Gee begins with some phylogenetics, presenting the broadly accepted tree of the major metazoan groups, some embryology, and an explanation of the concept of homology. He includes a bit of the history of the science here, but Across the Bridge mostly eschews that; it was the subject of his earlier book Before the Backbone.
Across the Bridge then proceeds through progressively closer groups, looking in turn at deuterostomes, echinoderms, hemichordates and then, within the chordates, at amphioxi, tunicates and finally vertebrates. Then comes a tour of vertebrate anatomy, first "from the outside in" (the notochord, segmentation and the head, the nervous system, neural crest, and the skeleton) and then more briefly "from the inside out" (the gut, the immune system, the pituitary gland). Gee finishes with a survey of the fossil evidence for the different groups, and a summary of how our current understanding of their relationships and key features has changed since the 1980s.
"Vertebrates have pursued a strategy of integration whose energy requirements are supported by an obligately predatory habit, which in turn imposes further energetic requirements such as the development of a sophisticated brain and sense organs. It could be that the duplication of the pre-vertebrate genome and the expansion of the neural crest effectively imposed this course on our distant ancestors, forcing them to be mobile to make a hard living, banishing them from the effectively mindless Eden the tunicates still inhabit."
Gee assumes only basic general biology in all of this, but he goes into technical material in far too much density for Across the Bridge to be a popular book.
"The expression of Hox genes in vertebrates has important consequences for the disposition of neural crest, as it is from the borders of the rhombomeres that neural crest flows ventrally and anteriorly to populate the pharyngeal arches and thus form the head and face. ... Crest from rhombomeres 2, 4, and 6 respectively populate the first, second, and third pharyngeal arches, taking their distinctive patterns of Hox expression with them. The first pharyngeal arch eventually develops into the upper and lower jaws, the palate, and, through Meckel's cartilage, the hammer and anvil bones of the mammalian middle ear. The second develops into the stapes in the middle ear, parts of the hyoid bone that supports the tongue, and many facial muscles; the third, much of the rest of the hyoid. The neural crest also contributes to the skull vault."
All this detail helps to illustrate a fascinating broader story, however. A few diagrams help a lot — more might have helped in places — and the material is clearly structured.
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