Convergent Evolution on Earth Lessons for the Search for Extraterrestrial Life

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.

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.

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.

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 look out for the massive, upcoming book 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, which I had a chance to flick through, though not yet read in-depth. For those wanting to get to grips with this topic more in-depth, I end this three-part series with a 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.