Btw, what I mean by "evolutionary theory" is what I consider macroevolution* or the theory of evolution - ie, that all life on earth evolved from LUCA via a process of natural selection.
The theory of evolution says nothing at all about weather all life evolved from a single thread of descent.
The theory is identical if there is just one line of descent or a dozen.
The only reason that the existence of a LUCA is accepted is that we haven't found any reason to think there is another line of descent evolving in parallel with the one we see. That is, there is ample evidence that all know living organisms are related very deeply in time.
If under some rock, somewhere an organism was found that was clearly not related to all the rest of us then that interesting new fact would have an impact on our understanding of the history of the evolution of life unfolding on earth but have absolutely no impact on the theory of evolution.
If you have trouble understanding that last paragraph then you need to back up a loooong way and learn a lot more.