This is the second of a two-part dive into the story of oceans on Earth and elsewhere, following my review of Ocean Worlds. That book gave a deep history of how our oceans shaped Earth and life on it and briefly dipped its toes into the topic of oceans beyond Earth. Alien Oceans is the logical follow-up. How did we figure out that there are oceans elsewhere? And would such worlds be hospitable to life? Those are the two big questions at the heart of this book. If there is one person fit to answer them, it is Kevin Peter Hand, a scientist at NASA’s Jet Propulsion Laboratory and their deputy chief for solar system exploration.
Alien Oceans: The Search for Life in the Depths of Space, written by Kevin Peter Hand, published by Princeton University Press in March 2020 (hardback, 248 pages)
A major question in astrobiology is whether the evolution of life on Earth is a fluke, or whether life is bound to pop up wherever conditions are favourable. Hand very neatly frames this in the bigger history of science. Over the centuries, we figured out that the laws of physics, chemistry, and geology work beyond Earth. But “when it comes to biology, we have yet to make that leap. Does biology work beyond Earth?” (p. 15). What we have learned is that life as we know it needs water. And though there is no shortage of theories on the origins of life, oceans are very likely where it started, and thus a logical first place to start looking for answers.
If you have any interest in astrobiology, you will probably have heard of the concept of a habitable zone or Goldilocks zone where, based on the distance to a star, conditions for life are just right. Not so close as to be too hot, nor so far as to be too cold. Earth obviously falls in that zone. Next to many minor insights, Alien Oceans had three major eye-openers for me. This was the first one:
There are other Goldilocks zones.
Depending on the details of their orbit, moons can experience such strong tidal tugs from their parent planet that the constant squeezing and stretching of the rock creates enough heat through internal friction to sustain liquid water. The physics of water helps, as it has a seemingly mundane but rather unusual property. Ice floats. When water solidifies, its density decreases slightly. What this means for moons is that the liquid water exposed to the cold of deep space freezes and forms a protective icy shell. Most liquids do not have this useful property. When they freeze, they sink to the bottom exposing more liquid until all of it is frozen solid. To top it off, ice is also a good thermal insulator, helping such ocean worlds retain heat. Maybe I have been hiding under a rock, but this was revelatory for me. Suddenly, the amount of cosmic real estate suitable for life has increased quite dramatically. And we have some of it right here on our doorstep.
“Over the centuries, we figured out that the laws of physics, chemistry, and geology work beyond Earth. But “when it comes to biology, we have yet to make that leap. Does biology work beyond Earth?“”
The existence of oceans in our solar system and how we gathered the evidence is one of the two major threads running through this book. Hand examines this in detail for Jupiter’s moon Europa, which has been studied in the most detail. Three types of data are typically gathered: spectroscopic, gravimetric, and magnetometric. This is where Hand gets fairly technical, though, fortunately, he extensively uses comparisons with everyday concepts and technologies to help you understand the underlying (astro)physics. Without retreading his careful explanations, in Europa’s case, these different strands of data all converge on a moon with an icy shell and a substantial subsurface ocean some 80–170 km thick as the best explanation. Mixed in with this narrative are the details and many technical setbacks of the Galileo mission that are nail-bitingly tense in places.
Similar missions and measurements have been done for Saturn’s moons Enceladus and Titan, Jupiter’s moons Ganymede and Callisto, Neptune’s moon Triton, and Pluto. The evidence for oceans gathered so far gets less robust in this order, but there are some notable variations on the theme. Enceladus ejects spectacular plumes of water and carbon compounds that were photographed and sampled by the Cassini–Huygens mission. Ganymede, meanwhile, is so large that the bottom of its ocean might consist of an exotic form of dense ice, ice III, formed at very high pressures not seen on Earth, meaning its ocean is sandwiched between two layers of ice.
So you have found exo-oceans. Now what? Can we expect to find life here? That is the second major thread. Hand identifies five conditions for life to emerge: a solvent such as water, chemical building blocks, an energy source, catalytic surfaces, and time. Interestingly, there is a gap between two schools of thought. The top-down explanation deconstructs life backwards in time until we arrive at an RNA world, but how did that get started? The bottom-up explanation has shown that life’s basic building blocks such as amino acids exist in space, but how do we go from there to larger functional molecules?
