Marine biologist Helen Scales returns for her third book with Bloomsbury’s popular science imprint Bloomsbury Sigma. After shells and fish, she now drags the reader down into the darkest depths of the deep sea. Both a beautifully written exploration of the ocean’s otherworldly wonders and a searing exposé of the many threats they face, The Brilliant Abyss is Scales’s most strident book to date.
The Brilliant Abyss: True Tales of Exploring the Deep Sea, Discovering Hidden Life and Selling the Seabed, written by Helen Scales, published in Europe by Bloomsbury Publishing in March 2021 (hardback, 352 pages)
Sir David Attenborough has probably said it best: “No one will protect what they do not care about; and no one will care about what they have never experienced“. Both Scales and the publisher have taken that message to heart and the book is neatly designed. As with her previous book, illustrator Aaron John Gregory is involved again, this time providing two beautiful end plates and an eye-catching cover, while the colour plate section contains some outstanding photos. But at the heart of The Brilliant Abyss is Scales’s captivating writing.
First, consider the landscape. As she explains, the seabed, shaped by plate tectonics, is far from a featureless bathtub. Spreading centres create mid-ocean ridges, colossal mountain ranges that girdle the planet, while subduction zones where oceanic crust plunges back into the planet form deep-sea trenches of terrifying depths. The abyssal plain in between is studded with active or extinct underwater volcanoes that form seamounts of great import to marine life. Wherever magma approaches the surface, percolating seawater becomes superheated, rising back to the surface laden with dissolved minerals and metals. They form hydrothermal vents: towering structures that are home to unique fauna and are “the deep-sea equivalent of hot springs and geysers on land” (p. 97). Woven throughout is a history of scientific exploration, from the first oceanographic expeditions to today’s robotic submersibles, and from pioneering deep-sea explorers to today’s trench-diving billionaires.
Otherworldly as the landscape is, the real stars of this realm are its fauna. Scales’s knowledge and love of marine biology shine through here, as she populates the pages with a bewildering cast of creatures. Notable examples of bizarre deep-sea fishes are included, but she gives you so much more. Whale carcasses, so-called whale falls, become complete ecosystems, home to bone-eating Osedax worms with unusual sex lives. Large gelatinous members of the drifting plankton, such as colonial siphonophores and giant larvaceans, form previously underappreciated links in the food web. Hydrothermal vents are crowded with worms and furry Yeti crabs that domesticate symbiotic bacteria capable of chemosynthesis, the “dark alternative to photosynthesis” (p. 104). Meanwhile, one species of snail makes its shell out of iron! And then there are the corals. No, not the familiar tropical corals who “hog not only the sunlight but the limelight” (p. 129); the lesser-known cold-water corals that occur at great depths and grow even slower.
“[…] the real stars of this realm are its fauna […] whale falls become complete ecosystems, home to bone-eating Osedax worms with unusual sex lives. […] Meanwhile, one species of snail makes its shell out of iron!”
And if the intrinsic value of biodiversity does not sway you, Scales is no stranger to discussing the deep’s instrumental values. The capacity of seawater to absorb heat and carbon dioxide. The role of global oceanic currents in regulating our climate. Or the carbon pump provided by marine snow; the constant rain of dead plankton, fish poop, and other organic debris that descends into the depths. And what of the quest for new classes of biological compounds with antiviral, anti-bacterial, or anti-cancer properties that could form the pharmaceutical drugs and antibiotics of the future?
Two-thirds through the book Scales switches gears. Now that she has your attention, it is time to highlight the many dangers the deep faces. Deep-sea fishing targets long-lived, slow-growing species such as orange roughy. Vulnerable seamounts with millennia-old corals are destroyed by trawlers in a matter of hours. Meanwhile, the promise of food for everyone is not being met. Vast catch volumes are being turned into fish meal for aquaculture and pet food, or questionable nutraceuticals such as omega-3-oil supplements. And where Daniel Pauly already gave me reason to be suspicious of the Marine Stewardship Council, Scales lays bare their dubious raison d’être: funded by royalties from sales of their eco-labelled fish, there is an imperative to keep certifying fisheries. She calls their scandalous certification of the “recovering” orange roughy population a “case of a dead cat bouncing, with a green-washed eco-label tied to its collar” (p. 204).
