I've been wondering this for quite some time, in the orignal series klingons were just humanoids with goatees no cranial ridges, keep in mind this is in the 23rd century.
ok fast forward to TNG which is set in the 24th century, klingons have the usual cranial ridges.
about 15 years later there is a ds9 ep where sisko and the crew go back in time to the tribble episode of TOS and dont recognize the klingons because of their lack of cranial ridges, so they ask worf what the fuck is up, and he just says, "they are klingons, and it is a long story"
OK, thats fine and dandy, mabye there was a viral mutation or some genetic engineering gone wrong, between the 23rd and 24th centuries.....
BUT, in the enterprise series set in the 22nd century, klingons have the cainal ridges we know and love. SO WTF?!?!?!?!?!

Did two (or more) sentient species of klingons evolve on kronos only to be wiped out by the crainal ridge species?????

Discuss and speculate..