Complex sentence: Nordics entered Sumerian plains (modern day Iraq) from the Iranian plateau, marking the birth of Sumerian civilization, preceding Ancient Egypt by 3 thousands years, migration to Egypt 3 millenias later marks the birth of Egyptian civilization.
Split into simpler sentences: Nordics entered Sumerian plains (modern day Iraq) from the Iranian plateau, marking the birth of Sumerian civilization, preceding Ancient Egypt by 3 thousands years. Migration to Egypt 3 millenias later marks the birth of Egyptian civilization.