Two papers report evidence in ancient rocks for microfossils dating from around 3.4 billion years ago, almost as soon as the Earth solidified and cooled sufficiently to allow the presence of liquid water.
The second paper finds evidence in rocks from Western Australia for the presence of an ecosystem including primitive photosynthesisers, methane producers, and methane consumers, implying life had already had the time to diversify from the last universal common ancestor into species with different metabolic niches.
The closer the evidence for life on Earth gets pushed back to the first possible moment it could have survived, the better panspermia looks to me as a theory of its origin.
Prof. Gold’s hypothesis (The Deep Hot Biosphere, ISBN 978-0387985466) was that life started deep underground in porous rock, where there was (a) a clay-type substrate on which organic molecules could assemble, (b) steady warm temperatures to encourage chemical reactions between said organic molecules, (c) protection from destructive solar ultraviolet radiation. Eventually, these life forms migrated to the surface, where life was much more perilous but the resources were much larger.
Under that hypothesis, a seafloor hydrothermal vent might be a good place for life to emerge from the subsurface.
The problem with all these theories of the spontaneous generation of life is that, if it could happen once, it should have happened many times; we should be able to see competing forms of life derived from entirely different parent lines.
Well, not necessarily. Life may have originated many times and, indeed, have still be originating many times today but eventually, in the beginning, one form out-competed the others and drove the others to extinction, just as Google Chrome did to Internet Explorer. Today. a newly-born life form may divide one or two times before chomp, it gets eaten by an incumbent organism.
There is the possibility that Earth is home to one or more “shadow biospheres” which are sufficiently different from our own that we do not recognise them as living and our DNA-based surveys do not detect. These may have co-existed since the early Earth, especially in environments like the deep, hot biosphere or hydrothermal vents.
