Hi,
The one bit I balk at is calling it something as complex as a microbe,....
If we relax the demand for proper and completely viable lifeforms having come from somewhere else and instead just suggested that some biologically relevant molecules fell on planet earth from somewhere else, then we're really in with the most current mainstream of thinking.
....Analyses of three meteorites suggest that nucleobases, the crucial components of DNA, could have formed in space and then fallen to Earth to provide the raw material for the origin of life itself.....
[Taken from "Meteorites could have brought DNA precursors to Earth",
a Natural History Museum article, available at this URL:
https://www.nhm.ac.uk/discover/news/2022/april/meteorites-could-have-brought-dna-precursors-earth.html ]
Let's make it clear what is being said here: It's not that whole, intact and viable lifeforms fell to earth, instead it's the much weaker assumption that some very important organic compounds developed elsewhere and fell to earth. We are not suggesting that any of these nucleobases had to be strung together in the same way as modern DNA or that any of the genetic code that may have been there if they did join together (which they would naturally tend to do) would have survived and will be present in modern day life. We only have to go so far as to suggest that some raw materials, some nucleobases did develop elsewhere and fell to earth.
In effect this would have given evolution a significant headstart or boost. You should note that these organic compounds could have developed on earth and there are some sensible theories for how that may happen (or has happened)....
The primordial soup hypothesis, suggests that intense ultraviolet radiation and lightning on the early Earth provided the energy for chemical reactions between compounds such as water, ammonia and methane to begin.
The theory suggests these reactions then led to the creation of molecules such as DNA and RNA, which subsequently became part of a self-replicating system which would ultimately become life....
However, there are some gaps between the conditions simulated in these experiments and what we now understand about the early evolution of Earth's atmosphere.
[Extract from the same Natural History Museum article previously cited]
The idea that some biologically relevant molecules, such as Amino acids and DNA-like material, fell to earth in meteorites is one of the most likely and reasonable ways to "shore up" the theory for the evolution of life on earth. It is this view which now seems to be favoured over the notion of an entirely terrestrial evolution.
It's no longer a huge leap to assume a bit more, maybe there were/are some forms of life as suggested by @neilep that fell on planet earth and remained viable. However, this is one of the main sections of the forum, the mainstream scientific views must take precedence and we just don't have the evidence to support this much (yet).
Best Wishes.