If anything, conceptions of ethnicity have gotten even more abstract as categories have advanced to "national origin," and the rapid and wide movement of peoples across the globe has encouraged rapid and widespread intermixing. Though even earlier iterations have proved to be just as murky. Yes, people from certain broad geographic regions have developed certain distinguishing physical features and cultural traditions, which were more concentrated in the premodern era when people didn't move quite as far as quickly or intermingle quite as much.
But even in the ancient and Medieval worlds people could move around quite a ways: the Vikings showed up anywhere from Byzantium to Iran and Arabia and to North America for example. And oftentimes a certain ethnic group may have acquired a reputation as being based out of a certain geographic area, but through their increasing prominence and expansion, their ethnic designation would become more determined by a way of life or an ethic or mode of labor. Their ethnicity would morph into a more abstract way of being. The Germanic tribes, like the various Goths for example, would take in runaway slaves and political dissidents, and those peoples would intermarry and effectively become Goths.
Ethnicity was always a shifty designation. Though that doesn't stop us from still seizing upon visible and immediately apparent grouping differences and then dividing and manipulating those groups in order to control the flow and maintenance of wealth and power, a la the construction of race, which has proven to be perpetually useful for the ruling elite.
