I don't think I've ever seen any that offered an explanation for the phenomenon, though I think it's a really fascinating question.
... I mean, if I were to handwave it myself, I guess I'd go with it being an outward manifestation of the cultural context the child has soaked up (think Temeraire and dragons in their shells). So it's something that started slowly with symbols and progressed to actual writing as the societies became literate. Maybe in the beginning it was only like a bruise. And gradually it became legible writing in whatever writing system was widespread in the child's culture (or the soulmate's -- it seems to be universal in MCU fandom that Bucky gets Russian writing if Natasha is his soulmate).
But this just raises SO MANY QUESTIONS TOO! Like, even today literacy isn't entirely universal, but it's only been even common for a couple hundred years. In the 1200s, did people get soulmarks? Were local priests constantly tasked with reading them to peasants? Honestly, considering that prior to the last few hundred years people didn't generally have free choice of life-mates anyway, how did anyone EVER figure out what it meant? Back in the 1000s or 1200s or 1400s, was it regarded as a mark of the devil? Like, was it the person who was going to tempt you to sin, when you yourself had been married from the age of 12 but didn't have anyone speak those words to you until you were 26? What if people are STILL misreading what it means, and whatever the hell it is just gets filtered through the cultural context? In 1200, it was taken as some kind of warning about the person who spoke those words to you; for the last 150 years or so, with the trend towards free choice of life-partners and idealization of romantic love, it's been taken as a soulmate mark; but that's not any more accurate -- it just means it's someone you're entangled with, in some way, positive or negative. In 100 years, they might have a whole different take on it.
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... I mean, if I were to handwave it myself, I guess I'd go with it being an outward manifestation of the cultural context the child has soaked up (think Temeraire and dragons in their shells). So it's something that started slowly with symbols and progressed to actual writing as the societies became literate. Maybe in the beginning it was only like a bruise. And gradually it became legible writing in whatever writing system was widespread in the child's culture (or the soulmate's -- it seems to be universal in MCU fandom that Bucky gets Russian writing if Natasha is his soulmate).
But this just raises SO MANY QUESTIONS TOO! Like, even today literacy isn't entirely universal, but it's only been even common for a couple hundred years. In the 1200s, did people get soulmarks? Were local priests constantly tasked with reading them to peasants? Honestly, considering that prior to the last few hundred years people didn't generally have free choice of life-mates anyway, how did anyone EVER figure out what it meant? Back in the 1000s or 1200s or 1400s, was it regarded as a mark of the devil? Like, was it the person who was going to tempt you to sin, when you yourself had been married from the age of 12 but didn't have anyone speak those words to you until you were 26? What if people are STILL misreading what it means, and whatever the hell it is just gets filtered through the cultural context? In 1200, it was taken as some kind of warning about the person who spoke those words to you; for the last 150 years or so, with the trend towards free choice of life-partners and idealization of romantic love, it's been taken as a soulmate mark; but that's not any more accurate -- it just means it's someone you're entangled with, in some way, positive or negative. In 100 years, they might have a whole different take on it.