It seems pretty unlikely, albeit not impossible. Life on Earth has evolved over an extremely long period, and advanced species have very robust immune systems against cellular life. When you consider that Earth already has viruses and bacteria that adapt very rapidly, that have spent eons competing with existing life — I think it's unlikely we'd find an bacteria or virus that could successfully infect Earth based organisms in a way that threatens us.

We're going through a pandemic right now because a virus has successfully found a way to evade our robust immune systems.

You can take a country as deadly as Australia and introduce something as innocuous as 24 rabbits, and it can have an absolutely devastating effect on the ecosystem.

Frankly it's beyond hubris to suggest that we'd be fine just because of the way we've evolved.

A virus that co-evolved along with us, specifically being able to infect complex cellular life in the way it has evolved on earth. Anything from another planet wouldn’t have developed in a way that lets it get past our immune system not because or complexity, but because of lacking the shared evolutionary history.

Something that represented a danger to us from another planet would have to be an issue in how simplistic it was...like mad cow disease style prions, or a parasite that happens to attack us in a novel way. Something virus like just wouldn’t be an issue.

>Anything from another planet wouldn’t have developed in a way that lets it get past our immune system not because or complexity, but because of lacking the shared evolutionary history.

Based on? Rabbits didn't co-evolve with australia's deadly fauna, and yet...

The rabbit example is a matter of ecology. A viral infection is a matter of immunology.

They are not analogous and depend on completely different factors.

If you have ever done any work or reading on immunology then you wouldn't think it to be far fetched to claim that immunology IS in fact ecology.

Indeed, microecology is a thing.

They did if you go back far enough.

We are talking compatibility on a protein level.

Well, this also works from the opposite perspective: we have evolved to combat earth virusses, but haven't evolved to combat anything from another planet. Lacking the shared evolutionary history may leave us more vulnerable.

An alien virus would have to be very lucky indeed to have the necessary machinery to dock to our cell's and inject itself into the cells replication mechanism. Viruses are often not even able to bridge the gap between relatively closely related species. A alien virus being able to infect Earth life seems vastly less likely than a virus that infects sea sponges being able to infect humans and nobody really worries about that.

It wouldn't have to be some form of virus to be deadly though, right? Something like prions or unlike anything we know could be equally (or more) deadly to us or even all life on earth. Our imagination is quite limited in this domain and well let's hope it stays this way..

(not that I find any such scenario to be even remotely likely)

Edit: well, that's basically what's being said in a parent comment

Yes, alien bacteria have a much better chance at surviving.

if an alien life-form is DNA based, maybe other similarities are shared with Earth based-lifeforms, even if it doesn't share an evolutionary past with it.

Right, I just saw this after replying to a different comment, but I think a COV-19 type scenario is probably the most likely of a bunch of really unlikely scenarios. Its unlikely an alien virus would be able to interface with a human virus, but if they managed to exchange genetic material, then it's possible the hybrid could infect a human. The same reason makes these types of pandemics from local viruses nearly inevitable. The proximity accelerates how viruses evolve and jump species. We obviously don't know enough about life to know if viruses are a universal phenomenon such that they could interface on first contact.

It's super unlikely that a virus that evolved outside of Earth would (on its own) have a mechanism to infect cells they hadn't encountered before in order to propagate. Again, nothing is impossible, but physics, chemistry and biology would all have to line up in a very specific way for that virus to infect a human cell.

If the standard expectation is to find DNA based life-forms, it could be fairly rational to suppose that alien life shares other evolution tricks with the Earth based life-forms.

So alien viruses could directly and initially interface with Earth based life-froms cells (humans,the biota inside the humans, etc.), with probably also, somewhat predictable results. Or at least novel coronavirus like outcomes.

> So alien viruses could directly and initially interface with Earth based life-froms cells (humans,the biota inside the humans, etc.), with probably also, somewhat predictable results. Or at least novel coronavirus like outcomes.

Entirely possible, but COV-19's novel spike protein was developed through brute-force trial and error. An alien virus would need to do so via happenstance.

