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Geoscientist Online

Across the Bridge: Understanding the Origin of the Vertebrates

Gee across the bridgeThis book brings together some of the latest research in zoology (including the fields of anatomy, palaeontology and molecular genetic data), whilst examining classical hypotheses, to discuss the evolutionary origin of vertebrates.

This work investigates the deuterostomes, including tunicates, lancelets, and hemichordates, at crucial positions in the evolutionary tree. In discussing these animal groups and their evolution with respect to the vertebrae, the author has developed an evolutionary scenario establishing the tunicates as the closest relatives of vertebrates. He traces the origin of the vertebrate to a common starting point with hemichordates and echinoderms, to around 600 million years ago, with an early chordate ancestor.

The link back from the tunicates to the early chordate ancestor can be interpreted as the author's bridge crossing a morphological divide between chordates and their closest relatives.
 
The book is structured to examine deuterostome animal groups, breaking down the development of the vertebrae into its individual parts, including the notochord (a flexible rod made from a material like cartilage) and neural crest (a grouping of cells, unique to chordates, giving rise to a diverse cell lineage).

The author fascinatingly reviews fossil forms in an evolutionary context to help bridge the gap from invertebrate to vertebrate. The text outlines the difficultly in interpreting fossils in the search for the source of the vertebrae and explores some of the well-known fossils from the Burgess Shale and Chengjiang fauna and mysterious remains that might bridge the gap.

Through these investigations, the book provides a glimpse of the early chordate ancestor, the earliest origins of the vertebrates, through palaeontological, developmental and genomic data. The data support the notion that the common ancestor of chordates, hemichordates and echinoderms had pharyngeal gill slits for filter feeding.

After reaching the above conclusion, the author considers the evolution of each characteristic of the early chordates independently and as a component of the vertebrate body plan.

In my opinion, this is a recommended read. However, if one is not familiar with the terminology of anatomy and palaeontology, I would suggest making notes of some of the key terms (it is worth it!) to unlock what is a fascinating theory into the origins of our most fundamental physiological feature; the vertebrae.

Reviewed by Simon Kettle

ACROSS THE BRIDGE: UNDERSTANDING THE ORIGIN OF THE VERTEBRATES by Henry Gee, 2018. Published by: The University of Chicago Press, 312pp. (pbk.) ISBN: 9780226403052 List Price: $25.00 W: https://www.press.uchicago.edu/ucp/books/book/chicago/A/bo24570319.html