Spirit Energy was the key to unlocking this era, the energy that allowed life to evolve.
And leaps in the level of life were often accompanied by the awakening of one's bloodline, tracing back toward an older, more powerful source.
This was atavism.
For the Uchiha Clan, the source of the power flowing through their blood was the Otsutsuki.
Even without the immense life energy of a Sage Body, without Hashirama's Cells serving as a catalyst and balance, an Uchiha Bloodline refined to its utmost purity could still knock upon the gates of the ultimate under the nourishment of Spirit Energy and the continual elevation of its own life level.
After obtaining Hashirama's Cells, Madara Uchiha had only developed the Rinnegan in his twilight years, after a long passage of time.
Afterward, he transplanted them into Nagato Uzumaki, relying on the Uzumaki Clan's powerful vitality and the traits of a Sage Body descendant to wield that excessively burdensome power.
But Uchiha Ten was different. He did not need to transplant another person's cells to develop the Rinnegan. Under the watering of Spirit Energy, his bloodline broke through its shackles.
Spirit Energy permeated all things without distinction, driving evolution.
It was nourishment for every inch of his flesh and every shred of his ocular power, while also indirectly stimulating the dormant power deep within his bloodline.
As his own life level continued to rise alongside his control over the Eternal Mangekyou and his deepening understanding of the world, bloodline atavism became a natural outcome rather than a shackle that required external force to pry open.
Spirit Energy replaced the immense vitality of a Sage Body, nurturing his body in a grander manner and supporting the terrifying demands that bloodline atavism placed upon the essence of life.
He and Madara Uchiha were