Two overarching questions confront would-be human spacefarers: where to go and how to get there.
Much attention has been given to the latter question. For interstellar travel to become a reality, major engineering advances are required, probably involving radically new propulsion systems. Many proposals are highly speculative, but we know of no fundamental physical principles that forbid interstellar travel; whether or not it becomes a reality boils down to technology, cost and motivation.
I wish to address the oft-neglected first question: the destination. Leaving aside fantastical speculations about faster-than-light travel, it is clear that journeying between the stars will take a very long time, even on the most optimistic estimates of technological advance. Therefore, interstellar tourism, or trade in physical substances (as opposed to information), is inconceivable.
Travel beyond the Solar System will be one-way only. Two possibilities then arise: that spacefarers will seek out and colonise other worlds, or that they will create permanent artificial habitats in space. Both scenarios have been popular in science fiction. I’m going to leave aside the vast engineering issues involved and dwell instead on a much trickier and more basic problem – the ecological requirements, especially those relating to microbiology.
Long-term human survival means more than growing enough food to eat and making enough oxygen to breathe. It demands creating a complete self-sustaining ecosystem. On Earth, complex multicellular organisms (e.g. animals, plants) form merely the conspicuous tip of a vast biological iceberg, the majority of which is microbial.
Almost all terrestrial species are microbes – bacteria, archaea and unicellular eukaryotes – and to date microbiologists have scratched only the surface of the microbial realm.
Microbes are everywhere – in the soil, in the air, in water, in the rocks beneath our feet, in the Earth’s crust to a depth of several kilometres. These busy little creatures are a vital part of the life-support system of our planet, both via their metabolic activity (such as recycling material) and through the exchange of genetic components. Even within your own body, microbes play a crucial role.
The microbial inhabitants of your gut, lungs, etc – known as your microbiome – outnumber your own cells. Without them you would die. So astronauts cannot be sent to the stars without, at the very least, their own microbiomes.
It’s not just bacteria – a fraction of space worms came back with two heads.
But it doesn’t stop there. Microbes do not live in isolation; they form a vast network of biological interactions that remains very ill-understood. The basic Darwinian process – replication with variation plus natural selection – is now recognised as an incomplete account of evolution. Darwinism can be regarded as the vertical transfer of information (from one generation to the next), but there is also much horizontal information flow, via gene transfer, cell-cell signalling, collective organisation of cells and much else.
Interwoven into this network are the activities of viruses, which infect microbes just as they do larger organisms. The subtle interplay of viruses, microbes and metazoa constitutes an ecological web of such staggering complexity that scientists have hardly begun to unveil it. The daunting nature of the problem may be glimpsed from the work of my Arizona State University (ASU) colleagues Hyunju Kim and Harrison Smith, who compiled data from over 28,000 genomes and 8658 biochemical reactions to create a map of information flow taking place, not just in localised ecosystems, but on a planetary scale. The biosphere, it seems, is the original World Wide Web.
Given that we can’t send the entire biosphere to another world, a fundamental problem arises: what is the minimum complexity of an ecosystem necessary for long-term sustainability? At what point, as more and more microbial species are dropped from the inventory of interstellar passengers, does the remaining ecosystem become unstable and collapse? Which microbes are crucial and which would be irrelevant passengers, as far as humans (and their animal and plant food supply chain) are concerned?
This is a Noah’s Ark conundrum with a vengeance: which species get chosen to go? Not only have we no clue as to the answer, we have little idea of the solution to a much simpler problem: identifying the smallest self-sustaining purely microbial ecosystem.
Can we pull the web of life to bits, extract a tiny subset of it, and expect such mini-webs to function forever in isolation? Any plan to terraform a planet ahead of human colonisation cannot proceed without a far deeper understanding of microbial ecology.
Imagine making a list of the minimum number of plants and animals needed to accompany humans on a one-way mission, and leave aside the logistics of growing, feeding and breeding all these organisms in space, many of which may require environmental conditions (temperature, pH, oxygen levels, etc.) very different from those congenial to humans.
We might think of cows, pigs, sheep, chickens, some fish, a few vegetables – that would do for a start. But how many, and which, microbial species do these animals and plants depend on? How many, and which, other microbes do those animal-and plant- servicing microbes depend on? Which of these might be pathogenic to humans, yet vital for some other part of the ecosystem? Without a full understanding of the principles of the networks involved, how can we be sure that we have done our ecological accounting exercise correctly?
