From everything I have read so far, it seems that this is not so much a case of life evolving separately, but rather a simple, albeit unique, case of natural selection.
If a bacteria could evolve to tolerate arsenic, it could then use arsenic as a substitute for phosphorus. That appears to be the case with the Mono Lake species.
Yes, it opens up the possible chemical signatures for life to be found elsewhere, but I would hardly call it "alien life".
Depends on how you define "alien", right? Alien as in different from human, then this qualifies. Alien as in extraterrestrial (ie. not originating on Earth), then this doesn't.
The bigger question, in my mind, is can you evolve a sentient species (in a few million years) from this microbe? And what would happen if we threw this microbe on Mars or Venus? Would natural selection allow it to thrive there?
If a bacteria could evolve to tolerate arsenic, it could then use arsenic as a substitute for phosphorus. That appears to be the case with the Mono Lake species.
Yes, it opens up the possible chemical signatures for life to be found elsewhere, but I would hardly call it "alien life".