Travels on the Backbone Crest (A Group of Embryonic Cells)

BOOKS & ARTS COMMENT EVOLUTION non-deuterostomes. In the most effective section, he strips vertebrates down to their parts, such as the nerve cord, notochord (fore- runner of the vertebral column) and neural Travels on the backbone crest (a group of embryonic cells). He even delves into the largely ignored gut and viscera. Chris Lowe lauds a study of vertebrate origins that Gee discusses how data from living and brings us up to date with a shifting field. fossilized hemichordates and echinoderms have facilitated the search for the origins of the defining chordate anatomies. He ome of the great remaining mysteries groups belong in the highlights how palaeontological, develop- in zoology concern origins — of multi- deuterostome lineage mental and genomic data now all support cellularity, complex nervous systems, of animals alongside the idea that the common ancestor of chor- Slife cycles and sex, for example. The chordates — have dates, hemichordates and echinoderms had evolutionary origin of vertebrates is among largely been resolved. pharyngeal gill slits for filter feeding. That the most intractable of these, despite more Others are as intrac- gives us a glimpse of the early chordate ances- than a century of work spanning a range of table as ever, including tor, which lived around 600 million years ago. disciplines and animal groups. where to place key fos- Other features, such as a complex brain, prob- In Across the Bridge, Henry Gee reviews sil groups such as the ably emerged much later. Having established the most recent research in this area. Gee curious vetulicolians, Across the Bridge: a hazy picture of the earliest chordates, Gee (the senior editor responsible for palae- which lived during Understanding focuses on building vertebrates and their the Origin of the ontology and evolutionary development the Cambrian period, Vertebrates defining features from the basic chordate at Nature) synthesizes contributions from some 541 million to HENRY GEE body plan, for example through spectacular anatomy, developmental biology, genomics, 485 million years ago. University of Chicago innovations in the vertebrate head. palaeontology and the study of evolutionary Nevertheless, Gee Press (2018) Gee considers the evolution of each charac- relationships using DNA-sequence data. effectively develops teristic independently and as a component of He also puts forward his own ideas on this his own evolutionary scenario. He traces the vertebrate body plan. He includes alterna- fascinating conundrum. vertebrate origins from the common ances- tive interpretations and areas in which data The book is a follow-up to Gee’s 1996 Before tor of chordates, echinoderms (which include are weak or missing, allowing the reader to the Backbone, which was one of the reasons I starfish and sea urchins) and hemichordates, think hard about some of the more specula- decided to pursue research into the origins of deep in animal history, to establish a start- tive parts of his arguments. Various bizarre chordates (the group including vertebrates, ing point from which innovations of the first animals illustrate particular points. For exam- the fishlike lancelets and marine invertebrates chordates arose. Having sketched that picture, ple, the strange gelatinous marine inverte- called tunicates). At the time, the discipline he traces the evolution of definitive vertebrate brates called larvaceans secrete a mucus was intimidating, involving impenetrable characteristics by mining rich new studies ‘house’ for filter feeding, which they shed and papers and a dizzying array of contradic- from neglected chordate groups, from lance- rebuild every few hours. They are an extreme tory hypotheses. Gee tamed and synthesized lets and tunicates to hagfish and lampreys. His example of how our closest invertebrate rela- the literature to lay out the history and logic well-argued rationale draws from anatomy, tives have taken their own evolutionary tan- of the most significant hypotheses, such as palaeontology and molecular genetic data. gent, partly driven by genome simplification. Walter Garstang’s 1894 auricularian theory. The book is not a revision of Before the My main criticism of Across the Bridge is (This posited that the chordate body plan Backbone. But one of its goals seems to be that for a subject this visual, I would have evolved through transformation at the larval, to examine how the most influential classi- liked more illustrations. It is, after all, the rather than adult, stage.) Before the Backbone cal hypotheses fare in the light of new data, oddness of the main groups and their stem inspired novices like me to get excited about which is one of Gee’s unique perspectives. fossils that has made vertebrate origins such the challenges in vertebrate origins. He begins with a group-by-group examina- a difficult nut to crack. Is a major new summary of the field tion, from echinoderms to hagfish and a few Do I buy Gee’s original synthesis? warranted, just two decades on? In my Mostly. The morphological divide between view, yes. (Among many others in the field, chordates and their closest relatives I encouraged Gee to write this book, and remains perilously large, and even with he thanks me for so doing in the preface.) new advances, Gee’s hypothesis is a good There has been significant progress across contender; but it is not the only one, so the disciplines on research into important, yet bridge to early chordates remains a little previously neglected, animal groups at cru- wobbly. (An alternative treatment of recent cial positions in the evolutionary tree. Along progress is outlined in Noriyuki Satoh’s 2016 with tunicates and lancelets, these include Chordate Origins and Evolution.) But Across NOAA OKEANOS EXPLORER PROGRAM OKEANOS EXPLORER NOAA hagfish, lampreys and marine worms the Bridge is a deft and well-argued distilla- called hemichordates. The publication of tion of how advances have shifted the field the genomes of two hemichordates — the to the point of dispatching some of the most acorn worms Saccoglossus kowalevskii and influential and elegant classical hypotheses, Ptychodera flava (O. Simakov et al. Nature and it is a bold attempt at developing a new 527, 459–465; 2015) — rounded out the synthesis. It thereby deepens understanding representative genomes of almost all major of our own evolutionary origins. ■ animal groups with direct relevance to vertebrate origins. That draws a line under a Chris Lowe is an associate professor at 20-year burst of productive research. Hopkins Marine Station in the Department Some of the problems that once stalled of Biology at Stanford University, California. progress — such as uncertainty over which Hagfish have been neglected in vertebrate studies. e-mail: [email protected] ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts rese5rv eJULYd. 2018 | VOL 559 | NATURE | 31 .

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