Is There a Shadow Biosphere on Earth?

Is the Earth home to a shadow biosphere? If life originated on Earth (as opposed to having been delivered from other sources in the cosmos), was there a single origin-of-life (abiogenesis) event, or more than one? Singular events are rare in nature, and if conditions on the early Earth permitted the formation of life, there’s no reason to believe it didn’t happen multiple, perhaps innumerable, times. If so, then why does all present-day life appear to be descended from a last universal common ancestor (LUCA), and not show evidence of multiple trees of descent? A common argument is that present-day life out-competed all competitors which eventually went extinct, leaving only it. But, just as life has repeatedly been found in environments where it was previously believed impossible such as deep underground, in Antarctic ice, and at vents in the deep ocean, perhaps descendants of other origins of terrestrial life have survived in extreme environments where we haven’t yet looked or, they might be all around us and overlooked because their chemistry is sufficiently different our tests for biological activity don’t detect them.