So the origin of the Sinhalese and indeed the rest of the inhabitants of Śrī Laṅkā is a contentious issue that has wide reaching implications for politics both within and outside the island. It is also clouded by nationalist histories from multiple groups, with the Laṅkān Tamils and the Siṅhalese both asserting domination of the island, though both have elements of truth.

In all actuality, neither the Siṅhalese nor the Laṅkān Tamils are the original inhabitants of the island. Śrī Laṅkā has been consistently inhabited for some 34,000 years.^8 The oldest known culture was the Balangoda culture, who inhabited it from that early date until the iron age. We don't know the genetic or ethnic background of the Balangoda, however their features seem to have been close to the modern Vedda people.^6,7,8 However, I would hesitate to draw a direct link between neolithic and modern populations, as notions of ethnicity and identity are quite fluid. However, it is likely that the Vedda represent the remnants of the indigenous group that predated the Siṅhalese migrations to Śrī Laṅkā.

The Vedda speak a language that is largely an isolate with a Sinhalese superstrate, or rather, a creole based on that language.^2,7 As contact with the economically dominant Siṅhalese became more and more important, the creole was developed as a trade language and eventually became the dominant language of the Vedda People through a process of assimilation. The most assimilated of the Vedda follow Buddhism, but many still practice their indigenous religion and even those who practice Buddhism have some underlying religious beliefs based on the traditional Vedda way of life.^7,8 Their religion is follows a rather interesting mixture of ancestor veneration, shamanism, and a cult of the dead. The dead are believed to become yaku when they die and are given offerings while a priest or shaman can commune with them and channel them. Bad yaku are driven out with stones however appeasement is considered the best means and the newer dead are most revered. Certain yaku are especially revered through their deeds, becoming gods. There is some Indian influence on the religion, notably in concepts of Metta, Karma, and Merit which have entered the religious language, especially with regards to offerings, though it may be superficial.^7

The Siṅhalese appear to have arrived some time in the 5th century BCE, establishing a small kingdom in the east and quickly pushing the natives further inland.^1,3,8 The Mahāvaṃsa records that a Prince Vījaya arrived on the island after his exile from Vanga with a retinue of 700 attendants and defeated the Yakkas who ruled the island and married a Yakka princess. Regardless of the truth of the Vījaya story, it does seem to record some truth. Genetic studies of the Siṅhalese indicate that the largest contribution to their genetics comes from the region of modern day Bengal.^9 Furthermore, the Yakka are often identified with the Vedda people and the Vedda in their own histories assert this descent.^8 It is highly possible that this story represents some history of a prolonged colonization by people from Vanga into Śrī Laṅkā that where there was considerable mixing with the indigenous tribes. This is also reflected in the genetic legacy as the Vedda and Siṅhalese are the closest genetic groups on the island in the modern day and both groups share a not insignificant genetic link. They also bear some link to the populations of Gujarat and Malaysia, likely due to trading that has happened on the isle.^9 The Siṅhalese also adopted some customs from the Vedda people, including the spirit worship in the form of the cult of a local god called Bandar.^5

The Mahāvaṃsa records an invasion by the Tamils in the 2nd century, however, it is probable that they had some presence on the island before that, but left little material or genetic legacy until after the Siṅhalese became the dominant population on the island.^8 The Śrī Laṅkān Tamils are somewhat genetically distinct from their mainland and recent (200 year) brethren having mixed somewhat with other populations on the island.^9 A major import of Tamils came during the late tenth and eleventh centuries CE during the South Indian invasions of the isle, notable that of Rajaraja Coḷa, who overthrew the kingdom of Anurādhapura. Following this there was a large colonization of the Jaffna peninsula. This doesn't mean that Tamil culture wasn't already influential though, many in the north were already "Tamilized" to some extent and it is likely that Tamil was the major language of trade in the north by the tenth century.^5,8

The Siṅhalese bear a not insignificant Tamil genetic contribution and the Tamils, especially early on, often intermarried with the Siṅhalese people. However, the modern Śrī Laṅkān Tamil population bears less Siṅhalese contribution, due to there being a greater connection to the mainland following the Coḷa period.^9

Other groups arrived later and have had a smaller genetic and cultural impact, like the "Moors" (who are unrelated to the Berber descended rulers of Spain during the medieval period and are actually of Arab and Persian descent) and the Indian Tamils (Tamils imported as workers by the British in the 19th century). However they have also played a major role in the island's history.

Sources and further reading

1 Strathern, Alan. "The Vijaya Origin Myth of Sri Lanka and the Strangeness of Kingship." Past & Present, no. 203 (2009): 3-28.

2 Dharmadasa, K. N. O. "The Creolization of an Aboriginal Language: The Case of Vedda in Sri Lanka (Ceylon)." Anthropological Linguistics 16, no. 2 (1974): 79-106.

3 Biedermann, Zoltán, and Alan Strathern, eds. Sri Lanka at the Crossroads of History. London: UCL Press, 2017.

4 Peebles, Patrick. "Colonization and Ethnic Conflict in the Dry Zone of Sri Lanka." The Journal of Asian Studies 49, no. 1 (1990): 30-55. doi:10.2307/2058432.

5 Seligmann, C. G. "77. Note on the "Bandar" Cult of the Kandyan Sinhalese." Man 9 (1909): 130-34. doi:10.2307/2840268.

6 Allchin, Bridget. "The Late Stone Age of Ceylon." The Journal of the Royal Anthropological Institute of Great Britain and Ireland 88, no. 2 (1958): 179-201. doi:10.2307/2844251.

7 Seligman, C.G. and Seligman, Brenda. The Veddas. Cambridge: Cambridge University Press, 1911.

8 De Silva, KM A History of Sri Lanka. 2nd Edition. London: Penguin Books, 2005.

9 Ranaweera, Lanka et. all "Mitochondrial DNA history of Sri Lankan ethnic people: their relations within the island and with the Indian subcontinental populations." Journal of Human Genetics volume 59, 2014. 28-36.