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.