This question assumes that the LUCA hypothesis is correct: that all organisms now living on earth are descended from a single organism (the precise age of which being debatable and not germane to this question, but we can assume for the sake of argument that it was prior to 3 billion years ago). Carl Woese's theory that LUCA was "a communal, a loosely knit, diverse conglomeration of primitive cells that evolved as a unit" also does not change the nature of the question.
What has prevented an individual, single celled, DNA-based organism, essentially identical to the LUCA, living say 500 million years ago, from producing offspring that successfully survived to the present day, in perhaps an example of concurrent evolution? What is preventing the same thing from happening today?
