That life exists elsewhere in the universe seems almost inevitable. That we could know anything about it seems almost impossible. But my goal is to show you that we can actually say a great deal about what aliens must be like, how they live, and how they behave.
We are increasingly confident that there is life elsewhere in the universe and, even more excitingly, that it might be possible to find it. In 2015, NASAO's chief scientist, Ellen Stofan, predicted that we will have evidence of life on other planets within twenty to thirty years. Of course, she was talking about microbes, or their alien equivalent, not necessarily intelligent life. But the principle is still staggering. We have gone from being obsessed with the possibility of alien life at the beginning of the twentieth century, to complacent pessimism in the seventies and eighties, and now back to a realistic, scientific optimism. This book is about how we can use that realistic scientific approach to draw conclusions, with some confidence, about alien life - and intelligent alien life in particular.
Short of aliens landing in New York, how can we know what they're really like? Do we need to rely on the imagination of Hollywood, and of science fiction writers? Or perhaps alien animals are likely to be no more bizarre than a kangaroo hopping on giant feet, or a squid jet-propelling itself through the sea, with skin that flashes in rainbow colours. Trusting in the universal laws of biology that bind us - all life on Earth - and also those creatures on alien planets, we will see that the reasons for those adaptations that animals have adopted on Earth are likely to be reasons for adaptations on other planets as well. Hopping and squirting will be perfectly reasonable ways to get around on many different planets, just as they are on Earth.
How rare is life in the universe? Until the 1990s, the presence of planets around other stars (exoplanets) was a matter for speculation, and some mathematical calculations. We had no solid idea how many planets there might be in the galaxy, and what they would be like: how hot, how much gravity, what atmosphere, which chemicals? As technology reached a level where it was actually possible to detect planets around other stars, excitement began to grow. Perhaps there was indeed a possibility of detecting planets that could host alien life.
First indications were disappointing. The few planets discovered were large, hot and made of gas - not very amenable to life, either as we know it or otherwise. But less than twenty years after the first exoplanet was discovered, there was a major breakthrough. The Kepler space telescope was launched to search for possible planets; staring at stars in just one tiny fixed region of the sky. Within just six weeks of Kepler beginning operations, five new exoplanets had been discovered. By the time Kepler ceased operating in 2018, it had discovered an unbelievable 2,662 planets around the stars in just that tiny corner of the sky: about as much as you can cover with your fist at arm's length.
The implications are staggering. There are far more planets in the galaxy than we had previously thought, and with improved measurement methods we now know more about what those planets are like. We have found the full range of planetary conditions, from hot, Jupiter-sized gas planets, to those that are remarkably similar to Earth. The universe is now a lot more crowded than it appeared to be in 2009, and our grandchildren will likely not even believe that we once said, 'Earth-like planets are rare.' We no longer have any excuse to say that the universe lacks possible homes for alien life.
We now have a much better understanding of the physical environmental conditions that are likely to exist on alien planets and, increasingly often, we can even measure them directly. New instruments being developed will be able to detect the chemicals in a planet's atmosphere by spotting changes in the light passing through the atmosphere from the star it's orbiting. We'll be looking for oxygen, of course, but also for complex chemicals that might indicate industrial development. Ironically, pollution is a sign of cosmic intelligence.
Somehow, life came into existence at least once in the universe. We are proof of that. But how that happened, we donÕt know. There are certainly many theories on the mechanisms by which life may have arisen on Earth. Most likely, the basic chemicals needed for life formed at random and then, by a lucky chance, combined into a special kind of molecule that could make copies of itself. Overall, a fairly unlikely set of circumstances. Does that mean life on other planets arose the same way? Absolutely not. We really donÕt know how relevant the processes that we think took place on Earth may or may not be on other planets. Aliens may be based on carbon chemistry like ours, carbon chemistry unlike ours, or something altogether different.
The principles of chemistry are pretty well understood, so many of these ideas can be tested in the lab, seeing which chemicals might be stable and which not. We think that chemicals like the ones that make up our bodies are pretty good ingredients for something that is 'alive'. But beyond the most basic ideas of what alien biochemistry might be like, a thick fog descends. We have no examples of alien plants or alien animals to examine - nor even any idea whether the terms 'plants' and 'animals' would have any meaning on another planet. Despite NASA's optimism that we will discover signs of alien life, the vast distances between the stars means that it would take a huge technological leap to visit planets outside our solar system. We can mix alien chemicals in a lab, but watching alien birds with binoculars will be a much harder proposition.
One problem with understanding the nature of aliens is that our starting point for comparison is just one single type of life - that on Earth. How much can we use our single example of life to draw conclusions about other planets? Some people claim that speculation about the nature of alien life is futile; that our imagination is too tied to our own experience to be able to encompass the staggeringly diverse and unfamiliar possibilities that may be reality on other worlds. The science fiction writer and author of 2001: A Space Odyssey, Arthur C. Clarke, said, 'Nowhere in space will we rest our eyes upon the familiar shapes of trees and plants, or any of the animals that share our world.' It's a popular belief that alien life is too alien to imagine. I don't agree. Science has given us the opportunity to move beyond such a pessimistic outlook, and we do seem to be able to identify some clues about what alien life may be like. This book is about using our understanding of how life works and, most importantly, how life evolves, to understand how life will be on other planets.
How did an Earth-bound zoologist like myself - who is more used to following wolves over the snows of the Rocky Mountains, or tracking furry hyraxes in the hills of Galilee - get involved in the search for extraterrestrial life? One of the things I study is animal communication, and why animals make the sounds that they do. In 2014, I gave a talk at the Radcliffe Institute in Harvard entitled, 'If birds could talk, would we notice?' It may seem obvious to us that humans have language and other animals do not - but how do we know for sure that is the case? I was looking for mathematical fingerprints of 'language' in the communication of animals - a clear-cut measurement that would say: 'Yes, this is language', or, 'No, this is not language'. With the encouragement of some good but somewhat eccentric colleagues, the obvious next step was to ask the same question about signals from outer space. Is this a language? If so, what kind of creatures might have produced it? From there, it becomes clear that we can extend our understanding of other aspects of life on Earth - finding food, reproducing, competing and cooperating with others - to alien planets as well.
