david_christian's picture
Director, Big History Institute and Distinguished Professor in History, Macquarie University, Sydney; Author, Origin Story
Extraterrestrials Don’t Land On Earth!

Yesterday, no extraterrestrials landed! Or the day before! Or, despite many claims to the contrary, in any earlier period of human history. Or Earth history.

This is odd. There are several hundred billion stars in our galaxy and at least a hundred billion galaxies in our Universe. In the last twenty years, astronomers have detected lots of planets around nearby stars, so we know planets are common. In fact, there could be tens of billions, or even a hundred billion Earth-like planets in our galaxy alone.

It’s hard not to think that a lot of these Earth-like planets (a few million perhaps?) may have had histories a bit like our Earth. They may have spawned living organisms. On Earth, we have found life in many extreme environments, from deep sea oceanic vents (where the current record-holder can survive at 120° C.), to the inside of rocks, where they have to live very very slowly in order to survive. Endospores can temporarily stop living (well, metabolism ceases) until things improve. Some bacteria may have jumped from Mars to Earth. So, life can exist in a wide range of environments, and today, many astrobiologists believe that life might have existed on Mars and Venus, and could exist even now on some of the moons of Jupiter and Saturn, such as Io and Europa, which have lots of ice. All in all, it’s beginning to seem that life of some kind could be common in the Universe. It may be that the Universe is quite bio-friendly.

If Simon Conway Morris and others are right, and there is a limited number of pathways along which life can evolve, then any organisms that exist on other planetary systems may have evolved in ways not too dissimilar to the organisms on our Earth. Evolution may have converged on similar solutions in different star systems. Perhaps multi-cellular organisms have evolved many times. Perhaps many had ways of detecting light waves (eyes?); and perhaps many developed ways of computing or thinking (brains?). Our galaxy is 13 billion years old, and most of its stars are older than our Sun, so most of its planetary systems should be older than our Earth, which means they would have had much longer to evolve complex life forms.

Here on Earth, life got going more or less as soon as our young planet was cool enough to have liquid water. That’s fast, and hints that simple forms of life may be common. Four billion years later, large, intelligent creatures have appeared, and lots of them. One of those species crossed a critical threshold when it evolved such a powerful form of language that its members began to share their ideas and accumulate more and more information from generation to generation. As a result of its ability to learn collectively, that species (us) has built an astonishing store of knowledge, which enables us to control more and more of our environment, until now we dominate the planet. We have become a planet-changing species and we now live in what many scholars call the "Anthropocene Epoch." We’ve even launched a few of us short distances into space, and sent robots throughout our solar system.

On planets where evolution began millions of years earlier than on Earth, you’d think evolution might have gone well past the crucial threshold of collective learning, past the production of a planet-changing species, and on, perhaps, to the point of colonizing nearby star systems. Could there be thousands of planets with species capable of collective learning? We can’t know, but such an estimate is not impossible, and many of these planets could be orbiting the 4,500 star systems within 60 light-years of our Earth that make up our galactic neighborhood.

So where are the extraterrestrials? This was Fermi’s famous question. The SETI program has been scanning the heavens for evidence of alien life since 1960. We haven’t seen them. We haven’t heard them either, or detected any other signs of their existence. Frank Drake, inventor of the "Drake equation," which lists the factors we must take into account to estimate the likelihood of encountering other species like ourselves, thought that one of the crucial factors might be how long planet-changing species like us could survive.

And there’s the rub. We are so clever that we have invented weapons that could ruin the biosphere in a few hours; and our energy-hungry civilizations seem to be degrading the biosphere and the climate-systems on which we depend. Is it possible that planet-changing species like us never get past this stage? Do they all hit a wall when they reach their local Anthropocene? If so, such species may last for a few centuries or a millennium or two, and then flicker out, perhaps after retreating to impoverished niches where they eke out a miserable existence before going extinct. That would mean that even if planet-changing species—species capable of telling stories and jokes, of painting and dancing, and building pyramids and space ships—are quite common, they would all self-destruct. That would solve Fermi’s problem! 

Or perhaps some other planet-changing species actually learnt their lesson, maybe after a few self-inflicted catastrophes. Perhaps they decided not to aim too high, not to try to dominate their planet or their solar system or neighboring star systems, but to live more sociably with their home planet and the other organisms that surrounded them, after realizing this was the only way of surviving. Perhaps we don’t see them because, like Candide at the end of Voltaire’s novel, they are all happily cultivating their own gardens. That would also solve Fermi's problem!