Scales made me shudder with her stories of pollution, especially the persistent legacy of the large-scale dumping of chemical weapons. But the topic that concerns her most is the looming spectre of deep-sea mining. Though much is still on the drawing boards, mining licenses are being issued and exploratory missions are taking place. What for? The minerals and metals contained in seamounts, hydrothermal vents, and the polymetallic nodules littering the seabed, which take millions of years to form. As with fishing, “the slow pace of the deep is out of step with the timescale of impatient human demands” (p. 205). Here too, the position of the body that oversees protection of the seabed, the International Seabed Authority, is incredibly compromised. Next to issuing mining permits they unbelievably have already assigned areas to be exploited by their own mining company!
“To think that [deep-sea mining] will result in anything but the rapacious plundering of ecosystems we have seen on land seems highly unlikely in her eyes. Meanwhile, the mining PR-machine is already running at full tilt […]”
Scales’s focus on deep-sea mining is urgently needed. Scientists have been sounding alarm bells in the peer-reviewed literature regarding its impact, but this topic is still mostly hidden from the public at large. Her descriptions of the destructive practices and the size of the machines involved are chilling. To think that this will result in anything but the rapacious plundering of ecosystems we have seen on land seems highly unlikely in her eyes. Meanwhile, the mining PR-machine is already running at full tilt, and Scales deftly disarms their arguments as to why deep-sea mining is necessary. She agrees that the shift to renewable energy requires infrastructure that needs tremendous amounts of diverse metals. However, as a detour into the design of wind turbines shows, predicting which ones will be needed is difficult. And whether the seabed is the best place to get them is highly questionable.
Scales tackles many of the same topics that Alex Rogers covered in The Deep. Her tone is more strident but no less knowledgeable and, as opposed to The Deep, her book does include endnotes with references. I recommend them both highly. Meanwhile, her call “to declare the entire realm off limits [to] extraction of any kind” (p. 286) meshes seamlessly with Deborah Rowan Wright’s bold vision laid out in Future Sea.
Whether you enjoyed her previous books or are new to her brand of writing about marine biology, I urge you to read this book. Next to an unforgettable trip, she provides a rousing rallying cry for the preservation of the deep sea. The Brilliant Abyss is, true to its title, brilliant.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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Planet Earth might just as well be called Planet Water. Not only is our planet mostly ocean, life also started out here. Following his 2011 book Convergent Evolution, palaeobiologist George R. McGhee returns to MIT Press and The Vienna Series in Theoretical Biology to expand his examination to oceanic lifeforms, with the tantalising promise of applying the insights gained to astrobiology. I was particularly stoked for this second of a three-part dive into what I consider one of evolutionary biology’s most exciting topics.
Convergent Evolution on Earth: Lessons for the Search for Extraterrestrial Life, written by George R McGhee, Jr., published by MIT Press in November 2019 (hardback, 317 pages)
Just to get you up to speed, convergent evolution refers to the ubiquitous pattern of evolution repeatedly hitting on the same or similar solutions to a problem in different organisms. McGhee’s coverage of this topic in his 2011 book was wide. Next to morphologies and behaviours in terrestrial animals, he examined convergent evolution in ecosystems and molecules such as DNA and protein. He also introduced the abstract concepts of theoretical morphology and the hyperdimensional morphospace where life is probing all possible and allowed options.
Convergent Evolution on Earth can be thought of as an extension of his previous work. There is no repetition of these concepts and the coverage across different levels of organisation is absent. McGhee assumes familiarity with this and readers would do well to read the two books in sequence. If you do, the approach here will feel familiar, as most chapters again revolve around lists with examples. What is new is that McGhee broadens his examination of convergent evolution to behaviours and morphologies in marine organisms.