But why study aliens in the context of zoology, when we haven't seen any aliens, and don't even know for sure whether they exist? When undergraduate students arrive at university, fresh from school and from exams that rigorously tested their ability to remember a long list of facts, our first job as their educators is to convince them that facts are all very well, but that they must understand concepts; not what happens in the natural world, but why. Understanding processes is the key to zoology on Earth - but it can also help us with understanding the zoology of other planets. As I write this, our second-year students here at Cambridge are preparing for a field trip to Borneo. Some of them are leaving the UK for the first time in their lives. Do we expect them to memorize a field guide to the hundreds of birds and thousands of insects of Borneo? Of course not. Like future explorers of an alien world, they must go equipped above all with an understanding of the evolutionary principles that led to the diversity of life that they will encounter. Only once the concepts are clear will interpreting the animals we find be possible.
Most people are confident that the laws of physics and chemistry are unequivocal and universal. They work here on Earth just as they would do on any exoplanet. Predictions that we make here about how physical and chemical materials will behave in different circumstances, are going to be good predictions about how those same materials would behave in those same circumstances in other parts of the universe. We rely on science to work in that way. Biology, however, is seen by some people to be an exception. We find it hard to believe that the laws we derive on Earth about how biology works would also apply on an exoplanet. Carl Sagan, one of the most famous astronomers of the twentieth century, and a fervent believer in intelligent life elsewhere in the universe, nonetheless wrote, 'For all we know, biology is literally mundane and provincial, and we may be familiar with but one special case in a universe of diverse biologies.'
When dealing with the unknown, there are indeed good reasons for caution. But there are also reasons for optimism; we just need to be careful to choose those laws of biology that are truly universal, in the same way that the laws of physics are universal. Why should biology be 'mundane and provincial', rather than universal? Surely the laws of nature - physical, chemical and biological - are common across the universe? The Earth is unlikely to be so exceptional that the rules here are different to every other planet. Lucretius, the Roman philosopher who died in c.55 bce, commented, 'Nature is not unique to the visible world.' Exoplanets have 'nature' as well, even if we've never seen them.
Contrary to what some people think, zoologists such as myself don't just spend our time identifying and classifying animals. Like scientists in all disciplines, we attempt to explain what we see in the world around us. Zoology, and evolutionary biology in general, is about proposing mechanisms for explaining the nature of life. Why do lions live in prides, but tigers hunt alone? Why do birds have only two wings? Why, for that matter, do the vast majority of animals have a left side and a right side? Observation is not enough. We want to derive a set of rules for life, in the same way that physicists derive rules for planets and stars. If those biological rules are universal rules, they will work as well on another planet as the law of gravity.
Yet there is no doubt that biology appears to be flighty and unpredictable. A physicist understands exactly how a ball rolls down a hill, and can give you a set of equations you can use to predict the motion of balls on hills everywhere in the universe. Physics experiments rely on highly controlled and simplified conditions - not at all what we find in the biological world. A well-known joke tells of a physicist trying to derive equations to predict the behaviour of a chicken, and declaring that this is possible, but only for a spherical chicken in a vacuum. Real chickens are out of the realm of 'physics' and so, a physicist would say, are unpredictable. But why can we predict the motion of a ball, and not the behaviour of a chicken?
Biological systems seem to avoid following strict rules because they are, in a profound way, complex. In mathematical terms, a complex system is one where multiple subsystems are mutually dependent on each other. It turns out that it doesn't take very much dependency between relatively simple systems for the overall behaviour to be utterly complex and unpredictable - chaotic, in the technical jargon. Imagine trying to predict the behaviour of all the interacting organs in your body. Better yet, imagine all the cells in all the organs - or all the proteins in all the cells in all the organs, and so on ... The slightest change in one element can have a cascading, unpredictable effect. Even the simplest life is clearly complex. And complex systems are hard to predict.
One of the frustrating properties of an unpredictable complex or chaotic system is that no matter how hard you study it, you will never unlock all its secrets. We are used to the idea that if we investigate something carefully enough, we will come to understand everything about it. Science seems to be based on this idea. But chaos theory tells us that sometimes you can investigate a system a hundred times more carefully, and only gain ten times as much ability to predict what it's going to do. You can put more and more resources into understanding a complex system, but only get very marginal returns. That's clearly a mug's game. Fortunately, complex systems also have what are called emergent properties: you may not be able to predict exactly what they are going to do, but you can get the general idea. The chicken will look for seeds, even if I don't know exactly which seeds. In practice, being able to say 'the chicken will look for seeds' is more useful to me as a biologist than 'the chicken will look for that seed'. Rather than being able to predict how the biochemistry of alien life will work, or what their eyes will be made of, we can make general predictions that their biochemistry will provide them with energy, and also whether or not they will have eyes of any sort.
What then are these universal laws of biology, using which we can make confident predictions about life on other planets? The first and most important law is that complex life evolves by natural selection. It is hard to overemphasize the importance of this process, which has been the cornerstone of all biology since the seminal work of Charles Darwin. Natural selection is not just the only mechanism we know for creating complexity out of simplicity (if we reject the explanation of a divine force pushing complexity to develop), it is also an inevitable mechanism, not just restricted to the planet Earth, or to 'life as we know it'. If we see complexity in the universe - complexity of the kind that we would call 'life' - it is because natural selection has been operating.