Most family names in Europe were not introduced until Napoleonic times. What do you enter for a famlly name, when a person was only know by his/her first name and partonymic? So Jason son of John is know as Jason Johnsson. And his father John was known as John Jacobson and his father as Jacob Robertson. No family name, althogh there is a clear family lineage.
If you don't use a family name and upload a GEDCOM, you end up with many UNKNOWN's which wikitree suggest skipping. So how do most of you deal with this?