Nobody knows the answer to this. It's surely possible, but whether it is as you say almost certain, or very unlikely, nobody knows. There have been several reviews of this in the context of a Mars sample return, and each new review seems to bring up new possibilities that need to be looked at, for ways that extra terrestrial life could be harmful to Earth life.
So far the general opinion in the reviews so far seems to be that this is possible, though low probability, nevertheless probability high enough so that we need to take precautions to make sure it doesn't happen. But it is important to realise, this assessment is based on no experience at all of any extra terrestrial life form other than Earth life, and so far, I don't think anyone is prepared to give a definite probability such as say 1 in a million or one in a thousand or whatever.
So far most of the discussion has focused on harm to human hosts by ET parasites, though that is just one of many things that could go wrong. We could be harmed also if our ecosystem is damaged, or if the life produces chemicals that are harmful to humans. But let's look at the possibility of direct harm first.
Most science fiction scenarios are implausible, hard to see them work. E.g. a virus from space able to infect humans. So it can sometimes be hard to take this seriously.
But there are things we should be concerned about, which for some reason are rarely looked at in science fiction movies or stories. And it might seem at first improbable that an ET microbe would harm humans. But when you look at it more closely, it's not as clear cut as it would seem at first.
Joshua Lederberg took a special interest in this
In "Parasites face a perpetual dilemma", he said about Mars life, which might not produce the same peptides and carbohydrates as Earth life:
So, you could have life that lives on our skin, in our sinuses, in our stomachs, our eyes, lining of our lungs, even in our blood or internal organisms, even starts eating our cells - and our body might not recognize it as life at all if it doesn't produce any of the chemicals characteristic of Earth life. Because our protections against diseases and other life forms have only needed to evolve to be able to recognize Earth type life. Even if benign, it might produce chemicals that are harmful for human based life.
As Carl Sagan (who was himself an astrobiologist, one of the first) wrote in his book Cosmic Connection:
Carl Sagan said once for Mars, that it could be that you can ingest kilograms of extraterrestrial life without any ill effects. But if you don't know what is in it, you have to be prepared for anything.
It's also worth noting that parasites when they first infect a new host tend to be at their most deadly. Because it's not in the interest of a parasite to kill its host, and over time they adapt and may eventually even become symbiotes that are non harmful or beneficial to their host. So in the same way, if extra terrestrial life was able to live in and on our bodies, by whatever method - even if our immune system ignored it, it is nearly always going to be to its advantage if it can keep humans alive, to provide this habitat for it to live in. But to start with it wouldn't be adapted to do this.
Then there's another possibility, that the extraterrestrial ecosystem is closely related to Earth life. If we have a common ancestor, even several billion years ago, then it may be able to exchange capabilities with Earth life using GTAs,
A Gene Transfer Agent (GTA) is a DNA fragment combined with a protein that is able to transfer capabilities between totally unrelated species of microbe. It could introduce extraterrestrial capabilities to Earth life, or Earth capabilities to introduced extraterrestrial life.
And of course the problem is not just infections of humans, but of animal and plant hosts generally since we depend on them for food, apart from anything else.
Also you can't say that, for instance because a species is ideally evolved to live say on the cold and dry Mars surface - or adapted to live in the Enceladus or Europa oceans that it can only cause problems for hydrothermal vent communities and cold areas of the Earth.
Life might find it easier to live on Earth than in their native habitats. Some extremophiles on Earth can live anywhere. As an example two species of microbes typically found in hydrothermal vents and ice caps, also found living in someone's belly button
This is something that hasn't been looked at in the sample return reviews so far, but I think may be a priority in future reviews. It's been a concern for XNA synthetic life in a laboratory, that escaped synthetic life might set up a self sustaining ecology "in the wild" that could take over from Earth life because it is better than it. In that case, you can do something about it - design the XNA based life so that it can't reproduce in the wild. But life from Mars or Europa say is of course not designed by synthetic life researchers so there is nothing to stop it from reproducing in the wild on Earth. I think future sample return hazard reviews might need to look at this possibility.
Anyway the discussions I've seen for XNA life don't go into specifics about how the XNA life might have advantages over Earth life, so this is my own example to make the discussion more concrete.
We have three main forms of photosynthesis on Earth – the type used by plants, with variations, the type shown here which doesn’t produce oxygen, and uses hydrogen sulphide in place of water,
and the method used by the Haloarchaea, which turn the red sea red. This last one converts light directly to electric potential by a similar process to the way that we see light.
Or more generally, ET based life might be all round better than Earth life. What if it has cells that are smaller and more efficient, have a better metabolism than Earth life.
