Final answer:
Muhammad was the key figure in unifying the Arabian Peninsula, including Mecca and Medina, through his leadership and the spread of Islam.
Step-by-step explanation:
The individual responsible for first unifying the people of the Arabian Peninsula, including Mecca and Medina, was Muhammad. He served as an impartial mediator in Medina, later becoming its de facto leader and establishing the Constitution of Medina.
In 630 CE, Muhammad successfully conquered Mecca without significant bloodshed, largely due to skilled negotiation with his former enemies. The conquest of Mecca marked the destruction of idols and the elevation of the Kaaba as the center of the Islamic world.
Muhammad's leadership and the spread of Islam unified the Arabian tribes under a single religious and political authority. By the time of Muhammad's demise in 632 CE, most Arabian tribes had willingly converted to Islam, solidifying the unification of the region.