We had a post here on 2022-03-25, “Is There a Shadow Biosphere on Earth?” about the possibility that Earth may be home to one or more forms of life (shadow biosphere) which are sufficiently different in their metabolism and genetic information transfer that they escape detection by the tools we use to survey for new species of microbes. This might especially be the case if organisms from the shadow biosphere primarily inhabit environments thought inhospitable to known life such as deep underground, mid-ocean vents, or in Antarctic ice.
We did not discover extremophile bacteria and archaea until the 1960s and 1970s, and they were, by comparison, abundant and right under our noses, using conventional metabolic pathways and genetics.
For years, it has been suggested that desert varnish might be an example of a shadow biosphere, but it is now believed to be formed chemically and not a living organism.