According to traditional genealogies, Somerled, who is said to have died in 1164 after ousting the Vikings from Argyll, Kintyre and the Western Isles, was descended from an ancient royal line going back to when the Scots were living in Ireland.
But Bryan Sykes, an Oxford University professor of human genetics who set up a company called Oxford Ancestors to research people’s DNA past, has discovered that Somerled’s Y-chromosome - which is inherited through the male line - is of Norse origin.
Prof Sykes’ studies of three Scottish clans have also led to the conclusion that some 500,000 people alive today are descended from Somerled - a number only bettered by Genghis Khan, who, among historical figures studied to date, has an estimated 16 million living descendants.
The MacDonald, MacDougall and MacAllister clans all claim descent from Somerled and Prof Sykes found that between 25 and 45 per cent of them shared the same Y-chromosome, of a kind normally found in Norway but rare in Scotland and Ireland.
By analysing the rate of mutation in DNA samples from clan members, Prof Sykes was able to show that the Y-chromosome came from a common ancestor who lived roughly 1,000 years ago.
He then tested five chiefs from the clans and discovered they all shared the same chromosome, which convinced him that the common ancestor must be Somerled, Lord of the Isles, in keeping with clan histories.
However, the analysis threw into doubt Somerled’s own origins. Prof Sykes told The Scotsman: "In the traditional genealogy, Somerled is a great Celtic hero who drives the Norse from Scotland, but his Y- chromosome is definitely Norse. The genealogies trace him back to a long line of Irish kings. But that’s not what the Y-chromosome says.
"He is certainly of Norse Viking paternal origin."
It is open to question whether Somerled, who made driving the Vikings from western Scotland his "cause clbre", would have known the truth.
But Prof Sykes said: "I think it is something you would want to keep quiet."
The fact that clan chiefs still share the same basic Y-chromosome after some 87 generations shows that high-status women in the MacDonald, MacDougall and MacAllister clans were extremely faithful.
However, the large number of people alive today with the same Y-chromosome means the men in the family did not share this virtue to the same extent.
Maggie Macdonald, archivist of the Museum of the Isles on Skye, said Somerled was traditionally viewed as a Celtic hero.
But she added: "Maybe at that time it was more important who it was said you were descended from than who you were actually descended from.
"People may well have known his great-great-grandfather was a Viking.
"But it could have been that his great-great-grandmother had relations with someone who wasn’t her husband - it could be Somerled wouldn’t have known and thought he was this great Celtic hero."