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.