So however life on Earth originated, whether by panspermia or not, however many times, one ancestor won out over all the others and evolved DNA and life as we know it?

Presumably once life has got started (for example on Earth) then nobody else gets a look in? I mean we don't see any 'new' life spontaneously originating on Earth today (only evolution). Do we know the mechanism behind this? Is it just that new life would not be able to compete; or have I got it wrong and it could still happen in some quiet corner of the planet?