The Swahili people originate from Bantu inhabitants of the coast of Southeast Africa, in Kenya, Tanzania and Mozambique. These Bantu-speaking agriculturalists settled the coast at the outset of the first millennium. Archaeological finds at Fukuchani, on the north-west coast of Zanzibar, indicate a settled agricultural and fishing community from the 6th century CE at the latest. The considerable amount of daub found indicates timber buildings, and shell beads, bead grinders, and iron slag have been found at the site. There is evidence for limited engagement in long-distance trade: a small amount of imported pottery has been found, less than 1% of total pottery finds, mostly from the Gulf and dated to the 5th to 8th century. The similarity to contemporary sites such at Mkokotoni and Dar es Salaam indicate a unified group of communities that developed into the first center of coastal maritime culture. The coastal towns appear to have been engaged in Indian Ocean trade at this early period, and trade rapidly increased in importance and quantity between the mid-8th and the 11th century.
Many Swahili claim a Shirazi origin. This forms the basis of the Shirazi era origin myth that proliferated along the coast at the turn of the millennium. Modern scholarship has rejected the veracity of these claims. The most likely origin for the stories about the Shirazi is from Muslim inhabitants of the Lamu archipelago who moved south in the 10th and 11th centuries. They brought with them a coinage tradition and localized form of Islam. These Africans migrants seem to have developed a concept of Shirazi origin as they moved further southwards, near Malindi and Mombasa, along the Mrima coast. The longstanding trade connections with the Persian gulf gave credence to these myths. In addition, because most Muslim societies are patrilineal, one can claim distant identities through paternal lines despite phenotypic and somatic evidence to the contrary. The so-called Shirazi tradition represents the arrival of Islam in these eras, one reason it has proven so long lasting. Extant mosques and coins demonstrate that the "Shirazi" were not Middle Eastern immigrants, but northern Swahili Muslims. They moved south, founding mosques, introducing coinage and elaborately carved inscriptions and mihrabs. They should be interpreted as indigenous African Muslims who played the politics of the Middle East to their advantage. Some still use this foundation myth a millennium later to assert their authority, even though the myth's context has long been forgotten. The Shirazi legend took on new importance in the 19th century, during the period of Omani domination. Claims of Shirazi ancestry were used to distance locals from Arab newcomers, since Persians are not viewed as Arabs but still have an exemplary Islamic pedigree. The emphasis that the Shirazi came very long ago and intermarried with indigenous locals ties this claim to the creation of convincing indigenous narratives about Swahili heritage without divorcing it from the ideals of being a maritime-centered culture.
There are two main theories about the origins of the Shirazi subgroup of the Swahili people. One thesis based on oral tradition states that immigrants from the Shiraz region in southwestern Iran directly settled various mainland ports and islands on the eastern Africa seaboard beginning in the tenth century. By the time of the Persian settlement in the area, the earlier occupants had been displaced by incoming Bantu and Nilotic populations. More people from different parts of the Persian Gulf also continued to migrate to the Swahili coast over several centuries thereafter, and these formed the modern Shirazi. The second theory on Shirazi origins also posits that they came from Persia, but first settled in the Horn of Africa. In the twelfth century, as the gold trade with the distant entrepot of Sofala on the Mozambique seaboard grew, the settlers are then said to moved southwards to various coastal towns in Kenya, Tanzania, northern Mozambique and the Indian Ocean islands. By 1200 AD, they had established local sultanates and mercantile networks on the islands of Kilwa, Mafia and Comoros along the Swahili coast, and in northwestern Madagascar.
The modern Swahili people speak the Swahili language as a mother tongue, which belongs to the Bantu branch of the Niger-Congo family. The language contains loan words from Arabic and Persian.