> brute-force trial and error

That's just another nice way to describe evolution. It's just a storm of probabilistic biochemistry. I think if you read about how fast and crowded biochemical interactions in the cell are [1], you will quickly appreciate that's it's basically just quadrillions of hypersonic processes doing what you describe as "brute-force trial and error".

For example, that's how a good number of drugs work that attempt to bind to some protein, like Aspirin. They don't go and find it. The drug molecules enter the cell space and just bang around at incredible speeds until they finally just jam into the protein they need to. Brownian motion and similar effects and not some taxi service to the target protein. It's basically just probability which is where concentration comes from. Increasing the probably of the drug molecule hitting the target.

That's also why I really dislike the personification of things. SARS-CoV-2 didn't consciously do anything. I know most people know, but the language constantly insinuates just that.

[1] http://www.righto.com/2011/07/cells-are-very-fast-and-crowde...

There's a very very big difference between a virus who has evolved alongside other living beings on this planet for millions of years and finally made the jump from one specie, vs some form of life that has probably been separated from us for billions of years. The former has very specific mechanisms that has evolved to perfectly trick our defense system, not too much to kill us instantly and not too little to not spread. It's perfectly in the goldilock zone. The latter on the other hand has never come in contact with anything anywhere close to our life form.

It really depends on what ET life looks like. If it shares a common set of DNA for example then it would be much more likely that it could be dangerous. But if it’s not even Carbon based then it could be totally inert to us and vice versa. The biochemistry might literally prevent all interactions. We won’t know until we can examine it up close.

Australia is a small continent isolated from a big world. Australian animals did not have as much competition as rabbit had during his evolution. So rabbit from Eurasia in Australia is like Earth bacteria wreaking havoc in some small oasis of life at some distant planet. It works that way, but it won't work other way. It took hundreds of thousands of years of constant evolution for COVID virus to break through our immune systems. Martian bacteria won't have that time.

Now if some otherworldy life had billions of years of evolution similar to ours or exceeding it, well, it might present some threat.

I agree with you. In the vastness of time we've been here just a moment and we are precariously balanced on the top of a food chain that we take for granted.

I second this - think about threats from other animal kingdoms, say plants - they get infected by bacteria, viruses or parasites. Yet no-one has ever managed to infect an oak with flu, or to get yellow rust from wheat. Anything actually alien is going to be further removed from me and you, than we are from jellyfish. The bodies will be just too different.

Maybe some kind of invasive organism could exist that spreads like kudzu and plain outcompetes existing life -> however this sounds like something that could be controlled by brute force.

This is the same reason humans can eat most fish and nearly all plants raw. The evolutionary distance is so high that infection is basically impossible.

What could still be very dangerous, is otherworldly defence mechanisms like toxins. Plant and fish toxins are commonly deadly to all higher life forms

"... slain, after all man’s devices had failed, by the humblest things that God, in his wisdom, has put upon this earth." - I suspect this might be a more likely outcome (courtesy of Wells). A single strain or a novel virus is far more likely to have a predator on earth than be a predator to something we need to survive.

That said, it would be prudent to take anything that may be living to the ISS first.

The ISS is coming down, one way or another. Better going to the moon.

I'd like to say that I disagree here with your assertion that this may be an unlikely scenario. I think it's more of an unknown. We don't know what unique capabilities an independent virus or bacteria may have generated. If you look at human history, contacts between peoples with different immune system resiliencies proved quite deadly. It's possible that we encounter a novel virus or bacteria and it may be extremely lethal or damaging with no ability for our immune system to fight off the infection, or it may not have an ability to interact with our cells and thus pass through without harm. Or anything in between. This goes both ways.

The infections that happened throught the history of human colonization do not apply here. Those were just what microbes had evolved for, finding untrained or weakened immune systems to infect.

Martian life, if any, should simply be "trained" to pry on the particularities of its own environment, not ours. There's a extremely low chance that it could survive and thrive on our warm planet. Add to the fact that lots of precautions will be in place, Martian life exploration can be considered very safe for Earthlings.

