Board Thread:Naruto Discussions/@comment-26135923-20150707130017/@comment-734582-20150709124405

I think you can go a long, long way by explaining Transmigrants with Kekkei Genkai and chakra. but first:


 * judging from what happened in the Hashirama flashbacks, the Senju likely got absorbed into the village and so "died out" through marriage. Being the strongest shinobi within the village the World Wars might've helped too, with the Senju on the frontlines.
 * Pure chakra seems to be very powerful (that is, straight from the Chakra Fruit) and going from there, increasingly dilute chakra seems to yield increasingly weaker ninja. The strongest ninja clans are those with the least dilute chakra (e.g. Senju and Uchiha) and the weakest possess chakra as given by Hagoromo. This is further emphasized by Hyuga since they use a main/branch construct to maintain their chakra purity.
 * Kekkei Genkai seem to be explained by special chakra (e.g. Sharingan) that, similar to genetics, give special abilities to people when acquired. E.g. Sasuke gets the Rinnegan and Naruto doesn't despite both getting chakra from the same man with a rinnegan. Sasuke got chakra that awakened the Rinnegan while Naruto did not (in stead, the chakra that awakened TSB's). Thus, a person's chakra can be seen as a mix of many sub-chakra's (like kekkei genkai chakra).
 * The Senju's power thus likely died out because their high-purity chakra was quickly diluted beyond recognition.
 * So, Transmigrants can be explained as the people who acquire both pure chakra and kekkei genkai. I'm not sure but given the definition, "longevity"/strong lifeforce is essentially a Kekkei Genkai. Furthermore, there seems to be an element beyond mere mixing (powerful ninjas and abilities can be born from mediocre people and vice versa) hence why Naruto can be a transmigrant despite being only superficially connected to Hashirama.

This model doesn't exclude the idea of souls and such, as Transmigrants definitely seem to have more going for them that what i explained. But the people who descend from Indra and Asura do have strong kekkei genkai (remember "when you face a sharingan alone, run!" ?) and strong chakra and so a child of children of both transmigrants has the potential of immense power.