The question originated with Cornell astronomer Thomas Gold in a 1992 paper and was developed in a book, Underground (2019) by Will Hunt.
As science writer Ross Pomeroy explains at Real Clear Science, much of the controversy revolved around claims made by Hunt that oil and gas dated to Earth’s formation…
The main effect was to encourage looking deep in Earth for life forms. If you don’t mind that they are all microbes, it has been a rich haul:
Microbes have been found as far as 2.8 kilometers underground, munching on a range of minerals for “food” and generally subsisting at a much slower pace compared to life up above. Some are so different from surface microbes that they’ve garnered a catchy nickname: “intraterrestrials”. Crucially, in regards’ to Gold’s bold hypotheses that deep microbes could have started life and may exist in the interiors of far-flung planets, scientists have found a bacterium that seems to exist totally independent from Earth’s surface. Candidatus Desulforudis audaxviator was found under a deep gold mine in South Africa and exists at temperatures as high as 140 degrees Fahrenheit in the absence of organic compounds, light, and oxygen.
Ross Pomeroy, “Did Life Start Deep Below Earth’s Surface?” at RealClearScience (April 12, 2022)
Talk about an extremophile deep in the Earth! Trouble is we don’t know that life started out like that. It could just as easily be that one late-arriving microbe could inhabit that territory but nothing else could.
Some hope to find evidence of life forms like audaxviator on exoplanets.