Whatever the advantage, it might not manifest right away. Perhaps for decades whatever we return is a rare, inoffensive microbe that just manages to get by, in the soil, maybe a bit like radiodurans. It's not adapted to Earth and so to start with it finds conditions here too warm, or it can't face the competition - all the reasons people suggest that extraterrestrial life is not likely to be a problem.
But then - something flips. Some gene changes expression. Or it exchanges a GTA fragment with Earth life. Or it just evolves. Or even, that the life doesn't change at all, it just "flips" into a new state due to external conditions.
On Earth, harmless seeming short horned grasshoppers in the Acrididae family, for many years, causes no problems to anyone:
But then for reasons little understood, with changing conditions, they turn into this
Or maybe nothing flips. Maybe it is just a very slow exponential growth. Like the green curve in this diagram.
Suppose those figures in the x axis are decades, the y axis shows population, and this is the exponential growth curve for a photsynthetic XNA based lifeform in our oceans. You may notice nothing for three or four decades, it's an insignificant microbe in low populations. But, even if it is just a fraction of a percent better than Earth life, then eventually, the bulk of the photosynthetic life in the ocean may be XNA based. And, since it's not DNA based life, it could be inedible to Earth based life, or indeed produce chemicals that are toxic to Earth life.
In all this - not at all saying that any of this is likely. Just it's a possibility and we have to look at all of these whenever human do something unprecedented such as potentially return life to Earth from another planet.
See also: "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus
Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
I've created this answer mainly by copy / paste of sections from those articles. As I'm the author of those articles - that's a legitimate thing to do :).
So far the general opinion in the reviews so far seems to be that this is possible, though low probability, nevertheless probability high enough so that we need to take precautions to make sure it doesn't happen. But it is important to realise, this assessment is based on no experience at all of any extra terrestrial life form other than Earth life, and so far, I don't think anyone is prepared to give a definite probability such as say 1 in a million or one in a thousand or whatever.
So far most of the discussion has focused on harm to human hosts by ET parasites, though that is just one of many things that could go wrong. We could be harmed also if our ecosystem is damaged, or if the life produces chemicals that are harmful to humans. But let's look at the possibility of direct harm first.
Most science fiction scenarios are implausible, hard to see them work. E.g. a virus from space able to infect humans. So it can sometimes be hard to take this seriously.
But there are things we should be concerned about, which for some reason are rarely looked at in science fiction movies or stories. And it might seem at first improbable that an ET microbe would harm humans. But when you look at it more closely, it's not as clear cut as it would seem at first.
Joshua Lederberg took a special interest in this
Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others...
On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse
So, you could have life that lives on our skin, in our sinuses, in our stomachs, our eyes, lining of our lungs, even in our blood or internal organisms, even starts eating our cells - and our body might not recognize it as life at all if it doesn't produce any of the chemicals characteristic of Earth life. Because our protections against diseases and other life forms have only needed to evolve to be able to recognize Earth type life. Even if benign, it might produce chemicals that are harmful for human based life.
As Carl Sagan (who was himself an astrobiologist, one of the first) wrote in his book Cosmic Connection:
…Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high.
…The likelihood that such pathogens exist is probably small, but we cannot take even a small risk with a billion lives.
Carl Sagan
Carl Sagan said once for Mars, that it could be that you can ingest kilograms of extraterrestrial life without any ill effects. But if you don't know what is in it, you have to be prepared for anything.
It's also worth noting that parasites when they first infect a new host tend to be at their most deadly. Because it's not in the interest of a parasite to kill its host, and over time they adapt and may eventually even become symbiotes that are non harmful or beneficial to their host. So in the same way, if extra terrestrial life was able to live in and on our bodies, by whatever method - even if our immune system ignored it, it is nearly always going to be to its advantage if it can keep humans alive, to provide this habitat for it to live in. But to start with it wouldn't be adapted to do this.
Then there's another possibility, that the extraterrestrial ecosystem is closely related to Earth life. If we have a common ancestor, even several billion years ago, then it may be able to exchange capabilities with Earth life using GTAs,
A Gene Transfer Agent (GTA) is a DNA fragment combined with a protein that is able to transfer capabilities between totally unrelated species of microbe. It could introduce extraterrestrial capabilities to Earth life, or Earth capabilities to introduced extraterrestrial life.
Credit: Zina Deretsky, National Science Foundation
The red colouration of this pea aphid comes from a unique ability to generate carotenoids itself. It got this ability through horizontal gene transfer from a fungi.
Archaea can also transfer genes between phyla that are as different from each other as fungi are different from aphids. It is an ancient mechanism and so may also be able to transfer genes from life that had last common ancestor with us in the early solar system.
