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.
Other recommended books mentioned in this review:
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