They were copied, and copied and copied through the centuries. By contemporaries, who thought of those texts as of great importance. In medieval times, a lot of this work was done in the monasteries by monks. A lot of texts that we know existed were lost to not being copied enough. Alternatively there are Roman texts which only one instance of exists and was found in monastery in Ireland - which was never a part of Roman world, but was a center of monastic activity at one time.

Hi, it would be worth x-posting to /r/AskAnthropology or /r/AskScience (Social Sciences) or /r/Archaeology. While there are several anthropologists here, there may be more on those subs, especially that work with ancient artifacts.