The standard narrative that you read in the history books, as passed down through Islamic tradition and historiography, is that in the first decades of the 7th-century the religion of Islam was promulgated by Muhammad and his followers from the cities of Medina and Mecca. Muhammad brought the tribes and peoples of the Arabian peninsula together under the umbrella of Islam. After his death leadership of the community passed to the Caliphs. Under Umar in the second half of the fourth decade of the 600s, the armies of the Islam defeated both the Byzantines and Persians and began the process whereby they would conquer much of the ancient lands of the Near East for the new religion (and beyond, from the Indus to the Atlantic!).
This is what you also read explicitly outlined in Tim Mackintosh-Smith’s Arabs: A 3,000-Year History of Peoples, Tribes, and Empires. But, lurking in the shadows of the text are nods to the sort of revisionist narrative outlined in Tom Holland’s In the Shadow of the Sword: The Birth of Islam and the Rise of the Global Arab Empire. There are many flavors and shades of revisionism, with some scholars changing their views over time. But the major insight and argument of all of the disparate views are that Islam, as we understand it today, was not the religion of the Arabs who conquered the Near East in the ten years after 636. Rather, Islam developed in the decades after the conquest, as an Arab response to the threat of assimilation into the conquered peoples.
Mackintosh-Smith’s book is very expansive, and I suspect he didn’t want to get bogged down in discussions about the “origins of Islam.” I think he did an excellent job balancing the ethnic and cultural aspect of being Arab, with the centrality of Islam in Arab civilization being included but not overwhelming the narrative.
But the chapters on the Umayyads do nothing to dissuade you from the view that the early Arabs were actually on their way to being assimilated into the matrix of the Near East, in particular, the Greco-Aramaic culture ascendant in the Levant. The main seed for revision is the fact that in the early decades of Umayyad rule non-Muslim sources invariably fixate upon the ethnic element of the conquest, rather than the religious one. Byzantine Christian sources seem to indicate that the religion of the Arabs was a heresy of imperial Christianity, rather than a separate and distinct religion (this was a period when there were many heterodox Christian and quasi-Christian sects in the Near East).
My own view, weakly held, is that self-conscious Islamic identity as adherents to a separate and new religion is probably a feature of the decade around ~700 A.D. This is when we start to see archaeological references to Muhammad as the prophet of God, and, the Umayyads rapidly shift the apparatus of imperial control away from Byzantine precedents (e.g., the quick phasing out of Greek in the bureaucracy, and native Christians and Arabs stop worshipping in the same buildings in Damascus). More strongly held is my position that what we substantively associate with Islam qua Islam beyond identity is really developed and fleshed out during the Abbasid period, after 750 A.D.
Though I would grant the Muslim Arabs around 750 had had a clear self-conscious religious identity distinct from Christians, Jews, etc., for several generations, many of the aspects of Islam which we take for granted developed well after the group identity became crystallized. Only after the mawali reappropriation of the religion does Islam in full form emerge from Late Antiquity (e.g., the centrality of hadith, the institutional emergence of ulema, and the sharp sectarian lines separating Shia from Sunni). An analogy here might be the fact that Jewish Christians had a coherent sectarian identity in the decades after the death of Jesus, but once gentiles became numerically dominant within Christianity they transformed it in fundamental ways (e.g., Trinitarian theology seems far more elaborate and abstract than anything conceived of in the mind of St. Paul).
And yet it is unlikely that Islam was created as propagandistic fiction by the Umayyads. Rather, the expansive narrative presented by Mackintosh-Smith makes it clear that the early Muslims were appropriating and refashioning elements and currents of Arabian culture in the 6th and 7th centuries organically. For example, one of the alternative names for Allah turns out to derive from the name of God within an indigenous monotheistic religious movement in southern Arabia, which was an alternative to Christianity and Judaism in the 6th-century.
Which brings me to the historicity of Muhammad. There are tentative references to the prophet of Islam in very early non-Muslim sources, so I believe that the figure as depicted in orthodox Islam is drawn from a historic individual with broad biographical similarities to that described in the Koran and Islamic tradition. The world of Late Antiquity was filled with apocalyptic prophets and visionaries. We have extensive documentation of this from Muslims, who had to deal with revolts in Iran from syncretistic cults and movements that fused native Zoroastrian religion with Christian, Indian, and Muslim ideas, in the first few centuries of Islam.
The most likely scenario to me is that in the late 7th-century the heterodox monotheism of the Arabs fixated upon one of the most prominent early 7th-century prophetic warlord, and fashioned a distinct religion around this individual. The division between the Alids and the Umayyads very early on suggests to me that the centrality of Muhammad may have been motivated in part by this conflict, as it was a convenient way to reappropriate the prestige of the family of Muhammad, by universalizing his message and relevance outside of the context of lineage.
But for the purposes of this post, I want to focus on the fact that the descends of Muhammad are still alive around us. Broadly termed “Sayyids,” friends in the genetic genealogy community have confided to me that a branch of J1 is the modal haplotype among Arab Sayyids (interestingly, the haplotype in question is a “brother” lineage to that of the Cohens among Jews). If people who claim descent from Ali do seem to descend on the whole from a particular male who lived in the 6th and 7th centuries, then to me that definitely increases the veracity of the biographical elements of the prophet’s life in the Islamic story (in this case, it would be an ancestor of Muhammad and Ali, since they were paternal first cousins).
To my knowledge, the inference of a particular J1 as that of Muhammad has been assessed through surveying supposed descendants (in India 90% of supposed descendants of Muhammad are not even of the J1 haplogroup, but J1 is far more common among them than it is in India as a whole). But Muslims do not engage in cremation but bury their dead (and it is likely that most Arabs before Islam were already monotheists of some sort and buried their dead). This means that very early ancient DNA could be retrieved from individuals reputedly descended from Muhammad, or even of the broader Quraysh tribe.
Combined with the phylogeography of those who carry the very specific J1 haplotype of Muhammad and the Quraysh, one could probe the traditions of the emergence of the Muslim movement out of the Hejaz, or, revisionist contentions that it was North Arabian.
We have the technology.