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Our understanding of vertebrate origins and the backbone of human history evolves with each new fossil find and DNA map. Many species have now had their genomes sequenced, and molecular techniques allow genetic inspection of even non-model organisms. But as longtime Nature editor Henry Gee argues in Across the Bridge, despite these giant strides and our deepening understanding of how vertebrates fit into the tree of life, the morphological chasm between vertebrates and invertebrates remains vast and enigmatic.
As Gee shows, even as scientific advances have falsified a variety of theories linking these groups, the extant relatives of vertebrates are too few for effective genetic analysis. Moreover, the more we learn about the species that do remain – from sea-squirts to starfish – the clearer it becomes that they are too far evolved along their own courses to be of much use in reconstructing what the latest invertebrate ancestors of vertebrates looked like. Fossils present yet further problems of interpretation. Tracing both the fast-changing science that has helped illuminate the intricacies of vertebrate evolution as well as the limits of that science, Across the Bridge helps us to see how far the field has come in crossing the invertebrate-to-vertebrate divide – and how far we still have to go.
Chapter One: What Is A Vertebrate?
1.1 Vertebrates in Context
1.2 What Makes a Vertebrate?
1.3 Breaking Branches
Chapter Two: Shaking the Tree
2.1 Embranchements and Transformation
2.2 Evolution and Ancestors
Chapter Three: Embryology and Phylogeny
3.1 From Embryos to Desperation
3.2 Genes and Phylogeny
Chapter Four: Hox and Homology
4.1 A Brief History of Homeosis
4.2 The Geoffroy Inversion
4.3 The Phylotypic Stage
4.4 The Meaning of Homology
Chapter Five: What Is A Deuterostome?
Chapter Six: Echinoderms
Chapter Seven: Hemichordates
Chapter Eight: Amphioxus
Chapter Nine: Tunicates
Chapter Ten: Vertebrates
Chapter Eleven: Some Non-deuterostomes
Chapter Twelve: Vertebrates from the Outside, In
12.2 The Organizer
12.3 The Notochord
12.5 Segmentation and the Head Problem
12.6 The Nervous System
12.7 Neural Crest and Cranial Placodes
12.8 The Skeleton
Chapter Thirteen: How Many Sides Has A Chicken?
13.2 The Enteric Nervous System
13.3 The Blood and the Heart
13.4 The Urogenital System
13.5 The Gut and Its Appendages
13.7 The Pituitary Gland
Chapter Fourteen: Some Fossil Forms
14.1 Fossils in an Evolutionary Context
14.2 Meiofaunal Beginnings
14.9 The Earliest Fossil Vertebrates
14.11 Ostracoderms and Placoderms
Chapter Fifteen: Breaking Branches, Building Bridges
15.1 Defining the Deuterostomes
15.7 The Common Ancestry of Tunicates and Vertebrates
15.12 The Evolution of the Face
15.13 Crossing the Bridge
Henry Gee is a senior editor at Nature and the author of such books as Jacob's Ladder, In Search of Deep Time, The Science of Middle-Earth, and, most recently, The Accidental Species: Misunderstandings of Human Evolution, the last published by the University of Chicago Press. He lives in Cromer, Norfolk, England, with his family and numerous pets.
"An excellent addition, complementing Gee's earlier book Before the Backbone, which provided a historical perspective on ideas surrounding vertebrate origins. Gee addresses an important topic for biologists and zoologists about vertebrates' place in the 'grand scheme.' We are familiar with vertebrates, or think that we are. However, Gee shows beautifully, as a group we are just as strange in many ways as other groups appear to us. Across the Bridge takes on a very esoteric subject and is genuinely witty and charming. The book really is magnificent."
– Neil J. Gostling, University of Southampton
"In Across the Bridge, Gee writes a beautiful ode to some of the least appreciated animals, who reveal our own evolutionary origins: the deuterostomes. Combining a sense for detail and prosaic ease, Gee guides the reader joyfully through deuterostomes – weaving disparate elements of embryology, paleontology, and morphology into an unprecedented and accessible narrative. This book not only gives a state of affairs – being an excellent primer for anyone interested in early animal evolution – it also proposes novel, compelling, and challenging hypotheses for researchers to test for decades to come. As senior editor of Nature, Gee has had a first-row seat to the revelations made across the disciplines of evolutionary biology for almost thirty years. Here, he has applied his polyhistoric expertise in this field to propose a vision for future interdisciplinary research, as well as created what will surely become a classic textbook for future generations of students."
– Jakob Vinther, University of Bristol