One of the strangest aspects of human history is the fact that periodically groups on the margins seem to rise to the fore and enter into a phase of rapid expansion into virgin territory. By “virgin” I don’t necessarily mean uninhabited, but rather virgin in relation to the mode of production which defines the expansionary group. A classic illustration by this is the rise of the Anglo-Saxon Diaspora between 1600 and 1900, as it settled territories inhabited by other populations at much lower population densities. The Bantu Expansion is another case in point. What you see in both cases is the migration of a population which has found a way to produce more calories per unit of land, and the weight of numbers resulted in the marginalization and/or absorption of the native populations, to varying degrees. In the Anglo North America and Oceania the admixture of indigenous ancestry is relatively low, at least into European populations. In East and Southern Africa the admixture of non-Bantu populations is definitely somewhat higher.
This dynamic has old roots in our lineage. It goes back at least to the rise of modern humanity on the fringe of Africa 50 to 100 thousand years ago, and its subsequent expansion across the world (with some assimilation of older hominin lineages). A more recent case is the Austronesian expansion out of Taiwan, which encompasses a longitudinal gradient from East Africa all the way to South America, and a latitudinal one from Hawaii to New Zealand. Even today I suspect people would be impressed by this, but it is all the more amazing when you observe that modern humans seem to have stabilized their range in Near Oceania for ~30,000 years. Unlike the “first farmers” of the Middle East the expansion of the Austronesians had less to do with a mode of production, than pioneering navigational skills and a lack of all sanity and rationality when it came to venturing across great expanses of water.
The question of why a small group of Southeast Asian people in Taiwan began to move in a manner which would trigger a world-wide cultural and demographic revolution is still an open one. But a second issue which can be explored is the nature of who these seafarers came into contact with. Of course most of the discussion has been around the uptake of Melanesian admixture in Near Oceania. A second question for me has always been the nature of the dominance of Austronesians in maritime Southeast Asia. Basically, Indonesia and Malaysia. The mainland of Southeast Asia was dominated by Austro-Asiatic peoples until the arrival of Tai, Miao, and Tibeto-Burman groups over the past few thousand years. Did Austro-Asiatics populate maritime Southeast Asia at one point? A preprint on bioRxiv aims to explore this question, Reconstructing Austronesian population history in Island Southeast Asia:
Austronesian languages are spread across half the globe, from Easter Island to Madagascar. Evidence from linguistics and archaeology indicates that the “Austronesian expansion,” which began 4-5 thousand years ago, likely had roots in Taiwan, but the ancestry of present-day Austronesian-speaking populations remains controversial. Here, focusing primarily on Island Southeast Asia, we analyze genome-wide data from 56 populations using new methods for tracing ancestral gene flow. We show that all sampled Austronesian groups harbor ancestry that is more closely related to aboriginal Taiwanese than to any present-day mainland population. Surprisingly, western Island Southeast Asian populations have also inherited ancestry from a source nested within the variation of present-day populations speaking Austro-Asiatic languages, which have historically been nearly exclusive to the mainland. Thus, either there was once a substantial Austro-Asiatic presence in Island Southeast Asia, or Austronesian speakers migrated to and through the mainland, admixing there before continuing to western Indonesia.
In the discussion the authors clear come down on the side that Austronesian and Austro-Asiatic admixture occurred prior to the settlement of maritime Southeast Asia. Though their marker set couldn’t infer timing of admixture event the relatively evenness of admixture in western Southeast Asia and the archaeological evidence seem to point to the idea that Austro-Asiatic speakers did not push past peninsular Malaysia (where there are Austro-Asiatic speakers in the interior among the Negrito populations). To me this has always struck me as strange, because obviously the island of Java has been amenable to widespread rice farming, and Indonesia today is as populous as all of mainland Southeast Asia. But it seems that the spread of populations over water can be highly contingent, and not inevitable. For example barbarian incursions into mainland Italy often stopped at the straits which separated the continent from Sicily, and the Vandal adoption of seafaring has always been somewhat mysterious. Though there has been gene flow across Gibraltar, it does seem that the existence of a water barrier has resulted in a major genetic discontinuity. And yet tens of thousands of years ago in prehistory the ancestors of the Australian and Melanesian peoples crossed from Sundaland to Sahul.