Being able to communicate with another is key to a culture's survival. As family groups clustered [the kindred], their ability to get along with one another would certainly increase this survival rate. A common language would accelerate this process, and as the family groups isolated themselves among the geography that surrounded them, a core method of communicating sounds and expressions would develop. The following table gives an outline [over time] for the folks who were to become the Welsh.
The earliest form recorded appears to be "Brude mac Muthut (fl. 330 AD) in the kingdom of the Picts (1), and "Cynan map Eudaf" (fl. 420 AD) in the kingdom of Dumnonia.(2) The next appears to be "Cynlas ab Owain" (fl. 530 AD) in the kingdom of Powys.(3) For the Picts and Scots the form "mac" is used to means "son of", and "ap" is used among the Welsh. [The Irish also used the form "mac".]
Thus begins the language roots to the lineal male descent among the kindred groups scattered about the islands.
The book by Mike Ashley, "British Kings & Queens", Barnes & Noble Books, 1998 was the source of this analysis.
(1) = p.169
(2) = p.114
(3) = p.150