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?

I might be wrong, but there probably isn't the ecological niches available now on Earth to develop without competition from the existing organisms, or to avoid interacting with them in a  parallel evolutionary path.

Edited February 11, 20223 yr by StringJunky

1 hour ago, charlie_sar said:

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?

Discovery of such organism would simply move LUCA back in time, to a common ancestor of the 'current LUCA' and this organism.

In terms of modern biology such an organism would be defenseless and have no advantages over competing organisms; most likely it would be an easy meal for the abundant array of highly evolved predatory organisms we have now. The (unknown) conditions for such an organism to evolve and survive would have been very different to most of the current biosphere - even without competition and predators they may be unable to survive the chemistry of present day Earth.

There is some evidence LUCA would have been an extremophile, found around hydrothermal vents.  

https://astrobiology.nasa.gov/news/looking-for-luca-the-last-universal-common-ancestor/