“[…] the question “who is convergent on who?” is much more relevant [here, as] it is the land plants who convergently evolved similar forms to much older marine animals”
I will come right out and say that I found this book a more challenging read. The terrestrial species examined in his last book will be familiar to most, but this book deals with marine vertebrates and, mostly, invertebrates. There are numerous groups here that even biologists will not necessarily be familiar with, also because many extinct groups are discussed. Thus, the convergent evolution of chemosynthesis found in deepwater species far away from light covers ciliophorans, polychaete and oligochaete worms, and a wide array of living and extinct mollusc groups. The convergent evolution of different morphologies to deal with living on soft and unstable substrates covers sponges, corals, extinct bivalves such as bakeveliids, and all sorts of echinoderms. More familiar groups such as fish and cephalopods feature when discussing adaptations to moving and living in the water column (McGhee’s mention of the repeated re-evolution of the whole spectrum of ammonoid shell forms following mass extinctions made me smile, as it reminded me of Danna Staaf’s discussion of this phenomenon in her excellent Monarchs of the Sea). And the convergent evolution of fundamental organ systems (e.g. nerves, muscles, or immune systems) reaches all the way back in time to some of the earliest invertebrate groups such as ctenophores, cnidarians, and bilaterians.
Of course, our land-dwelling, backboned vantage point makes us biased—for the longest time these invertebrate forms dominated life on Earth, and they are still instrumental to our ecosystems. Even so, most of us will not know what they look like, and this where the lack of images is much more noticeable than in McGhee’s previous book. The recent The Invertebrate Tree of Life is a good reference work to have at hand, not just for the imagery, but also for the taxonomical content. Though it was published just after Convergent Evolution on Earth and McGhee will not have had access to it, the taxonomy he has adopted closely mirrors that of Giribet & Edgecombe, with some exceptions deep in the tree of life that are known areas of contention.
“[…] what of the promised lessons for astrobiology? […] I felt a bit let down by the subtitle—it promised more than the final […] chapter to which this discussion is now limited.”
Next to showing the very deep roots and fundamental nature of convergent evolution, the question “who is convergent on who?” is much more relevant and appropriate this time around. Though we have named many sea creatures after land plants (e.g. sea lilies and moss animals), this book makes clear that, to solve the same fundamental problems, it is the land plants who convergently evolved similar forms to the much older marine animals. A notable advance is the adoption of new, recently proposed terminology, distinguishing between iso-convergence, allo-convergence, and retro-convergence. These terms respectively describe whether convergent traits evolved from the same or different precursor traits, or are a case of re-evolution of ancestral traits.
But what of the promised lessons for astrobiology? There is a look at Mars’s geological history, the possibility of life on water worlds in our Solar System such as the moons Europa, Enceladus, and Titan, and there is the conclusion that biological signatures are likely found on water worlds and technological signatures on water worlds with landmasses (readers interested in this will want to check out the massive Life in the Cosmos). Although what McGhee covers here is interesting, I admit that I felt a bit let down by the subtitle—it promised more than the final, 25-page chapter to which this discussion is now limited. My feeling is that most general readers will be better served by Kershenbaum’s The Zoologist’s Guide to the Galaxy. For those wanting to get to grips with this topic more in-depth, I end this three-part series with my review of Contingency and Convergence which revisits the question of their relative importance and applies this to astrobiology in a thought-provoking manner.
Convergent Evolution on Earth is not for the faint of heart. For evolutionary biologists, this is an interesting add-on to McGhee’s previous book, though requiring a certain level of background knowledge. For many other readers, there is probably less astrobiology in here than they would like.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended books mentioned in this review:
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To outsiders, phylogenetics, the study of the evolutionary relationships between organisms, must seem like quicksand: the landscape is ever-changing and what you thought was solid ground can turn into contested and unstable territory overnight. Even so, we are getting an ever-clearer picture. In no small part this is due to new methods: the rapid technological progress in DNA sequencing has now made it both feasible and affordable to sequence whole genomes (all of a cell’s DNA) instead of selected genes for many taxa. And when you can bring multiple lines of evidence – morphological, developmental, genetic, and palaeontological – to bear on the question of evolutionary relationships, the resulting family trees become better supported and more credible. That is exactly what Gonzalo Giribet and Gregory Edgecombe, both experts in invertebrate biology and palaeontology, have done here in The Invertebrate Tree of Life – a work of dizzying scope since 96% of all known species are invertebrates. They have synthesized a truly monstrous amount of research to give an overview of our current thinking on invertebrate phylogeny, writing a new benchmark reference work for students of invertebrates.
