The most widespread fanon (fan canon) rationalization is that smoothies were genetically engineered Klingons to make subterfuge easier and that the turtleheads were always the natural state of their species which synchs up with what you read.
It’s fanwankery though, and not the “real” explanation.
Oh no…
(WAG)It’s going to be something about The Founders and ‘the next stage in Klingon evolution’. I bet it is.
The Founders? Changelings? How could they have anything to do with this when the wormhole won’t even open for another two hundred years?
Close. TV cameras in the 60s were not technically advanced enough to show the bumps. It’s believed to be a resolution issue.
I think it’s all tied in with Dr Watson’s war wound.
They exist outside of time, or some bullshit technobabble 
They had some contact before the official opening, didn’t they?
Don’t be silly! There’s no one on Earth more tanned or better-rested than George Hamilton.
Perhaps you are confusing them with ENT’s Sphere Builders? They looked very similar.
Real spoilers!
Similar but different. The Sphere Builders are more reptilian looking.
It’s a plastic surgery fad. Without anesthetic.
There is no honor in deadening one’s nerves.
My IMHO is that during Kirks time there was significant Klingon Romulan interbreeding especially in the highest Klingon families. Leading to the extreme similarity in the two races during Kirks time.
Will you people forget this horse puckey and worry about something important, like what happened to Kirk’s hair?
Er, what?
(Female) Romulan Commander and (Male) Romulan Commander vs. Kang, Kor, and Koloth.
OK not quite as similar as I remembered.
Actually (some trekkie I turned out to be), when I mentioned the founders above, I was thinking of that long-dead race that was cited as the reason why so many species are humanoid; they were humanoid and foresaw their own extinction, so they coded their DNA into a single-celled lifeform, then spread it all over the galaxy, (implausibly)giving rise to humanoids all over the shop.
That would be the Progenitors.
Trivia: Salome Jens, the woman in that picture, also played the Female Changeling in DS9.
I am intrigued by your ideas and wish to subscribe to your newsletter.
Of course, if we get rid of the tribbles, then we’ll have to Lucasfy the films and remove the one from Kirk’s head. 
Linky-Link no work for Baby Bosda.
Goo.
Sorry I’m late. I couldn’t find a place to park.
After TMP, I always used a slight mental cheat and thought that the Klingons interbred with conquered species. Then for the out-of-the-way or sesspool worlds they just let the best of the conquered species pretend to be Klingon and practice conquering on their own.
Now this is fine for a moments diversion - a thought to fill in the gaps - like a Kit-Kat Bar in the afternoon - but there are problems if you try to make it true.
What I hope they don’t do is use Phlox’s medical genius to develop some Human-Klingon DNA melding technique and actually explain it away that way.