However... since life evolves to be successful in (almost) any environment, Martian life could just as well be trained exactly for these rare moments: to be dormant then suddenly pry on unsuspecting alien visitors that come snooping around... But then that would be the argument of a successful Ridley Scott movie franchise.

If we assume that life readily emerges, that's evidence that the life that predominates on Earth is robust as presumably it has overcome many spontaneously emergent forms of alternative Earth life, and possibly even extraterrestrial life.

But if life doesn't readily emerge, then we don't have any evidence one way or another. However, the likelihood of encountering extraterrestrial life is also diminished.

I’m not sure that I understand your point here. Could you elaborate? I understand that life on Earth has gone through billions of years of intense competition for survival, but I’m not sure that we should assume any likelihood of how an interaction with an alien organism would go given we haven’t found life outside of our planet and we can’t be sure of the evolutionary path it may have taken.

If life is as common/likely as some believe (that is to say, "very likely" or even "inevitable if liquid water"), then it should be continually emerging on Earth in various forms. The persistence of life as we know it would therefore imply something about its robustness--its ability to outcompete those alternative forms. Such evidence certainly wouldn't be dispositive, but it would be evidence nonetheless, the strength of which would be a function of the likelihood of spontaneous life.

"But", someone replies, "I'm talking about life forced to evolve under extreme and alien circumstances." But Earth has plenty of extreme environments, both now and especially early on, and scientists optimistic about the likelihood of spontaneous life are constantly equivocating those environments to others in our Solar system and beyond. Plus, let's not forget that the modern, biologically created Earth atmosphere is something of an extreme environment of it own. So no matter how you spin it (I'm too lazy to put down several other scenarios I've had in mind), the more likely we are to encounter extraterrestrial life, the more robust Earth life likely is.

If random independent life emerges on Earth now it's not surprising old life that had billions of years to optimize for the environment and in general just outcompetes it immediately.

Expecting the opposite would be like betting on a random path in a graph with millions of edges to be faster than a path that was optimized by billions of computers for billions of years.

It's not that old life optimized for killing the new life (it probably didn't because it doesn't seem to be a common enough occurence). It's that the new life would be very likely very bad at everything.

Sorry, I should have been more clear in my post. I was responding to this comment specifically:

> I think it's unlikely we'd find an bacteria or virus that could successfully infect Earth based organisms in a way that threatens us.

Not my comment. :) My argument is more general and doesn't make any hard assumptions about similarities to bacteria, viruses, or even DNA-based life.

Yes but my post that you were responding to was a response to that comment specifically. I appreciate your comments here, I just want to make that clear.

> If you look at human history, contacts between peoples with different immune system resiliencies proved quite deadly.

Between human species on Earth. Humans in the Americas were not genetically distinct from their European cousins, thus a European virus could readily infect an American human and vice versa (Europeans were not wiped out because the diseases they acquired were not as deadly, such as syphilis).

Our immune systems are adaptive, but a bacteria/virus will be limited in it's ability to adapt. As such, it's pretty unlikely to be able to infect a complex, multicellular organism that it's never encountered before. The most likely vector for an attack is an alien bacteria that is able to rapidly transpose genetic code with earth-evolved bacteria and create a novel hybrid. Then you are dealing with a COV-19 type scenario.

Yes my point want that such a meeting would be similar, but that such a meeting between alien ecosystems could have a similar effect. To the larger point about the OP, my issue is the idea that any of this is unlikely, when there is a sufficient lack of information to base any sort of probabilistic assumption. They’re all simply incorrect because there is insufficient data. For all we know an alien organism may be specifically evolved to devour multicellular life, or pass into other organisms without detection. There is no reason to think the Ridley Scott scenario is any more or less likely than a harmless scenario. We just don’t know because our frame of reference is only life on Earth.

Sure, I get you. There are all kinds of possibilities in a vast universe. If, for the sake of discussion, we can assume that alien bacteria and viruses have roughly the same physical constraints — it's pretty unlikely you see that kind of adaptive behavior toward multicellular organisms. There is a physical limit on the size of bacterial genomes and viral RNA is a single set of instructions that get inserted into the host's own genome, so they literally can't be that complex. Every bacteria and virus on Earth targets a specific host, environment and/or life cycle - they have to specialize and adapt to survive.

