A lot of what people assume is history regarding this period is actually mostly theology or religious narrative. Western pop-culture understandings of early Islam often tend to be of the theological variety.
In recent years, scholars have increasingly used the label “proto-Sunni” to denote a
late Umayyad — early Abbasid period movement which played a pivotal role in the
formation of Sunni Islam... the category proto-Sunni usefully underlines the fact that, like other
religious traditions,5 Sunni Islam did not appear in history fully formed; but that it
emerged through a complex historical process, a process which yielded widespread
Sunni self-awareness no earlier than the late 9th century.6 As such, the designation
proto-Sunni underscores the provisional nature of the many competing versions of Islam
in this period, and against that background, the crucial role of those who prepared the
way for an eventual Sunni consensus... While much of the credit for the
“Sunni synthesis” must go to jurists and theologians of the 10th through the 13th centuries,
such as al-Ash‘arı¯ (d. 936) and al-Ghazza¯lı¯ (d. 1111), who absorbed many of the
cross-currents of early Islam into grand legal and theological schemes, it is important to
remember that those very schemes would have proven impossible without the
foundations laid by the early proto-Sunnis.83 Laying these foundations or plausibility
structures is their greatest achievement; an achievement which was the result of the
proto-Sunnis’ uniquely satisfying solutions to the historical pressures they faced on one
hand, and the increasing prestige they enjoyed on the other.
The Roots and Achievements of the Early Proto-Sunni Movement: A Profile and Interpretation - Matthew J. Kuiper
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Yeah well i provided a link too but didn't bother with all the pretentious copy and paste.
And i noticed you highlighted "While much of the credit..."
Not all the credit because, from my link
The original split between Sunnis and Shiites occurred soon after the death of the Prophet Muhammad, in the year 632.