Arab lineages allegedly originate from three groups:
Perished Arabs (العرب البائدة): These are the ancients of whose history little is known. They include ‘Ad, Thamûd, Tasam, Jadis, Imlaq and others.
Pure Arabs (العرب العاربة): They allegedly originated from the progeny of Ya‘rub bin Yashjub bin Qahtan bin Hud so were also called Qahtanian Arabs.
Arabized Arabs: They allegedly originated from the progeny of Ishmael and were also called ‘Adnanian Arabs. The Quraish are a branch of the "Arabized Arabs".
The Quraish had become a prominent tribe in Mecca before the birth of Muhammad and essentially ruled the city. Before Muhammad's birth, the tribe had split into different clans, each with different responsibilities. There were some rivalries among the clans, but these became especially pronounced during Muhammad's lifetime. Some clan leaders did not appreciate Muhammad's claim of prophethood and tried to silence him by putting pressure on his uncle, Abu Talib. Many of the clans also began to persecute the followers of Muhammad, for example by boycotting them. This response led Muhammad to initially send some Muslims to Ethiopia, and later would lead to his own emigration to Medina.
After Muhammad's conquest of Mecca in 630, he pardoned many of those who had oppressed him before, and peace among the different clans was maintained. After Muhammad's death, clan rivalries reignited, playing central roles in the conflicts over the Caliphate and contributing to the Shi'a-Sunni divide.
Clans
Quraish branched out into various sub-clans, who in turn branched out into yet further sub-clans. Roughly the division corresponded to the family lines of the current chieftain of that clan having sons.
Banu Quraish — Banu Quraish was divided into several sub-clans.
During the era of the Islamic prophetMuhammad, there where a powerful elit that ruled the Arabian city of Mecca. All of those people came from the tribe of Quraish and where accordingly referred to as the Leaders of the Quraish.
Part of Shi'a belief stems from the fact that Ali was a member of Muhammad's clan, the Banu Hashim. Abu Bakr, while a close companion of Muhammad, came from the Banu Taim clan.
When Ali was made caliph after the death of Uthman, the Caliphate was in the hands of the Banu Hashim, but he was almost immediately challenged by Muawiyah, who was a member of the Umayyad clan. After Ali's assassination at the hands of the Kharajites, the Shi'a hoped his son Hasan would become Caliph, but he deferred to Muawiyah, who established the Umayyad line of Caliphs.
After the death of Muawiyah, his son Yazid became Caliph but was almost immediately challenged by Ali's younger son, Hussein, who would not swear allegiance to Yazid for a number of reasons, among which that the Caliphate was not supposed to be hereditary, and that Yazid was said to be corrupt.[[Citing sources citation needed]] Yazid's forces were stronger than those of Hussein and Hussein was killed at the Battle of Karbela. This event would ultimately lead to a full schism between Shi'a Islam and Sunni Islam.
The fact that Muhammad's descendants through Ali would be persecuted by Umayyad Caliphs did not help the matter. It seems that initially, the difference between Shi'a Islam and Sunni Islam was purely political. Only under the later Abbasid Caliphs would actual doctrinal religious differences arise.
