Board Thread:Questions and Answers/@comment-29752445-20180827202712/@comment-29752445-20180829225215

Hoshido0Nohr wrote: that could be the case but it's likely just the wording, I believe Ymir is the spiritural ancestor of all eldians and the eldian race existed along side her.

had Ymir been the ancestor of all Eldians then Royal blood would not be special and you really can't argue incest or cousin marriages because that would still mean multiple families with enough Ymir DNA would be every where rather than being limited to the Fritz/Reiss family. and there is no question Zeke and Krista have royal blood and both have one random parent. the problem is that eren kruger specifically said that the subjects of ymir are ymir's descendants, and there is only one meaning to "descendants".

I have discussed few people about this and we came to the conclusion that there isn't really an answer.

but we think that the most probable answer is that once ymir died the 9 succesors ate her body (we can see a hint of that in the ending of season 2) which 3 of those 9 were her 3 daughters, and we believe the daughter, and each one of the 9 got one of the shifters power, and we believe that one of the 3 daughters inherited the founding titan power, by "fate" I guess, and then that daughter's bloodline and descendants are the "royal blood".