Across the Bridge Understanding the Origin of the Vertebrates

When you think of an animal, you will most likely think of a vertebrate. Since we are animals with a backbone ourselves, it is not strange that that which is closest to us comes to mind first. But when and how did vertebrates evolve? To answer that question, Nature editor Henry Gee takes a good hard look at invertebrates, convincing the reader that they are not all equal. More than 20 years ago, Gee wrote Before the Backbone, which took a look at historical explanations for the origins of vertebrates. Which group of invertebrates is closest to us remains a topic of active research and Across the Bridge brings readers up to date with our current thinking.

Gee uses the first few chapters to bring the reader up to speed on what a vertebrate actually is, how evolution works and how scientists make hypotheses about relatedness, how certain genes (Hox genes and others) influence gross morphology, and how embryos develop. That last topic is a fluke of history, for scientists in the 18th and 19th century used to think that the way an embryo develops mirrors evolutionary progress from simple to complex organisms. It gave rise to that hackneyed expression that “ontogeny recapitulates phylogeny”, an expression inseparable from the name of German naturalist Ernst Haeckel. His drawings of developing embryos are iconic, contested, but ultimately not entirely wrong (See Haeckel's Embryos for the definitive history of those, and my (long-winded) review of that book here).

The topic of embryology brings with it a lot of terminology that you may or may not remember from your zoology classes: the names of tissue layers (ecto-, endo-, and mesoderm), other structures (blastopore, coeloms, notochord), and processes (gastrulation, neurulation, enterocoely, and schizocoely). Luckily, Gee includes plenty of helpful schematic drawings.

Vertebrates, together with a number of invertebrate groups, belong to the deuterostomes, a distinction that is based on what an embryo develops first, the mouth or the anus. Adding more terminology, a substantial part of the book first reviews the morphology and development of those invertebrate groups closest to the vertebrates, including the echinoderms (think sea stars), hemichordates, tunicates, and Amphioxus. He then cuts the cake the other way and looks at typical vertebrate organs and features and asks what equivalents our closest invertebrate cousins have where, say, the heart, immune system, head, or brain is concerned.

All this terminology notwithstanding, Gee does a good job of keeping his text as readable as can be reasonably expected. He inserts a touch of humour here and there. He makes observations in his footnotes that delighted both my inner five-year-old (comb jellies have several anuses!) and my inner 38-year-old (would our thinking about evolution have such a close link with development if early naturalists had not been so interested in embryology?). But most important and helpful: most chapters end with a summary. Who said that summaries should only be something found in student textbooks?

The other refreshing part of this book is how it corrects many potential misunderstandings about how evolution proceeds. There is no linear progression from simple to complex organisms. Some lineages evolve faster than others. Invertebrates today are not some frozen primitive snapshot of life hundreds of millions of years ago but have kept on evolving since they split off from the last common ancestor between us and them (you are more likely to have heard this in the context of the evolution of humans and other primates ). And what can really throw a spanner in the works: organisms can lose complex traits again.

Technological advances over the last few decades have allowed detailed studies of genes and whole genomes, and have birthed the study of the evolutionary relatedness of genes between organisms – a discipline known as evolutionary developmental biology, or evo-devo for short (see Endless Forms Most Beautiful for an accessible introduction, and From Embryology to Evo-Devo for a deeper study). These advances, and the aforementioned spanner in the works (the loss of complex traits), have brought about at least one revolutionary change in our thinking which Gee relates here with gusto. Of all the invertebrate cousins described, tunicates (sessile or free-floating marine filter feeders such as sea squirts that look nothing like us) are more closely related to us than Amphioxus (segmented, vaguely fish-like invertebrates also known as lancelets that do resemble us). Closer inspection shows it makes sense, but also shows the remarkable evolutionary history of tunicates that have lost many traits. All this serves as a powerful reminder that evolution is far from a neat and tidy story of progression.

Gee closes out with a short look at fossils, which seems a bit of a non-starter for creatures that lack hard structures such as bones and teeth. The deuterostomes diversified in the Cambrian, between 541 to 485 million years ago, and there are a few localities on the planet that have yielded exceptional invertebrate fossils of the very distant ancestors of vertebrates and all the other deuterostome groups mentioned here so far. One of these is the Burgess Shale in British Columbia, made famous by Gould’s book Wonderful Life, but the Chengjiang biota of southern China is equally astounding, as documented in The Cambrian Fossils of Chengjiang, China.

If your interest in evolution goes well beyond extinct mammals and reptiles, you can handle a bit of zoological jargon, and Chordate Origins and Evolution strikes you as too technical, then Across the Bridge is a wonderfully readable overview of how we vertebrates relate to our closest invertebrate cousins. Although much remains unclear and there is plenty of scope for further research, this book is a timely overview of the current state of knowledge on this neglected topic.