It would be no good getting half-way to Alpha Centauri only to find that a key bacterium was overlooked and left back on Earth.
An added complication is that the activities of microbes depend on which genes they express (i.e. switch on). My ASU colleague Cheryl Nickerson found that bacteria can change their gene expression in zero g, and is working with NASA to study changes in astronaut microbiomes when they go into orbit. Of concern is whether a relatively benign bacterium might turn into a toxin in space.
And it’s not just bacteria that change under space conditions. Michel Levin’s lab at Tufts University experimented with planaria worms that had flown on the space station. Planaria can regenerate both head and tail if chopped up. Levin found that a fraction of the space worms came back with two heads.
Some of these difficulties might be mitigated by biotechnology. The visionary physicist and futurist Freeman Dyson has articulated a hope that we may eventually map the genome of the entire biosphere, then use the colossal computing power envisaged to be available in future decades to make sense of it all. We might then be able to design an ecosystem customised to a target planet.
I am far less confident, however, that the behaviour of an ecosystem can be captured in this manner by mere number-crunching. Even if it was, the supercomputer may tell us that there is no solution at all that matches the physical environment of our intended destination. Or it may specify that tens of millions of species are required. And that is not all. Because biological evolution involves a large element of chance, a transplanted ecosystem will not endure indefinitely as originally designed, but may evolve in ways incompatible with human habitation, requiring complex “mid-course corrections” entailing planet-wide bioengineering.
In my view, the best hope lies not with assembling an inventory of genes, but with our discovering the underlying laws and principles that govern the flow and organisation of information in living systems – what we might call “the software of life”. I believe that there are universal informational patterns or motifs in biology, which would be hallmarks of life whatever its chemical basis. If we understand the properties of these patterns and how they change with time, or how they can become disrupted, we might be able to create a transplantable ecosystem small enough to be transported off Earth and robust enough to withstand space conditions.
Rather like software engineers can design a computer game without mapping a computer’s circuitry, biological software engineers might be able to reprogram the organisation and management of information in terrestrial ecosystems without unravelling all the genetic details, and produce a system suitable for “playing” on another world.
Suppose the solutions for a sustainable miniecosystem are indeed one day worked out, and a mighty one-way mission departs for the stars, destination: a planet many light years away, where the spacefarers or, more likely, their very distant descendants, will make a new home. Astronomers are now fairly certain that the Milky Way contains millions, possibly billions, of Earthlike planets (depending a bit on your definition of Earthlike), so there’s plenty of real estate to choose from.
In many science fiction stories, the heroic adventurers touch down and step out onto an equable and verdant planet, hosting a rich indigenous biosphere, though preferably not a hostile civilisation, and take up joyful residence. Unfortunately it’s not that simple. There is a vanishing chance that the neatly-excised and self-sufficient truncated terrestrial micro-ecology would peacefully co-exist alongside the (presumably more extensive) alien equivalent, and proceed to carry on business as usual.
But this problem highlights a much deeper and more substantive obstacle to human colonisation of other planets, which is the very existence or otherwise of indigenous life.
Many fictional scenarios envisage humans in search of a planet with abundant life to take care of the colonists’ needs thereafter. An ideal world for human colonisation is one with oxygen to breathe and edible indigenous plants and animals. But this vision flies in the face of basic biology.
Organic matter is edible only when its biochemistry closely matches that of the consumer. Even on Earth, the vast majority of organisms are not suitable for human consumption. There is no reason to suppose that terrestrial biochemistry, which is highly specific to both the conditions on our planet and the accidents of evolutionary history, is universal.
It is easy to imagine carbon-based life on other worlds using different amino acids, different informational molecules, different membrane molecules, and so forth. It is also easy to imagine a mirror world in which familiar organic molecules are replaced by their mirror images (e.g. righthanded amino acids instead of left-handed). It is highly likely that this alien foodstuff would be unpalatable and indigestible. (The same reasoning makes nonsense of the whimsical suggestions that aliens coming to Earth might choose to eat humans.) Worse still, the indigenous biota would serve as a barrier to the establishment of a secondary transplanted terrestrial ecosystem.
There is, however, a flip side to the biological incompatibility problem. An alien biochemistry that offers little scope for consumption also poses little threat for infection. Alien microbes and viruses (if they exist) would probably be unable to invade terrestrial organisms, or to make much progress if they did. And vice versa. Wells’ “happy ending” to the War of the Worlds, in which the Martians succumb to terrestrial germs, is simply not credible.
