For nearly 1,000 years the kingdom of Champa occupied the coast of modern Vietnam, from Annam down to the Mekong delta. It was a maritime facing polity whose language was Austronesian. The Chams were closely connected to the people of Maritime Southeast Asia, and part of the broader Indic zone. They were Hindu. Unlike other people in the region, they never fully converted to Theravada Buddhism.
Today most Chams are Muslims due to contact with Islamicized Malays. But a minority in Vietnam remain Hindu. The reason for this is tragic and contingent. The final conquest and absorption of Champa by the expanding Vietnamese in the early modern period occurred during a period of Islamicization. Though the king of the Chams remained Hindu until the end, the court around him was converting to Islam. If the Vietnamese had not conquered the Chams and isolated them from the rest of the world, it is likely all of them would have converted to Islam.
As it is, a Vietnamese program of periodic genocide and forced assimilation produced an involuted Cham culture. That is, it turned inward and became a fossil society. The goal was survival. They stood in place.
Today the Cham are Saivite Hindus in Vietnam, with some elements of the varna system preserved.
This brings us to the question: how did Indian religious and cultural forms become entrenched on the coast of Vietnam 2,000 years ago? Two new preprints shed light on this, Reconstructing the human genetic history of mainland Southeast Asia: insights from genome-wide data from Thailand and Laos, and Indian genetic heritage in Southeast Asian populations. The second preprint is obviously more explicit.
Figure 5 is clear:
From the discussion:
355 Our study revealed substantial South Asian admixture in various populations across Southeast Asia (~2-16% as inferred by qpAdm). We observed South Asian admixture in some populations (Cham, Ede, Giarai, Khmer, Kuy, Nyahkur, and Thai) for whom the admixture was not reported before. Most populations harboring South Asian admixture were heavily influenced by Indian culture in the past or are related to descendants of ancient Indianized states in Southeast Asia. In contrast, we failed to detect South Asian admixture in most “hill tribes” and in present-day hunter-gatherer groups from Thailand. Consequently, the spread of Indian influence in the region can be explained by extensive movement of people from India rather than by cultural diffusion only.
The genetics suggest that the gene flow had to have happened between 0 and 1000 AD. This is basically prehistory in Southeast Asia, as most of our information comes from references in Chinese records. But the presence of R1a in particular across the region indicates to me serious elite migration from India during this period.
In 2000 BC the Sintashta warlord Indra* huddled in his tents some of the Volga, on the cold and open steppe. 2,000 years later his direct paternal descendants were overseeing a nascent thalassocracy on the South China Sea. The history of humanity is more wonderous than anything imagined in your philosophy Horatio…
* I am aware that he is likely a BMAC deity. This is a joke.