In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day
And of course the problem is not just infections of humans, but of animal and plant hosts generally since we depend on them for food, apart from anything else.
Also you can't say that, for instance because a species is ideally evolved to live say on the cold and dry Mars surface - or adapted to live in the Enceladus or Europa oceans that it can only cause problems for hydrothermal vent communities and cold areas of the Earth.
Life might find it easier to live on Earth than in their native habitats. Some extremophiles on Earth can live anywhere. As an example two species of microbes typically found in hydrothermal vents and ice caps, also found living in someone's belly button
EXAMPLE - WHAT IF IT IS BETTER AT PHOTOSYNTHESIS THAN EARTH LIFE?
This is something that hasn't been looked at in the sample return reviews so far, but I think may be a priority in future reviews. It's been a concern for XNA synthetic life in a laboratory, that escaped synthetic life might set up a self sustaining ecology "in the wild" that could take over from Earth life because it is better than it. In that case, you can do something about it - design the XNA based life so that it can't reproduce in the wild. But life from Mars or Europa say is of course not designed by synthetic life researchers so there is nothing to stop it from reproducing in the wild on Earth. I think future sample return hazard reviews might need to look at this possibility.
Anyway the discussions I've seen for XNA life don't go into specifics about how the XNA life might have advantages over Earth life, so this is my own example to make the discussion more concrete.
We have three main forms of photosynthesis on Earth – the type used by plants, with variations, the type shown here which doesn’t produce oxygen, and uses hydrogen sulphide in place of water,
Green sulfur bacteria in a Winogradsky column. One species of this type of bacteria is able to use the heat radiation of hydrothermal vents to photosynthesize. So photosynthetic life in the Europa and Enceladus oceans is possible as well as on Mars. See Infrared photosynthesis: a potential power source for alien life in sunless places
and the method used by the Haloarchaea, which turn the red sea red. This last one converts light directly to electric potential by a similar process to the way that we see light.
Salt ponds in San Francisco bay, pink and red with Haloarchaea (salt loving bacteria, the same ones that turn the Red Sea red). These photosynthesize using bacteriarhodopsin, which is what gives them their pink coloration. The light sensitive cells in our eyes use rhodopsin in a similar process. This method of photosynthesis doesn’t generate oxygen or fixate carbon but converts light directly into an electrical potential which the lifeform uses for energy.
Suppose for instance the life uses photosynthesis, but has a third method not yet explored by Earth life?
Or more generally, ET based life might be all round better than Earth life. What if it has cells that are smaller and more efficient, have a better metabolism than Earth life.
Whatever the advantage, it might not manifest right away. Perhaps for decades whatever we return is a rare, inoffensive microbe that just manages to get by, in the soil, maybe a bit like radiodurans. It's not adapted to Earth and so to start with it finds conditions here too warm, or it can't face the competition - all the reasons people suggest that extraterrestrial life is not likely to be a problem.
But then - something flips. Some gene changes expression. Or it exchanges a GTA fragment with Earth life. Or it just evolves. Or even, that the life doesn't change at all, it just "flips" into a new state due to external conditions.
On Earth, harmless seeming short horned grasshoppers in the Acrididae family, for many years, causes no problems to anyone:
Or maybe nothing flips. Maybe it is just a very slow exponential growth. Like the green curve in this diagram.
- XNA life could create molecules that closely resemble life molecules but are not identical and taken by mistake. Like BMAA.
L-serine, resembles
BMAA which is created by green algae. It's been suggested that BMAA can be misincorporated to cause tangle diseases like Alzheimers.Perhaps in a similar way (my own suggestion here) an XNA based lifeform could generate organic molecules that resemble amino acids used by Earth life and be misincorporated to cause protein misfolding and tangle diseases. It doesn't need to be adapted to Earth life to do this. - Or it could create chemicals that are allergens for Earth life - organisms can die from allergic reactions to unfamiliar chemicals.
- Or - it just produces chemicals in bulk that are useless to Earth life. E.g. a photosynthetic lifeform that takes over from the algae in our oceans, and is inedible by Earth life, because it uses mirror image DNA or XNA or has some other form that makes it inedible or poisonous to Earth life. See What Food Can You Safely Share With An ET?
In all this - not at all saying that any of this is likely. Just it's a possibility and we have to look at all of these whenever human do something unprecedented such as potentially return life to Earth from another planet.
See also: "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus
Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
I've created this answer mainly by copy / paste of sections from those articles. As I'm the author of those articles - that's a legitimate thing to do :).
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
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