The Invertebrate Tree of Life, written by Gonzalo Giribet and Gregory D. Edgecombe, published by Princeton University Press in February 2020 (hardback, 680 pages)
As the authors point out in their preface, it is exactly the pace of progress that makes this book relevant and gives it a reason to exist next to third editions of e.g. Nielsen’s Animal Evolution and Brusca’s Invertebrates – two works that have much inspired these authors. Those works date back to 2012 and 2016, respectively. The nearly 3,000 references here, packed in a 100-page bibliography, bring the reader bang up-to-date to mid-2019.
In 52 chapters, the authors navigate down the different branches of the invertebrate tree of life, for most groups summarising at least their systematics, genomics, and the fossil record where information is available. As they point out, that last one is often an either/or proposition. Either books discuss the biology of living invertebrates, or they focus on the fossil record. You might think that it is only invertebrates with shells or exoskeletons that would litter the fossil record. And they do. But several exceptional sites, so-called Lagerstätte, preserve invertebrate fossils in exquisite detail, revealing more than just the hard parts. Next to the well-known Burgess Shale in British Columbia, there is the Chengjiang biota found in the Chinese province of Yunnan.
For the better-studied groups, the authors provide additional information on morphology, physiology, neuroanatomy, reproduction, development, and life cycles. Given the scope of this work these sections are necessarily brief – this book is explicitly not intended as a complete textbook on invertebrate biology. But it will give you the basics with plenty of references to papers and books if you want to read more on, say organ systems or neuroanatomy. And if you want to go into the real technical details on any one group, the ongoing The Handbook of Zoology series published by De Gruyter is your next port of call.
“You might think that it is only invertebrates with shells or exoskeletons that would litter the fossil record. And they do. But several exceptional sites, so-called Lagerstätte, preserve invertebrate fossils in exquisite detail […]”
This brevity notwithstanding, there is plenty of absolutely fascinating material here even when invertebrate taxonomy is not your speciality. I knew that cnidarians have a complex life cycle, with free-swimming jellyfish alternating with sessile polyps. But wait, boloceroidid sea anemones can swallow their own tentacles that then grow into new polyps? Or what to think of all the invertebrate groups practising kleptocnidism: the stealing of the stinging nematocyst cells from Cnidaria, which are then used for their own defence? And what to make of the fact that in Platyhelminthes (flatworms) even a single cell can regenerate an entire animal?
The authors also do a good job balancing how much (or really, how little) information they provide for well-known groups such as arthropods and molluscs versus all the other lesser-known groups. Getting more familiar with the latter gives the reader a renewed appreciation of the diversity of life. So, yes, invertebrate organisms can be both bizarre 
and incredibly diverse, but communicating this is a but a secondary aim of this book.
The real value of The Invertebrate Tree of Life is the discussion of evolutionary relationships and how our thinking has shifted over time: how we used to think groups were related, what names have fallen by the wayside, and how other group names have come to encompass different groups. For example, Arthropoda used to include Onychophora (the velvet worm, pictured on the book’s cover), which is now considered a sister group. These shifts in nomenclature can make entering the literature a daunting prospect, so this historical review is invaluable.
“The real value of The Invertebrate Tree of Life is the discussion of evolutionary relationships and how our thinking has shifted over time […] These shifts in nomenclature can make entering the literature a daunting prospect, so this historical review is invaluable.”
