The origins of all the blue egg layers are the Quechua, from South America. This thread is quite thorough in talking about the history of the breed:
https://www.backyardchickens.com/t/402512/quechua-tojuda-ameraucana-easter-eggers-in-vino-veritas
In short, they came from South America, were exported around the world and crossed with all sorts of breeds, were brought to the U.S. and refined into a few different breeds. Remember, with chickens it's not so much that the parents are "purebred" as the offspring follow the standards of the breed. That's why you can have two purebred Ameraucana or Araucana and when crossed together, the offspring might not qualify for the breed (in the case of two colors being mixed that don't match the standard, or off-colored legs, or what have you). Of course, the longer you breed a line for a certain standard, the more likely you'll keep getting that standard with each generation, which is how you get a "breed".
The thing with Easter Eggers is that they're a land race (as someone else explained earlier) which means as long as they look generally alike then they still fit in with other Easter Eggers. The part that's confusing is that a lot of the traits that define an Easter Egger, fluffy cheeks, green legs, blue eggs, are dominant traits and are therefore spread to their offspring widely, making half-breeds look a lot like an EE so they're still called EE.
The other thing is that by default, any Araucana or Ameraucana that doesn't meet the standard is an Easter Egger since they fit into the "land race" description.
Clear as mud?