The foregoing issues would disappear if the host planet had no indigenous life; that is, if it was habitable but uninhabited – terra nullius on a planetary scale. Unfortunately, this scenario has its own difficulties, one of which is crucial to human survival: oxygen.
Oxygen is a very reactive element, and does not endure for long in planetary atmospheres unless it’s replenished. A planet with breathable atmospheric oxygen implies the presence of photosynthetic plants, or at least microbes. (An important project in astrobiology is the construction of a space-based optical system capable of detecting the spectral signature of oxygen in the atmospheres of extra-solar planets as a surrogate for detecting life.) If there is no life on the destination planet, then it very probably would not have breathable amounts of free oxygen in the atmosphere. Nevertheless, setting up home on a previously sterile planet, and breathing manufactured oxygen, would be far easier than coping with an indigenous biosphere.
Quite apart from the practicalities of colonising another planet, there are serious ethical issues at stake. If a planet already hosts some form of life, the question arises of whether human beings have the right to limit or threaten it by transplanting Earthlife in its midst. Attitudes to this issue will depend on how important human colonisation is deemed to be and how complex the alien life forms are.
One motivation for sending humans into space is as an insurance policy against a megacatastrophe on Earth. Often cited is the impact of a large comet or asteroid which might destroy our civilisation or even our entire species. More likely in my view is a sudden pandemic, either naturally occurring or through the accidental release of a virulent biowarfare pathogen.
In any case, over a period of millennia, there is no lack of potentially species-annihilating hazards. If all that stood in the way of human survival were some indigenous microbes on another world, few people would have scruples about ignoring their “rights”.
If a planet had complex plant and animal life, there should be strong ethical objections to contaminating it with terrestrial organisms. Even if the two forms of life were so biochemically different that direct infection was avoided, it may still be the case that the terrestrial invaders would plunder some vital resource and deplete the indigenous ecosystem. Earth organisms might spread like the rabbits in Australia, and elbow the indigenous life aside, driving it to extinction.
Earth organisms might spread like rabbits and elbow indigenous life aside.
That issue would be greatly sharpened if a target planet is found to host intelligent life. In the movie Avatar, resource-hungry humans muscle in on the planet Pandora to the extreme discomfort of its indigenous population, although in the interests of Hollywood-style justice, the pesky human invaders eventually receive their comeuppence. There is no guarantee that future generations of humans would exercise respect for the rights of alien beings, nor can we be sure that aliens would respect ours.
Even aliens far in advance of us in technology and social development may not share our ethical values. Because we cannot begin to guess the motives and attitudes of truly alien beings, when it comes to the prospect of humans encountering an extraterrestrial civilisation, all bets are off.
It seems to be generally accepted that interstellar travel should, and could, become part of our destiny. Why? A familiar answer is that humans have always had wanderlust, a sense of curiosity, a desire to explore the world about them and to push on to pastures new. That may be true, but people have always fought wars and oppressed minorities too; just because something is deeply ingrained in human nature does not make it a noble motivation.
Rather easier to justify is the argument that human society has produced much that is good, which it would therefore be good to preserve for posterity. Humans may choose to undertake interstellar colonisation to keep our species, and the flame of our culture, alive somewhere in the cosmos. By establishing a permanent settlement on another planet, human culture could continue even if disaster struck at home. It could be countered that this argument adopts an inflated view of human significance and human worth, and that it is life, as opposed to our specific species or culture, that should be perpetuated and perhaps disseminated around the cosmos.
We could already begin sending microbes in tiny capsules out of the Solar System if we were so minded, but it is hard to imagine much enthusiasm for the project. Seeding a barren galaxy with DNA may one day fire people’s imagination (assuming the galaxy is not already teeming with life), but today the appeal of interstellar travel is deeply rooted in ideals of human adventure and advancement.
When Neil Armstrong took that first small step on the Moon, it was widely hailed as the initial step on a stairway to the stars. Half a century on, with humans seemingly stuck in low-Earth orbit, the prospects for interplanetary, let alone interstellar, travel look bleak. These microbiology problems compound what is already a formidable challenge in spacecraft design, propulsion systems and medical technology. Yet if humans wish to secure a long-term future in an uncaring and occasionally dangerous cosmos, some form of cosmic diaspora needs to be part of our long-range plan.
This piece first appeared in Cosmos Magazine in January 2020.
Paul Davies is Regents' Professor and Director of the Beyond Centre for Fundamental Concepts in Science at Arizona State University. He is also a prolific author, and Cosmos columnist.
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