The Klingon forehead explanation was rubbish.
But I wouldn't have minded a story about the genetically enhanced Humans. They were very well cast and interesting to watch, when they weren't harassing scientists by stuffing them in tubes filled with death-dealing biological weapons. Some of the choreography of the fighting sequences was particularly effective ...
I believe it was actually a great explanation for the absence of ridges to mask TOS budgetary restraints and a 1960s audience being unable to face late 1970s alien prosthetics.
But another explanation could have been in the scattering of species by "the old ones" the further away in any direction the greater the difference in species.
In TOS neither the Klingon home world or the Romulan home world was in range of Federation space. I got the impression at least that the three bordered one another at their extreme outer edges.


So the closer to human occupied space one was the more human like the species. The deeper into the Klingon space one would go the more forehead ridges and spiny backs etc.
The same with Romulans, the deeper into Romulan space the Romulans had the V shaped forehead ridge and less like the more human looking Vulcans.
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