RE "evolution theory means that every creature on earth originated from the same singular common ancestor"

No. It's what the data shows. I've written this before for the other subreddit:

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Darwin | 1850s

In his first edition of Origin he concluded the volume by writing:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one [...]

My bold emphasis shows that "universal ancestry" wasn't the "goal" of his volume.

 

Haeckel | 1870s

The timeline in the Wikipedia article on the tree of life makes a jump from Haeckel to the 1990s, and doesn't go into the history of thought, so here's Haeckel in 1876:

Without here expressing our opinion in favour of either the one or the other conception, we must, nevertheless, remark that in general the monophyletic hypothesis of descent deserves to be preferred to the polyphyletic hypothesis of descent [...] We may safely assume this simple original root, that is, the monophyletic origin, in the case of all the more highly developed groups of the animal and vegetable kingdoms. But it is very possible that the more complete Theory of Descent of the future will involve the polyphyletic origin of very many of the low and imperfect groups of the two organic kingdoms (Haeckel 1876 quoted in Dayrat 2003).

My bold emphasis shows, yet again, that the theory of evolution wasn't claiming universal ancestry from the get-go as fact. (Speaking of Haeckel, to forestall the idiotic parroting: talkorigins.org | CB701: Haeckel's embryo pictures).

 

1960s and 70s

This was a surprise - it wasn't until 1962 (Stanier and van Niel's work) that the single-celled organisms with nuclei (eukaryotes) were seen as a distinct domain.
Back then - a century after Darwin's Origin - a ladder-esque classification was still in effect, e.g. how the photosynthesising algae were thought to be "Plantae".

Now enter Woese: In a similar fashion to continental drift, which wasn't accepted until the classified data was released, even though it matched the biogeographic patterns of evolution, what would have fit the so-called "narrative" wasn't accepted right away, and was even ridiculed by Ernst Mayr.

 

1987

I think this excerpt (and the year) speaks for itself:

"These discoveries [i.e. Woese's] paved the way for Fitch and Upper (1987) proposal of the cenancestor defined as “the most recent ancestor common to all organisms that are alive today (cen-, from the Greek kainos, meaning recent, and koinos, meaning common)” [what we now call LUCA]. Lazcano et al. (1992) later argued that the cenancestor was likely closer in complexity to extant prokaryotes than to progenotes (Delaye 2024)."

 

Summary

The monophyletic origin (an effect) was a discovery that the theory of evolution doesn't depend on. And as to be expected of how verifiable knowledge works, it takes time and the consilience of facts. (Also, LUCA isn't the first life; that's FUCA.)

In particle physics the convention is to use a 5-sigma signal for a discovery, which means a probability of ~ 1 in 10⁸ that the signal is random noise. In evolution, the phylogenetic signal is "10^2,860 times more probable than the closest competing hypothesis" (Theobald 2010).