I think the point of that episode was that the majority of humanoid species in the galaxy were related to the hologram lady’s species.
More like they are the artificial product of the engineering efforts of the hologram's species, not necessarily directly biologically related.
And it may well be that "natural evolution" exists in the Trek universe just as apparently in ours - it's just that this megaengineering project by these Ancients rides along that evolution, and then at the right moment grabs and perverts a suitable species into a sapient biped, the type favored by the Ancients. On Earth, it grabbed some dinosaurs and then, after they left, some apes. On other planets, it did its thing on different species, ranging from lizard-lookalikes to fish-lookalikes and no doubt beyond.
When the ancient alien lady hologram revealed the information about different species being originated from the same source, there should've been more species around to listen, now there were only few.
Now that would have ruined this religious-level revelation: you always have to choose whether to trust the word of a select few prophets or witnesses.
Can anyone recall when this issue came up in TOS? Kirk and Spock were discussing why so many species throughout the galaxy share a similar experience. Something in a TOS episode explained it. Some earlier species, maybe the one The Chase, seeded the galaxy with life that would develop similarly.
These early heroes had all sorts of theories. In "Bread and Circuses", Spock quoted a "Hodgin's Law" on parallel development, but that appeared to refer to cultures rather than biology. In "Return to Tomorrow", Spock said that Sargon's tall tales about their culture having seeded life 600,000 years ago might explain certain elements of Vulcan prehistory, while Dr. Mulhall hotly insisted humans on Earth evolved independently.
The heroes probably were wrong on all their assumptions, and the claims from "The Chase" take precedence in modern thinking. Although the holographic Ancient is as dubious a source as any, at least her story about extensive genetic engineering across the galaxy must be true, as it is through this engineering that the hologram was made visible in the first place.
Maybe what you have in mind are the Preservers. The Preservers were mentioned in "The Paradise Syndrome" (TOS). They were supposedly a species that rescued primitive peoples that were in danger of extinction and then spread those peoples around the galaxy. Presumably they were spreading humanoid type species - you know the type, a head, a body, two arms and two legs.
Their only claim to fame is the transplanting of some North Americans around the 1700s. Nothing suggests they would have done much to affect the spreading of sapient humanoids to the galaxy millions or billions of years ago.
However I don't think the Preservers are the same species as the changeling lady in "The Chase".
The hologram explained that their genetic engineering megaproject came to be because the culture knew that one day they would be gone. No doubt they are - the project dates back four
billion years! The Preservers are contemporaries to our heroes.
Timo Saloniemi