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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“Deep-Sea Fishes: Biology, Diversity, Ecology and Fisheries”, written by Imants G. Priede, published by Cambridge University Press, in August 2017 (hardback, 500 pages)
Priede’s work provides an academic treatise on the biology, ecology and diversity of deep-sea fishes. About half of the book is dedicated to a systematic summary of all the families to be found. As quickly becomes clear, the deep sea provides a home for fish right across the taxonomic spectrum, with the vast majority of families having at least some species living here. The book is not intended to allow species identification. Especially the FAO has published books and guides to that end, as this much information could never be contained in a few hundred pages. Instead, this book should allow you to look up fish caught at sea and retrieve information to genus level, with notable and important species described in some detail. Line drawings are provided of representative members of important families, revealing some of the weird and wonderful fish to be found. Clearly, there is much more to the deep-sea fish fauna than the famous anglerfishes, whose bizarre appearance have attracted so much attention that they have been the subject of a separate book (Oceanic Anglerfishes: Extraordinary Diversity in the Deep Sea). Although a plate section is included, this is no pictorial work. If you want to gawk at photos of bizarre-looking fish, you’re better off reading Creatures of the Deep: In Search of the Sea’s Monsters and the World They Live in or Life in the Dark: Illuminating Biodiversity in the Shadowy Haunts of Planet Earth. This part of Deep-Sea Fishes is really a reference work, not necessarily intended to be read, more to be consulted.
This core is bookmarked by chapters on the deep sea, its colonisation by fishes in evolutionary history, their adaptations to living here, a description of the faunas by area and depth zone, and, as seems unavoidable nowadays, a section about the impact of fisheries. This makes for informative reading and taught me a lot I didn’t know yet.
“[…] the deep sea is not just a featureless bathtub […] the land is topographically as varied as that above sea level”
Most of us will have at least a dim awareness that the deep sea is not just a featureless bathtub. Anyone familiar with plate tectonics will have heard of the Ring of Fire or the mid-Atlantic Ridge. But the deep sea has many underwater features, and the land here is topographically as varied as that above sea level. Special mention is made of hydrothermal vents and cold seeps, which provide oases of heat and chemicals in the deep and were only confirmed to exist in 1977 (see The Ecology of Deep-Sea Hydrothermal Vents). What is astonishing are the maps showing just how much of the planet is deep sea. The evocatively named abyssal (3000-6000 m) and hadal (>6000 m) zones make up more than 53% of the planet’s surface area. Fish do not occur uniformly throughout this enormous body of water though, with most of them hugging the contours of the bottom, creating vast swathes of water that sit over deep bottoms that are relatively devoid of permanent fish life, though there is plankton, vertically migrating fish, and eggs and juveniles galore in many areas. As the section on adaptations makes clear, there’s a physiological depth limit to fish life (about 8400 m) below which they cannot survive. Around the deepest trenches, such as the Mariana trench, this has interesting results, with the walls of trenches hosting fish fauna, but not all fish able to traverse the open deep-water chasms to the other side.
The chapter on adaptations to life in the deep sea is similarly fascinating in revealing the many outlandish solutions that have evolved to meet the unique challenges of this environment. The chapter is thorough, obviously covering well-known traits such as the extreme eyes or bioluminescent lures, but also providing in-depth accounts of physiological adaptations such as buoyancy mechanisms, and circulation and respiration.
“[…] areas that are repeatedly targeted for bottom trawling are akin to agricultural pastures on land […]”
The chapter on fisheries outlines the same story as All the Boats on the Ocean, describing the rise of intensive factory fishing by the USSR and other nations, and the collapse of fish stocks worldwide, with the industry exploiting and exhausting species after species. But there are many interesting details here, such as the different effects of the different types of fishing methods. One author has suggested that areas that are repeatedly targeted for bottom trawling are akin to agricultural pastures on land from the point of view of environments modified by humans. For the deep-sea fish fauna, the one saving grace is that physiological adaptations mean that many species are small, have a very low metabolism, low muscle mass and high water content in their body, making them commercially unattractive. This doesn’t stop destruction caused by by-catch, especially as rapidly being hauled to the surface from these depths is lethal in itself for many fishes.
Cambridge University Press has published a number of fascinating scholarly works on the deep sea in recent years (see The Hadal Zone: Life in the Deepest Oceans and Discovering the Deep: A Photographic Atlas of the Seafloor and Ocean Crust) and this book joins that line-up. The book has a pleasant weight and the binding and paper used make for a book that will lay open on a surface for reference and study. The production is slightly marred by some typos and figure references in the text not pointing to the correct figure panels, but these are infrequent enough not to be a big distraction. Deep-Sea Fishes might be too academic for a general audience, but if you really want to know the nuts and bolts of the topic, or you study this topic professionally in whatever guise, this book is an unparalleled source of information that you cannot do without. Even though new information becomes available continuously (for example, as I was writing this review, a study was published in Scientific Reports showing that hydrothermal vents could function as egg-incubators for a deep-sea skate), Deep-Sea Fishes is a synthesis that will be the go-to reference for years to come.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
Other recommended documentaries and books mentioned in this review:
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