What is a vertebrate?. Vertebrates in context ; What makes a vertebrate? ; Breaking branches ; Summary -- Shaking the tree. Embranchements and transformation ; Evolution and ancestors ; Summary -- Embryology and phylogeny. From embryos to desperation ; Genes and phylogeny ; Summary -- Hox and homology. A brief history of homeosis ; The Geoffroy inversion ; The phylotypic stage ; The meaning of homology ; Summary -- What is a deuterostome? -- Echinoderms -- Hemichordates -- Amphioxus -- Tunicates -- Vertebrates -- Some non-deuterostomes -- Vertebrates from the outside, in. Introduction ; The organizer ; The notochord ; Somitogenesis ; Segmentation and the head problem ; The nervous system ; Neural crest and cranial placodes ; The skeleton ; Summary -- How many sides has a chicken?. Introduction ; The enteric nervous system ; The head and the heart ; The urogenital system ; The gut and its appendages ; Immunity ; The pituitary gland ; Summary -- Some fossil forms. Fossils in an evolutionary context ; Meiofaunal beginnings ; Cambroernids ; Vetulicystids ; Vetulicolians -- Yunnanozoans ; Pikaia ; Cathaymyrus ; The earliest fossil vertebrates ; Conodonts ; Ostracoderms and placoderms ; Summary -- Breaking branches, building bridges. Defining the deuterostomes ; Ambulacraria ; Echinoderms ; Hemichordates ; Chordates ; Amphioxus ;The common ancestry of tunicates and vertebrates ; Tunicates ; Vertebrates ; Cyclostomes ; Gnathostomes ; The evolution of the face ; Crossing the bridge ; Conclusions.
Summary:
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 remainfrom sea-squirts to starfishthe 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.
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