Prior to the East-West Schism, and from the beginnings of organized Christianity, the entities which became the Eastern Orthodox Churches after the schism had the same Pope, with a somewhat different role, as Western Christianity.

Some of them also have had things not unlike the inquisition, though they don't call it that.

And, of course, outside of the Eastern Orthodox and maybe the “Old Catholics”, even those Christians who disagree with the Roman Catholic position don't see the Eastern Orthodox as having a particular claim to original Christianity.

EDIT: for example, the Oriental Orthodox Churches and the Church of the East are, in their current form, older than the Eastern Orthodox, and from their perspective the branch containing both sides of the 1054 Schism is a divergence from “original Christianity” in the same way that Western Christianity is viewed by the Eastern Orthodox.,,

So it's like the relationship between modern agile methodologies and the original agile manifesto then?

No, it's more immutable vs mutable... or accepting messages with bad public key signatures.

I can understand the belief that Eastern Orthodoxy is more of a "trunk" than more recent "branches", but what about core ideas that predate Jesus? e.g. the immortality of the soul was reasoned by Plato (Republic, circa 350BC); heaven and hell have been portrayed by Virgil (Aeneid, circa 19BC). Aren't these the "trunk", and the Hebrew Bible and Jesus another branch?

Trees have roots, too.

Yes, also Christians in the US are mostly Protestants, which purposely moved away from the things you mentioned.

I'm a Lutheran and I've studied the works of Luther. When I learned about the Orthodox church as an adult, I really liked what I saw. If Luther had been an Orthodox priest, I wonder if he would have launched his gentle rebellion (and, later, his not-so-gentle rebellion).