“There are other Goldilocks zones. Moons can experience such strong tidal tugs that the heat released by internal friction can sustain liquid water.”
This was the second major eye-opener for me: “Our environment is full of chemical disequilibrium […] there are reactions just waiting to happen. […] The metabolisms that drive life accelerate reactions in the environment, releasing energy faster than would have occurred without life” (p. 144). Hand takes a leaf out of Nick Lane’s book The Vital Question (which, shame on me, I still have not read) when he enthusiastically concludes that “the why of life is metabolism” (p. 146), offering the universe a pathway to increase entropy faster. These are remarkable ideas that give a whole new meaning to philosophical questions on the meaning of life.
The third and final eye-opener concerns the need for a catalytic surface, which is where Hand circles back to oceanographic exploration here on Earth, a recurrent theme in this book. When the submarine Alvin discovered hydrothermal vents in 1977 and found them teeming with life, these quickly became a popular alternative explanation to warm tidal pools as a place where life could have started. These so-called black smokers are powered by magma rising to the surface at mid-ocean ridges, jetting out superheated water of over 400 °C. Though volcanism and tectonics are, or sometimes were, common processes on many solar system bodies, there is another option. Alkaline vents, first discovered in 2000 at the Lost City hydrothermal field, are powered by exothermic (energy-releasing) reactions between water and mineral-rich rock, heating water to a more gentle 70–100 °C. All these need are the right rocks with cracks in them so water can percolate down.
“In addition to hydrothermal vents, alkaline vents are another place where life might have started. All these need are the right type of rock with cracks in them so water can percolate down, no plate tectonics needed.”
Hand raises many other interesting questions towards the end of the book, of which I will mention just three. One, life’s metabolic reactions require so-called oxidants, oxygen being “the most glorious of oxidant” (p. 162), but how would these get down into subsurface oceans? Two, how contingent or convergent is the evolution of life’s biochemistry? Carbon is a suitable building material for life as it is “hands down the best team player on the periodic table” (p. 212). But does physics restrict us to these options, or can we sketch a periodic table of life with other, weirder possibilities? And three, how should we seek for signs of life? What makes a good biosignature? This is discussed far more in-depth in Life in the Cosmos, but Hand considers three types of evidence.
Alien Oceans limits itself to oceans in our solar system, not touching on the topic of exoplanetary oceans. Given this is not Hand’s expertise, that is reasonable. He also glosses over the question of what aliens might look like, though he speculates on the likelihood of intelligent life in ice-covered subsurface oceans. Even without these topics, Alien Oceans is information-dense, requiring me to make a summary, and then a summary of that summary while preparing this review. Nevertheless, it is an intellectually very rewarding book and the many analogies make it accessible. I enjoyed it as a follow-up to Ocean Worlds but it is a fine standalone book. Terribly fascinating, Alien Oceans makes a convincing case for exploring the moons in our solar system in the search for extraterrestrial life.
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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Can we predict what aliens will look like? On some level, no, which has given science fiction writers the liberty to let their imagination run wild. On another level, yes, writes zoologist Arik Kershenbaum. But we need to stop focusing on form and start focusing on function. There are universal laws of biology that help us understand why life is the way it is, and they are the subject of this book. If you are concerned that consideration of life’s most fundamental properties will make for a dense read, don’t panic, The Zoologist’s Guide to the Galaxy is a spine-tingling dive into astrobiology that I could not put down.
The Zoologist’s Guide to the Galaxy: What Animals on Earth Reveal about Aliens – and Ourselves, written by Arik Kershenbaum, published by Viking in September 2020 (hardback, 356 pages)
Fundamental to answering the question of what alien life might be like, Kershenbaum argues, is to recognize that evolution by natural selection is the most important law in biology, “an inevitable mechanism, not just restricted to planet Earth” (p. 8). Rather than trying to answer particulars (Will aliens have two legs? Six? Or none?), he focuses on process: “Movement, communication, cooperation: these are evolutionary outcomes that are solutions to universal problems” (p. 11). Thus each chapter discusses “some feature of animal behaviour on Earth that is not unique to Earth—that can’t be unique to Earth” (p. 14). These three quotes nail down how Kershenbaum managed to hook me in right from the start of his book.
