Which lineage stretches back the longest unbroken?

[Tom Cruise]

Curious.

Middleweight goes back to the start of Hopkins lineal reign, whenever that was (early 2000's?). Is that the longest?

[gingerbreadman]

Pretty sure it's flyweight, according to boxingscene itself goes back to 1975

[Humean]

Pretty sure it's flyweight, according to boxingscene itself goes back to 1975

I think you are right, the flyweight lineage goes all the way back to Canto.