Who is the earliest recorded Douglas?

[William Douglas]

There is a story circulating that while researching the Vatican archives for a book on the topography of Lowland Scotland, the author Nigel Tranter found a document which stated that John, son of William Lord of Douglas attended a royal council at Scone in 1056. This would be more than 100 years earlier than the previously accepted first Lord of Douglas.  At that time, Macbeth would have been king.

There is the following tradition in regard to the origin of the name. In the year 770 Solvathius king of Scotland, obtained a victory over Donald Bain of the Western Isles, by the assistance of a man who was unknown to him. After the battle, being desirous to see one who had done him so signal a service, he was pointed out to him with these words: " Sholto Dhuglass," behold that swarthy man. 

But there are those that claim that the first recorded Douglas to stride across the stage of Scottish history was Sir William Douglas who fought and died for William Wallace. His son, Sir James Douglas was a supporter and lifelong friend of Robert the Bruce. The "Good Sir James" died taking Bruce's heart on a crusade to the Holy Land. 

Has anyone seen evidence of Tranter's research?