Having practiced both TKD and Karate for a long time and having black belts in both it's just my general observation that they are not very different. Think about it; a punch is just a punch, a kick is just a kick and so on. Once you can do a TKD kick you have no problem doing a proper karate kick and certainly not after a tiny bit of coaching. I've even gone back and forth between them and it was all seamless
Southern kung fu > proto Okinawan karate > Okinawan karate > Japanese karate > TKD (Korean karate).
..........1800...................................1850.......................................1890.............................1930................................1985
I think the above timeline of system evolution is a good (
rough) estimation, the dates after 1800 representing when the system subjectively came into its own as
a unique art, distinct from its predecessor. Yes, TKD started sooner, but previously was still much like Shotokan so I allowed an extra couple of decades for TKD to develop its own differentiating flavor.
The main point however is not the evolutionary timeline, but evolutionary
distance. IMO, each step as listed is
equally (more or less) different from its predecessor. It has been a step-by-step progression as one system flowed into the next.
If one is to say which is
most different from its ancestor, I don't think it would be TKD, but Japanese karate as institutional teaching and sport competition (a fairly sudden and impactful evolutionary stimulus) weighed heavily on its development as a style. One can
say TKD is a completely different kind of thing from karate for identity purposes, but physically it's just another step on the continuum. That's not a bad thing. It's true for all the other systems I listed above as well. We're all in the same family.