was to be the first to reach the South Pole. Nevertheless, in Scott's mind, his expedition was primarily a scientific journey of exploration, not merely a race with Roald Amundsen's Norwegian polar party. Wilson's satellite expedition to Cape Crozier was driven by the desire to test an evolutionary hypothesis about embryonic development and its relationship to evolutionary history. The reasons underlying the six-weeklong sled journey to a penguin rookery in the continuous dark of the Antarctic winter seem extraordinary, but they are not so different from the foundations of many other expeditions dispatched across the planet and into space on the basis of what later were shown to be very shaky theoretical concepts.

Darwin's most forceful adherent was the German zoologist Ernst Haeckel, who became the most prominent late-nineteenth-century evolutionary theorist. Haeckel's achievements included the coining of numerous scientific terms (including such enduring favorites as ecology, phylogeny, and heterochrony), the prediction of the discovery of a fossil link between apes and humans (he called it Pithecanthropus), and the inauguration of a passionate quest for phylogenetic trees. In 1866 he propounded the famous and overwhelmingly influential biogenetic law, which states that ontogeny (the development of the individual) results from phylogeny (the evolutionary history of the lineage). Haeckel's mechanism of evolution required that new forms appear as a result of the addition of new terminal stages to the ancestral ontogeny. Thus, all animals should recapitulate their phylogenies in an abbreviated form during development, and developmental stages should reveal those histories. In 1911 Haeckel's ideas still held powerful sway in the minds of zoologists, a vision potent enough to send men on a desperate journey through the Antarctic night. It was Wilson's idea that the emperor penguin is the most primitive living bird, an evolutionary relict, a creature pushed to the very farthest reaches of the south polar regions by competition with more recently evolved and more advanced groups of birds spilling out of Eurasia. In the first decades of the twentieth century, Eurasia was thought by zoologists to be the center of many evolutionary radiations, a cauldron of Darwinian competition. It was thought that humanity originated there as well. Less fit species either became extinct or sought refuge in peripheral parts of the world not reached by their superior competitors.

The winter trip to the penguin rookery was aimed at preserving eggs in fixative and bringing them back to England for study. Wilson thought that if the trip took place early enough in the breeding season, various early developmental stages would be represented in the sampled eggs. Because in Wilson's scenario the emperor penguin is a primitive relict, its embryos were expected to recapitulate the reptile-to-bird transition. As Cherry-Garrard put it, "it is because the Emperor is probably the most primitive bird in existence that the working out of his embryology is so important. The embryo shows remains of the development of an animal in former ages and former states; it recapitulates its former lives. The embryo of an Emperor may prove the missing link between birds and the reptiles...