Board Thread:Naruto Discussions/@comment-945885-20140709082448/@comment-24593592-20140714125254

Mrlmm0605 wrote: Jashin Uchiha wrote: If Kaguya was the first person to have chakra then everyone in the five great shinobi countries descended from her, and would have KKG.

Alright then, here's why that is wrong. Chakra has been shown to cause demonstrable physical changes in individuals when introduced (the most patent examples being Orochimaru's juinjutsu utilizing the Sannin's senjutsu chakra to cause mutations, or the introduction of Hagoromo's, offspring's, chakra creating a Rinnegan in a transmigrant). These examples are cases where the chakra has merged with and manipulated the genotype (the DNA) of an individual to affect their phenotype (expressed genetic traits) on an immediate level. It is therefore feasible that the Chakra Circulatory System (CCG) could have originated as just such a chakra mutation. The progenitor members of the population, those who found themselves with this trait, would have found themselves with an innately hyper-efficient way to transmit chakra throughout their bodies, rather than having it settle in eyes, bones, etc. as typical phenotypic Kekkei Genkai (KKG) do. This would have enabled them to better utilize a more varied genre of ninshu or ninjutsu, which would improve their odds of survival in a shinobi-vs-shinobi or shinobi-vs-normal combat situation. This would then allow for an increased lifespan and increased likelihood of reproductive success. Over the untold thousands of generations since Hagoromo founded the world civilization as the characters know it, these individuals would have bred like rabbits, becoming, over a long period of time, the example of a 'normal' Narutoverse dweller.

A similar track can be followed for the existence and variety of KKG in modern Naruto, which the revision that KKG are likely recessive traits that exhibit complete dominance in the genome.

Juinjutsu was based on a already existing KKG. Plus, Madara already had the genetics for the Rinnegan.