Let me explain.
To start, there have been various theories regarding Mokuton ever since its debut and more with every development. This theory is based for the most part on recent revelations in the manga that tie to information we learned in the past.
To start, there is an undeniable connection between Yang chakra and Mokuton. As witnessed when Naruto obtained KCM, i.e. acces
Kurama's Yang chakra, it affected wood and White Zetsu, both of which are products of Mokuton. This undeniable proves that Yang, the power gives form to nothingness, is the source of behind Mokuton, as we had already been told in chapter 510.
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Out of all the Senju clan members, only Hashirama was a Mokuton user. The kekkei genkai started and ended with him. None of his offspring inherited Mokuton genetically despite that being the prevailing trend of kekkei genkai. So what was so different about Hashirama?
As we recently found out, Hashirama was a transmigrant of Ashura.
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Ashura inherited Hagoromo's strong life force and vitality. In other words, Ashura inherited Yang Release from Hagoromo.
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Hashirama, being a transmigrant of Ashura, inherited that very same power.
This made Hashirama the only Senju that possessed Earth, Water and Yang Release, which would explain why he was the only Mokuton user and why none came after him because, unlike normal KKG, Yang Release is not a KKG but a specific power that can only obtain through transmigration or using Hashirama's cells.
Now, before anyone brings it up, yes, Yamato possesses Wood Release as well despite not being a transmigrant. Well, the thing about Yamato is that he is a clone of Hashirama, and like Madara, his power is a result from cultivating Hashirama's cells. Orochimaru created Yamato through similar methods.
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Further proof of Yang's involvement in Mokuton lies with those that have pursued Hagoromo's power. Madara, Obito and Danzō all pursued the power of the Rinnegan through combining Yin and Yang, with various levels of success, e.g. Danzō, who had Yin, Yang and Yin-Yang Release, had Hashirama's cells implanted in him but could not control it.
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I believe the evidence supplied conclusively proves Mokuton is created by mixing Earth, Wind
and Yang. As such, all those in possession of Mokuton possess Yang Release as well as Earth and Water (Hashirama, Madara, Tobi, Danzō) or possess Hashirama's cells (Yamato is a clone and Orochimaru has a Zetsu body).
This would, of course, contradict what we have been told by characters in the manga, however, it would be one of many contradictions in Kishimoto's manga, and further more, there is a mountain of contextual evidence pointing to this new truth.
