The discussion about Arthur’s existence has been active since Geoffrey of Monmouth made him a European sensation in the mid-eleventh century. For many, it revolves around his mention in Historia Brittonum and Annales Cambriae, which is now generally believed to have come from the Northern Memorandum. Personally, I’ve approached the question from several angles no one had ever thought to so it’s not necessary for me to believe that the hypothetical source is early to know Arthur existed. That also means I’m less likely to be biased toward making it any earlier than it has to be. To business; the veracity of both sources depends on the Northern Memorandum written within the lifetime of an individual.
I say this because it has been suggested that people during this period might have lived a hundred years or more, as long as they were able to avoid wars and disease. Between 526 and 547, Britain suffered three major plagues or famines. Between 410 and the time of Gildas’ De Excidio Britanniae, the Germanic migrants expanded west to control what in later times would be Northumbria, Essex, a good portion of Wessex, Sussex, East Anglia, and Kent. And the Germanic peoples were not Christian at that time so that they saw monasteries as storehouses of wealth only. By 632, when Cadwallon would run roughshod over Northumbria, northern Britain was cut off from Wales, suggesting Mercia had expanded to the western coast. Safe to say that nowhere on the island was there a region that could have avoided disease, poor crops, or war for more than a decade. And of course the very young and the very old would have been the most likely to die during plagues, famines, or conquests.
Statistical fact, the average male lived to be 32 and the average female 28 in the first couple centuries of post-Roman Britain.
All that out of the way, we know that the rare churchman could live to be as old as 80. Unlikely as that seems, it means he could have been witness to an event he saw at 5 and wrote it down at about 80. More likely, a person would remember an event from 15 on, but we’ll stick to the extremes. That way whatever dates we come up with won’t be edited for a wider range.
The first dated historical event in either Historia Brittonum and Annales Cambriae is the Battle of Arfderydd, dated to 573. A boy aware of the event at the time might have recorded it as late as 648. The last northern British dated event is the death of Dunod in 595, and therefore the earliest possible date would be 595. Thus Run, or a monastery associated with him, likely scribed the original Northern Memorandum in 595×648.
Professor Koch long ago suggested a means by which a British history could have been brought into Germanic hands and made use of without bending it to a strictly Germanic bias. At some time after 664, Alhfrith revolted against his father. He was dead by 671. Alfrith was possibly a descendant of Urien according to the Historia Brittonum, which means he might have had access to British records. Modernizing the history for the purpose of showing his ancestry and winning the support of British nobility would have been an excellent motivator for a Northumbrian to put in the details about Rieinmellth marrying a Northumbrian.
However, as he died in 671, the Germanic genealogies end during Ecgfrith’s reign, and the last event of Germanic concern is 685, it’s also a safe bet that the history was rewritten in 685 or later.
There is also one curiosity in the history of the theoretical Northern Memorandum; Patrick is one of three religious figures mentioned in prominent roles (Rhun as the author and Germanus as the nemesis of Vortigern being the other two). But Patrick serves no known purpose for any British history written between 595 and 648. Nor would Alhfrith or Ecgfrith have had any political motivations. In fact, one might argue the British would have had more reason to mention other British saints, while the Northumbrians would have been more likely to name Columba or Aedan. Add to this that Arthur’s presence seems to be interwoven with the Arthurian entries and we are given the suggestion that these two may have been part of a still earlier recension. Is there any evidence as to its date-range, or even that there was an earlier recension? Nope! Just a thought.