The Zoologist's Guide to the Galaxy What Animals on Earth Reveal about Aliens – and Ourselves

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.

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.

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.

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.