The only wrinkle is that biologists might decide, presumably on genetic simularity, that red junglefowls and chickens are still the same species, like has been done with dogs and wolves.
That would mean the chicken came first, because it was the taming that made it a chicken.
In such a case, we would simply need to look backward in history until we find an ancestor that doesn’t meet the chicken criteria. Fowl as a clade were separated from other bird clades before the K-T Extinction Event, and many such species before the event had teeth, which means they weren’t chickens.
The only wrinkle is that biologists might decide, presumably on genetic simularity, that red junglefowls and chickens are still the same species, like has been done with dogs and wolves.
That would mean the chicken came first, because it was the taming that made it a chicken.
In such a case, we would simply need to look backward in history until we find an ancestor that doesn’t meet the chicken criteria. Fowl as a clade were separated from other bird clades before the K-T Extinction Event, and many such species before the event had teeth, which means they weren’t chickens.