I anxiously await the day that our planet is dominated by an ancient race of giant tardigrades. Sure, the ones we know about are tiny and peaceable, but also nearly indestructable bad-asses, but who's to say they don't have larger, raging kindred roaming the cosmos that just haven't found us yet?

They may present themselves in a form that we would not be able to comprehend.

Im not sure it matters what they look like, it is more their temperament and their intentions that are relevant to us.
If there is ever a first contact, they will have found a way to fold space or use wormholes, or some other method of traversing vast distances... This would infer intelligence and technical abilitability beyond what we can comprehend.
I hope they are not moody and suffer from jet-lag, it may not end well..???

They could be our cousins from an ancient civilization from 50,000 years ago—not likely, but possible.

I have no science whatsoever to support my comments, but.... It seems to me that we may be very low on the intelligent life specter. We cannot recognize life in any forms other than what we can now recognize. I believe alien life is all around us and we simply cannot recognize or appreciate it. We are of no interest to alien life.

Bacteria

If our primordial genes didn’t arrive from Mars.. thus possibly from a far more distant origin, I honestly can’t imagine… Though, unlike Star Trek, I’m sure they won’t be speaking California English

Crickets