Simplify the following text for a Grade 7 native English-speaking person.
Original: In the historic period the earliest known settlers were Picts to the north and Gaels in the southern kingdom of Dalriada prior to the islands becoming part of the SuÃ reyjar kingdom of the Norse , who ruled for over 400 years until sovereignty was transferred to Scotland by the Treaty of Perth in 1266 .
Simplified: The islands became part of the Kingdom of Mann and the Isles , the SuÃ reyjar kingdom of the Norse . The Norsemen ruled for over 400 years until sovereignty was transferred to Scotland by the Treaty of Perth in 1266 .