I've been laboring under the impression that the most likely source of life on this planet is seeding. Not from travelers, but random pieces of rock or ice that just happened to land here. That said, I've heard discussion about the possibility that life began "spontaneously" near under-sea thermal vents. I gather this is abiogenesis?
I'm open to thoughts.
Life arising from an alien source (off-earth) only puts the origin of life somewhere else. It has to start in some way, either by natural processes (i.e. "abiogenesis" - from lifeless chemicals), or by supernatural means (i.e. "the magic wand" - choose anything you like - no proofs required!).
For the term "spontaneous", one definition is of a rapid event, occurring quickly - almost immediately some might say. This would be highly unlikely for the process of abiogenesis. It is simply not a rational concept based on life's extreme complexity. Such spontaneity falls under "supernatural means".
But "spontaneous" has other meanings, and one of these is most appropriate for abiogenesis: "occurring without apparent external cause" - that is, a natural event occurring on-earth (a magic wand need not apply).
For those of us who are fans of real science and what it all means, abiogenesis* is the most logical and rational approach because it relies on well defined chemical interactions, many found in standard chemistry books, particularly organic chemistry. Many of the reactions and interactions that occur in life are already known to occur in the absence of life. This clearly provides a solid, logical foundation for abiogenesis.
To be sure, laboratory experiments demonstrate that some of the fundamental building blocks of life, such as amino acids, can be produced by a simple mixture of chemicals subjected to energetic sources (lightning, radiation, etc.). Such experiments provide the best approach to establishing an origin of life, assuming logic is the primary driver of rationale and reasoning on this subject.
The devil, as it were, is in the details of how all this complexity arose. But that it arose without outside interference is accepted by almost everyone who works in the sciences - for obvious reasons.
An old, alternate concept of how life arose is known as "Spontaneous Generation". This notion suggested that all life simply popped up everywhere, and anywhere. Like magic! You just had to "catch it in the act" to prove this notion was correct. Such origins of life were largely disproven by Pasteur and Tyndall in the 1800s. Even a below-average grasp of logic dictates that Spontaneous Generation as the origin of life is simply not possible.
To be sure, the old and dead concept of Spontaneous Generation is in NO WAY associated with abiogenesis, and anyone suggesting otherwise is sadly mistaken.
*
https://en.wikipedia.org/wiki/Abiogenesis