That's not to say we couldn't inadvertently pick up a bioengineered complex organism from an advanced alien species - that would be a completely different story.

Frankly we won’t know until we find ET life. If we’re talking about something that is relatively similar to a Terran bacteria then perhaps it could interact with our cells.

Alternatively, it is probably far more likely that life from Earth would rapidly evolve to kill or replace any extant life on other worlds, especially microscopic life. We know that Terran life is tenacious. It readily thrives in hostile environments and has evolved under extreme adversarial pressures on Earth. It’s not a stretch to imagine that it could become the apex predator in an ecosystem devoid of such extreme evolutionary pressure.

Indeed, if alien species are so environmental constrained, the competition to thrive could be almost non-existant, and Terran life could have the winning hand in that case.

We should be careful about what we send to Mars. Something like candidas aureous could just wipe out the life on the planet.

This.

we know quite a lot about this, most life-forms on Earth almost continously try to kill humans or at least to mine its biological resources for its use.

I think the opposite is actually a much larger concern. It seems more likely that introduced Earth organisms would destroy native extraterrestrial ones. Or at the very least contaminate any samples.

Have you read Andromeda Strain? It's pretty terrific

Rocks have been exchanged between solar system bodies for millenia. We know that life can theoretically survive on these meteors. So it seems rather unlikely that a foreign life form that is not well adopted to life on Earth has a big chance of survival.

we could have had already some encounters, maybe some current life-forms in Earth were actually alien-forms initially (somehow octopuses comes to mind).

Some people on the net says that the current coronavirus situation could be an alien-species. I know it is FAR FAR from anything related to what we already know about the virus.

But it gives an idea about what could have looked an encounter with a new alien-patogen which just landed on Earth at some point at history and got to survive and thrive in the local ecosystem.

That's like asking if we could find a virus on some old mineframe that by accident works better than modern computer viruses on the newest version of linux :)

It's very very very unlikely it will even be able to run at all. It would use a different machine code, even if it was by crazy accident the same it would use some long forgotten binary format instead of elf and depend on different directory structure. It wouldn't be adapted to modern protection schemes like stack randomization.

I'd be very surprised if something from a completely separate evolutionary line was (1) interested and (2) capable of attacking Earth organisms.

I'd even be surprised, though less so, if they can even survive here.

Of course we have zero empirical observations of such life, so predictions are guesswork.

That said, if something managed to survive and multiply on Earth, I can imagine it could be a a huge nuisance without attacking our bodies directly.

What about something that eats proten and requires water. A fungus comes to mind. If you are talking Virus, it needs to be able to highjack teresterial DNA, a fungus or similar only needs to find us tasty.

Yeah, viruses need to coevolve with their prey. Which is why the computer virus in * Independence Day* was so silly.

And I think terrestrial immune systems can handle microorganisms, whether they're DNA based or not. It hunts based on shape, as I understand it.

I was thinking microorganisms, since anything big we can just kill on sight. But when you mention it, something insect size could be very hard to fight.

Our life has been here 4 billion years. What if it meets with life that has existed for similar timescales, or a billion times that?

What conditions on other planets would evolve human pathogens more effectively than the conditions very similar to those that humans live in?

No one knows.

One school of thought is that extraterrestrial pathogens could pose limited risk. Earthbound pathogens have co-evolved with humanity in order to harm us better, but xenobiological pathogens would not have this advantage and might not pose much of a threat.

On the other hand, it's entirely possible that the random chances of evolution could create a xenovirus that's airborne in humans and bootstraps a fatal prion disease in everyone it infects.

Something like a bacterial infection might pose a problem as it is just eating simple sugars and multiplying, but is completely novel to our immune system. A prion seems unlikely.

A virus wound be absolutely shocking.

> A virus wound be absolutely shocking.

Indeed, it would imply coming from an origin with reproducing DNA.