A key observation to support his argument that natural selection will not be limited to planet Earth is convergent evolution. I find this one of the most exciting topics in evolutionary biology and have written about it extensively last year when reviewing three books in The Vienna Series in Theoretical Biology from MIT Press. Brief refresher should you need it: convergent evolution refers to the ubiquitous pattern of evolution repeatedly hitting on the same or similar solutions to a problem in different organisms. Kershenbaum introduces it here with some examples and also touches on the contingency vs. convergence debate, of which Stephen Jay Gould and Simon Conway Morris are the most prominent spokesmen. Convergent evolution can hold lessons for astrobiology, though George R. McGhee’s book Convergent Evolution on Earth did not quite deliver on its promise to do so. Kershenbaum, however, does. There is no reason to think that convergent evolution would be limited to life on Earth because “we live in a universe where not everything is possible” (p. 46). The laws of physics circumscribe a limited set of possibilities, something that Charles Cockell so memorably expressed by writing that “physics is life’s silent commander“.
So what are these characteristics that we can expect to evolve universally? Kershenbaum considers six, from very basic to likely rarer: movement, communication, intelligence, cooperation, information exchange, and language. Even though these are very fundamental properties of life that you could talk about in abstract terms, what makes The Zoologist’s Guide to the Galaxy so accessible is Kershenbaum’s pithy writing style. I will highlight three examples to give you a taster.
“[Kershenbaum] focuses on process: “Movement, communication, cooperation: these are evolutionary outcomes that are solutions to universal problems“”
Take movement: “We move because we must, not because we can […] Life needs energy, and if energy is not evenly distributed, life must go in search of it” (p. 70–72). Earth life has tried pretty much every mechanism we can think of to move in a fluid medium (air or water) or on the interface between a fluid and a solid, so expect alien life forms to float, paddle, or develop legs.
Cooperation similarly seems likely. There is a range of benefits to individuals cooperating, not in the least the threat of predation. “Predation is universal, because no ecosystem can exist for long without someone trying to take a bite out of somebody else; the selective pressure on acquiring as much energy as possible is just too strong” (p. 171). When and whether it is evolutionarily advantageous to evolve cooperation is something we can answer mathematically using game theory, “a simple technique, applicable on any planet” (p. 192). We should not be surprised to find aliens with complex social structures, dominance hierarchies, and reciprocal behaviour.
A full-blown language, on the other hand, seems uniquely human. This chapter leads you through the difficulty in defining language and grammar, the contentious topic of language evolution, and an interesting dive into xenolinguistics, or how you would recognize whether a signal carries the signature of language. These are all areas of active research where no consensus has been reached between different schools of thought. Nevertheless, Kershenbaum identifies two fundamental features that an alien language would have: it is a means to communicate complex concepts, and it evolved by natural selection.
“There is no reason to think that convergent evolution would be limited to life on Earth because “we live in a universe where not everything is possible“. The laws of physics circumscribe a limited set of possibilities […]”
This core of six chapters is padded out with a fascinating chapter that considers artificial life forms. After all, evolution as we know it acts blindly, without foresight. “But what if it were all different? What would life look like if it did know where it was going?” (p. 258). Well, perhaps not all that different. “Game theory […] is ruthlessly inevitable” (p. 280), so expect conflict and cooperation. Furthermore “some things like mutation, and even death, can’t be eliminated just by being incredibly smart” (p. 286).
The whole is bookended by two more philosophical chapters. The first asks what an animal is and whether aliens would be considered animals. Though we would not share ancestry, the point of this book is to show that we would likely share fundamental processes and properties. The last chapter considers the impact that the discovery of alien life would have on us and whether we would recognize such life forms as a fellow form of humanity. Throughout, there are footnotes to general literature, and an annotated list of suggested reading provides plenty of material if you want to delve deeper.
Kershenbaum admits that you probably wanted him to tell you what aliens look like, and his book contains less speculative zoology than e.g. Imagined Life. However, by the same logic of giving a man a fish vs. teaching him how to fish, understanding the rules that life follows is ultimately more rewarding. Kershenbaum’s smooth writing style makes it a proper page-turner.
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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