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.