Similarly, the authors provide a good overview of current competing schools of thought on evolutionary relationships. For example, some authors talk of the clade Neuralia (Cnidaria + Ctenophora + Bilateria, but excluding Porifera and Placozoa), while Giribet & Edgecombe support the clade Planulozoa (Cnidaria + Placozoa + Bilateria). Similarly, which groups are part of Xenacoelamorpha (a clade of mostly marine worms)? That depends on who you ask. They also guide the reader to areas of consensus, and the kinds of data in support of it. Take Nemertea (ribbon worms), which look like flatworms. The current consensus based on phylogenomic analyses places them closer to molluscs and annelids.
The book is illustrated with a selection of photos of extant and extinct species, detailed trees for the internal organisation of prominent groups such as molluscs and arthropods, and useful schematic drawings of morphology, anatomy, and life cycles of other notable groups. My only criticism is that, for a book that wishes to be a textbook for students, there is no glossary – the authors assume familiarity with terminology. Some of it is explained, but can you tell paraphyly from polyphyly, synapomorphy from plesiomorphy, or schizocoely from enterocoely? I admittedly had to look some of this up, 
and some diagrams in the introductory chapters to get everyone on the same page would have been useful. But this is a minor quibble.
The Invertebrate Tree of Life is an invaluable work for anyone entering or already working in the field of invertebrate evolution, taxonomy, and phylogenetics. By unlocking and reviewing a huge body of literature, identifying knowledge gaps, and providing a balanced overview of both current consensus views and disagreements, Giribet and Edgecombe provide an incredibly useful community service, making this work a benchmark for the future.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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“The Deep: The Hidden Wonders of Our Oceans and How We Can Protect Them“, written by Alex Rogers, published by Wildfire in April 2019 (hardback, 366 pages)
I find the deep ocean one of the most fascinating ecosystems on our planet. Perhaps it is the final-frontier aspect, as so much of it remains unexplored (Rogers gives some mind-boggling statistics), or maybe it is the otherworldliness of its inhabitants, but I cannot get enough of reading more about it. Rogers’s fascination started at an early age, encouraged by summer holidays spent out on the ocean with his grandfather, an Irish fisherman. Although he recounts these formative experiences with great fondness, he also recognises the cruelty visited upon the unfortunate fish and lobsters that were caught.
A research career in marine biology followed and Rogers relates with great enthusiasm some of the highlights in marine science that he has lived through. Hydrothermal vents had only just been discovered in 1977 (see also The Ecology of Deep-Sea Hydrothermal Vents), and he participated in expeditions that first explored these deep-sea hot springs in the unruly waters of the Southern Ocean using remotely operated underwater vehicles. There was the ten-year Census of Marine Life (see also Discoveries of the Census of Marine Life: Making Ocean Life Count). And, of course, technological developments allowed both manned and unmanned submersibles to venture deeper, capture better footage, and even carefully return samples to the surface (see also Discovering the Deep: A Photographic Atlas of the Seafloor and Ocean Crust and In Oceans Deep: Courage, Innovation, and Adventure Beneath the Waves).
This is the wonder in this book.
The furry yeti crabs, the dream-like towers of black smokers hiding in the abyss, the millennia-old corals, the predators that are all mouth and stomach. And do not get me started on the fish (see also my review of Deep-Sea Fishes: Biology, Diversity, Ecology and Fisheries). Rogers’s eyewitness descriptions of the bizarre fauna are mesmerizing. The colour plate section is only all too small.
“The furry yeti crabs, the dream-like towers of black smokers […]The colour plate section is only all too small”
Here is another strong suit of The Deep: Rogers shows the relevance of seemingly obscure scientific research. The confirmation that the deep-water coral species Lophelia pertusa – so different from tropical species – builds reefs might seem like academic arcana to outsiders. But it was decisive in a UK court judgement to recognise the continental margin of the UK (and by extension that of the European exclusive economic zone) as habitat in need of protection from oil exploration and other activities under the European Habitats Directive.
Rogers is that rare breed of scientist who is not content losing himself in his research. He has advised conservation NGOs, provided testimony in court, and worked on projects for the UN to name but a few achievements. He cares intensely and his fighting spirit shines through in his writing.
This is the horror in this book.
