Board Thread:Manga/@comment-33873869-20200317050453/@comment-36306777-20200402115812

Razzylada wrote: Now, about the firsts SoY. From my POV :

We can debate about considering Ymir Fritz a SoY since she's the first, but we can say for that her three daughters were SoY by birth, the first ones if we don't consider their mother. And their children too were SoY, etc... But what about the people who ate the three daughters and yet weren't their children, which mean they weren't SoY ? Well, eventually, they still could eat the daughters, and gain a Titan Power, and with them and the children of the daughters, we have the first holders of the 9 Titans. I consider that those non-SoY who ate the daughters became SoY by eating them, for unknown reasons.

These non-SoY who ate the daughters were in a smilar situation as Ymir's : they were born as non-SoY, and gained a Titan Power. These are the only cases of people who weren't SoY by birth. These exceptions can be "explained" as it's still the genesis of the Titans.

Since this moment, every SoY is a SoY by birth.

I don't think the first SoYs became such by eating Ymir's daughters or the first 9 shifters. Remember when Ymir was hit and killed by that spear? Just before she finally closes her eyes and dies, the king orders her to stand up and work because she was born to be a slave. I think when she awoke in the Path Realm she chose to grant the power to turn into Titans only to Eldians because she was brainwashed to advantage Eldians for all her life. So I think that right after Ymir's Death all the Eldians of that time (which, given the more "tribal" society they lived in, were also ethnically Eldian) got the ability to turn into Titans all at once, thus becoming SoYs.

This is what I've always assumed, but now, re-reading chapter 122, especially the king's speech on his death bed, I see that it's kinda ambiguous and it could also be interpreted your way.