Citizenship first came about in the ancient Greek polis. Prior to the Greek invention of citizenship people were bound to their family groups as their strongest tie and they didn't really have a firm bond to their place of living. Greek citizenship changed that by establishing an official bond to their city-state. This type of citizenship arose with the development of nomos, or rule of law, as the guiding force behind Greek society. Under nomos every man was bound to the law and none were above it, all were equal in regards to the law within the polis. This equality under the law is what established citizenship. Of course, not everyone was equal, slavery existed and women couldn't be citizens, but everyone had to obey the laws. There is some debate on which city-state first conceived of citizenship, but Athens is usually considered the first. Some historians claim it was Sparta, but they're a minority.

Citizenship in Greece had nothing to do with discriminating by ethnicity. Citizenship was acknowledged by other city-states, because they too, had citizenship in their land. There were outsiders that had no concept of it, so if Jason was a citizen of Athens and he went out to meet Jean from Gaul, Jean wouldn't have any idea what Jason meant when he said he was a citizen. Citizenship was accepted by the people of the city-states, because it had to be if they were to be engaged in the political process.

Migration and adopting of a new citizenship did happen. Birthright citizenship was the most common method of obtaining citizenship, but if you were a man who wasn't a barbarian you could move to a different city-state and become a citizen in most cases.