See, one of the biggest problems with the open ocean is its remoteness and its lawlessness (see also my review of The Outlaw Ocean: Crime and Survival in the Last Untamed Frontier). Large parts of the oceans fall outside national jurisdictions and are effectively a marine wild west. Rogers effortlessly switches between the wonders of the denizens of the deep that we are only just beginning to discover, and the horrors of their destruction before we have even had the chance to properly take stock of what is out there.
“[…] the denizens of the deep [are destroyed] before we have even had the chance to properly take stock of what is out there “
The Deep is bang up-to-date on this front. There is the familiar spectre of overfishing: the rise of the industrial fishing complex, the harmful government subsidies, the collapse of fish populations, the destructive practice of trawling, the infuriating wastefulness of by-catches and simply catching more than the market can handle (see my reviews of All the Boats on the Ocean: How Government Subsidies Led to Global Overfishing, Daniel Pauly’s Vanishing Fish: Shifting Baselines and the Future of Global Fisheries, and The Curious Life of Krill: A Conservation Story from the Bottom of the World, and references therein). But Rogers is equally at ease explaining the details behind ocean acidification, eutrophication with concomitant episodes of oxygen depletion and algal blooms (a topic I am intimately familiar with from my own studies), changing ocean temperatures and coral bleaching, and palaeoclimatological evidence of past changes (see also my review of The Oceans: A Deep History).
And then there is pollution. So much pollution. Untreated wastewater and sewage, discarded and lost fishing gear, oceanic garbage patches and microplastics (see Microplastic Pollutants and Microplastic Contamination in Aquatic Environments: An Emerging Matter of Environmental Urgency), persistent chemicals that accumulate in the food chain. Even something as topical as the chemical oxybenzone in sunscreen, recently banned in Hawai’i, is mentioned. And Rogers is warily eyeing up the imminent threat of deep-sea mining, as the world continues to hunger for rare earth elements (see also The Elements of Power, and the double-act Deep-Sea Mining: Resource Potential, Technical and Environmental Considerations and Environmental Issues of Deep-Sea Mining: Impacts, Consequences and Policy Perspectives).
“[…] Rogers is warily eyeing up the imminent threat of deep-sea mining, as the world continues to hunger for rare earth elements”
Amidst this onslaught, Rogers continues to fight and eschews cynicism (though his success in court surprised even him). As opposed to terrestrial ecosystems, few marine species have yet gone extinct and fish populations can rebound if given the chance (see my review of Ocean Recovery: A Sustainable Future for Global Fisheries?). He convincingly shows that marine protected areas are a vital tool to offer fauna and flora a safe haven (see e.g. Marine Protected Areas: A Multidisciplinary Approach and Marine Conservation), and joins E.O. Wilson in his call to set aside large tracts of the planet (see Half-Earth: Our Planet’s Fight for Life).
The Deep is an absorbing and passionately written book that successfully combines an eyewitness account of the biological bounty of the deep sea with first-hand reporting on the threats, the conservation initiatives, the (failure of) policies and treaties, the fierce industry lobbying, and the political shenanigans. The Deep is aimed at a general audience and Rogers has decided to leave out literature references, footnotes, and an index. But those omissions are a small price to pay. At a time when we need as many people as possible to take notice, and when a documentary such as Blue Planet II can shake a nation out of its stupor on plastic pollution, good storytelling is a necessary and promising approach.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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“Across the Bridge: Understanding the Origin of the Vertebrates“, written by Henry Gee, published by the University of Chicago Press in July 2018 (paperback, 312 pages)
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: Images, Evolution, and Fraud 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.
“Would our thinking about evolution have such a close link with development if early naturalists had not been so interested in embryology?”
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: The New Science of Evo Devo and the Making of the Animal Kingdom for an accessible introduction, and From Embryology to Evo-Devo: A History of Developmental Evolution 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.
“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”
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: The Burgess Shale and the Nature of History, but the Chengjiang biota of southern China is equally astounding, as documented in The Cambrian Fossils of Chengjiang, China: The Flowering of Early Animal Life.
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: The Molecular Evolutionary Road to Vertebrates 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.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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