Upvote:1
Blood ties were just as strong in Rome, and Rome was only one element of Ancient times, all of which had dynastic lines just as much or more as France and England in the Middle Ages.
Emperors were chosen by the Military, and in times of trouble the current Emperor might be toppled and a usurper put in place, and the relatives of the former regime killed. This isn't a repudiation of the concept of lineal descent, but a confirmation of his power.
The one time, under Diocletian and the tetrarchy, that a true male heir was passed over to rule by someone else, two of them (Maxentius and Constantine) pushed their way into the rulership of the Empire and started a new round of civil wars to get power. Constantine the Great was the last man standing.
Historians vastly overplay the significance of the Adoptive Emperor effect as some universal Roman rule. Instead, it was coincidence. The first time one of these fellows had a son, Commodus, the rule was out the window. And this emperor was Marcus Aurelius, the philosopher.
Upvote:4
In a nutshell, Europe had a feudal system unlike the Roman world where the monarchy was added on top of the old republican system. The feudal system was based on personal relationships with a liege.