Age and the Silver Tribe girl (Yuti?) are the only ones who the same race as the ones they serve. The Golden Tribe told all the races to "move" and only 4 responded. The Heroic Tribe fought amongst themselves, and destroyed a lot, leaving only 5 survivors. The Golden Tribe decreed that they would be put into hosts in the other Tribes of the galaxy, and would serve under contract until one of them dominated the universe and succeeded the Golden Tribe. Penance for the destruction they wrought.

The Silver Tribe was the strongest of the remaining three (seeing just one of them can be a threat to a battleship) and quickly brought the Bronze Tribe under their yoke, and then the rest of the Tribes that didn't "move." This is how they gained control of the other 3 Nodos. Open threats (like Yuti always gives to Mehitaka) kept them all in line. The Iron Tribe lost their homeworld to the Silver Tribe in an invasion led by the Bronze Tribe and have been hunted by them ever since.

The Golden Tribe gave the strongest of the Heroic Tribe to the weakest of the Tribes, the Iron. All of the Nodos are bound under contract to protect and aid the Tribe they are bound to until only one of the three major Tribes dominates the galaxy. Then that winning Tribe must grant the Nodos a wish. The Silver Tribe pressured the others to work for them.

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I imagine Yuti is the second strongest of all the Nodos (Bellcross being #1), but for some reason, the leader of the Silver Tribe, or at least her superior, will not let her leave whatever planet they are on.