Creatures… that are born pregnant; with twenty different sexes; that eat their own children; that can survive without water for a quarter of a billion years. Absurd? Not at all.
These are creatures alive on planet Earth. And they show us just how different alien life could be from anything we know.
What does a Martian Look Like? The Science of Extraterrestrial Life (also known in other editions as Evolving the Alien) sets out to do something seemingly impossible: to scientifically describe something we have never seen. The question it asks is what we can know about extraterrestrial life. Of course, we have never found any of that. And yet, Jack Cohen and Ian Stewart also argue against imagining it will be just like life on Earth.
So what can they even say in this book? Mainly that we need to speculate, but speculate wisely; taking into account all that we really know, and seeing what follows logically, instead of getting trapped in what our imaginations naturally tell us.
I got to know the authors first from their collaboration with Terry Pratchett on The Science of Discworld, a work half allegorical fiction and half popular science. Nowadays, I will mention them if asked about my favourite philosopher, even though they’re not officially philosophers. Jack Cohen is a biologist, Ian Stewart is a mathematician, and together they write about complexity science. They possess both an acute, fascinating scientific vision and an entertaining, slightly quirky way of writing. This book is no exception.
Cohen and Stewart contrast their approach, which they call “xenoscience”, with traditional “astrobiology”. Astrobiology in this sense assumes that life in the rest of the universe must be very much like life on Earth, and so, for example, if we are to find any, we must look for planets just like ours. By contrast, this book argues that life could (for all we know) be based on something completely different than Earthly chemistry, as long as it can support the basics, like reproduction and evolution by natural selection. The authors point out that even less-known life on Earth does things that some astrobiologists would consider impossible. What looks to us like favourable conditions for life, like having water and oxygen around, could look impossibly hostile to some lifeform that evolved elsewhere. It’s only vital for us because we evolved here. Since we’ve seen no alien life, we don’t actually know that it could thrive in a totally different environment — but we certainly don’t know that it couldn’t.
Cohen and Stewart also contrast their approach with naïve depictions of aliens in science fiction and accounts of alleged real encounters with them. In fact, they spend quite a lot of time discussing why these depictions only betray our lack of imagination. I agree that it’s necessary for them to do this — you can’t explain how a thing really is if the person you’re telling it has the wrong idea to begin with and you never explain why it’s wrong. Besides, it’s fun and interesting. So they explain why we’re not to expect any cat people or little grey men. Speaking of science fiction, though, they also look at examples of different levels of credibility. The believable ones tend to be the most alien, but some of those don’t work either.
As I said above, Cohen and Stewart teach us to speculate, but speculate wisely. The way they put it, you could expect alien life to be practically anything… except that many specific ideas won’t work. There has to be a way something could evolve in the first place. No-one can say what real aliens will be like exactly, but the authors are able to give meaningful limits to speculation based on the distinction between universals and parochials: universally useful features for life — like wings for flying or eyes for seeing, provided there’s air to fly in or radiation to see, of course — and things that just happen to be like that because of evolutionary history, like our having five fingers. Just think of how sharks and dolphins have evolved into about the same shape for similar purposes, but their tails move in different directions because dolphin ancestors were running land animals in between. Based on this, the authors make many surprising but entirely logical observations about what is possible. I was amused and intrigued by the observation that you would never expect any aliens to have their nose above their mouths, because that seemingly natural arrangement is something we only have because all land vertebrates have descended from a particular species of fish that left our airways inconveniently crossing our foodways.
What Does a Martian Look Like? is very imaginative, but in a way that only makes it more rigorously reasoned; the creativity and imagination ensure that lack of imagination does not prevent us from seeing what is really logically possible. It’s entertaining and absolutely fascinating, not only as a study in xenobiology but also in human psychology, popular fiction, and in the principles of evolution. It’s also highly recommended for anyone planning to write at all realistic fiction featuring aliens. Cohen and Stewart are simultaneously able to imagine the truly alien — and yet to be critical about what ideas could really be realised.