However, that's the exact same answer that a Catholic or OO would give. What would be the argument that would settle the issue over which church is the original and which two are the ones that broke off and are in schism?
In school they'd teach us the Patriarch of Constantinople wouldn't acknowledge the Pope's authority, and that they had mutually excommunicated each other, and so the East seceded. It wasn't that simple from what I've found.
In the West it's known as the Eastern Schism, long in the making, rising with pride- Michael Cerularius persuaded the emperor to appoint him the patriarch of Constantinople and cut the golden thread for good when one of the Papal legates excommunicated him- which wiki says was really invalid since the Pope had just died and St.Peter's chair was empty. He saw Rome as a provincial pretender he didn't have to contend with. Died in exile less than five years after the fact, banished from his post and the city, accused of heresy.

Michael Cerularius | Byzantine Emperor, Iconoclasm, Excommunication | Britannica
Michael Cerularius was a Greek Orthodox patriarch of Constantinople from March 1043 to November 1058 who figured prominently in the events leading to the Schism of 1054, the formal severing of Eastern Orthodoxy from Roman Catholicism. Although Cerularius was educated for the civil service rather
