Answer: Klingon Cranial Ridges

In the original Star Trek television series, Klingons had smooth foreheads like humans—not a distinct ridge in sight. That changed with the release of Star Trek: The Motion Picture in 1979, where Klingons were depicted with their signature cranial ridges.

The only problem was that not a single word was spoken to explain why an entire alien species had gained a very distinct facial feature. The practical reason was that the motion picture had a much bigger budget than the television series, and with a radically expanded special effects budget, the makeup artists and designers ran wild. But an in-universe explanation for the change was absent, however. So glaringly absent, in fact, that the difference between the ridged and non-ridged Klingons became a running debate and source of rampant fan theories over the next decades.

It wasn’t until a pair of episodes from the Star Trek: Enterprise series aired in 2005, “Affliction” and “Divergence”, that the difference was fully explained. In the 22nd century, Klingons experimented with genetically engineering their own kind using DNA from genetically augmented humans. The experiments went wrong and a virus strain caused a series of mutations including smooth foreheads. Outside of the mutations caused by the experiments, all Klingons should have cranial ridges.