Post–Ice Age sea-level rise has been shown to do more than reshape the map of Southeast Asia, it also triggered large-scale human migration.
A research team from Nanyang Technological University (NTU), Singapore, integrating paleogeographic reconstructions with an analysis of 763 genomes from 59 Asian ethnic groups, demonstrates that the submergence of more than 50% of the Sundaland landmass coincided with major population splits and human dispersal to other parts of Asia.
These findings were published in Communications Biology.
When the Region Was Still Connected
Around 26,000 years ago, at the peak of the Last Glacial Maximum (LGM), Sumatra, Java, Borneo, and the Southeast Asian Peninsula were not separated by seas.
Together, they formed a vast continuous landmass known as Sundaland, which had been inhabited by modern humans since approximately 70,000–50,000 years ago.
Major changes began with the onset of the last phase of global warming. Between about 22,000 and 6,000 years ago, global sea levels rose by roughly 130–135 meters.
This rise was not gradual, but punctuated by two particularly rapid phases: around 14,500–14,000 years ago, with rates reaching up to 46 millimeters per year, and around 11,500–11,000 years ago, with rates of up to 22 millimeters per year.
The impact was profound. More than half of Sundaland was submerged. After the first rapid sea-level rise, the land area of Southeast Asia shrank by about 18%; following the second phase, it declined by a further 19%.
Land bridges that had once connected major regions disappeared, permanently transforming the landscape.
When Land Loss Reshaped Populations
To assess the human consequences of these changes, the study analyzed 763 high-resolution genomes from 59 ethnic groups across Southeast Asia and South Asia. The results indicate that major population separations occurred around 15,000–13,000 years ago, precisely during the periods of the fastest sea-level rise.
As landmasses fragmented and formerly continuous habitats were divided by rising seas, human groups that had once been connected became isolated. Demographic models based on whole-genome sequencing reveal that several population-splitting events took place almost simultaneously during this time.
The population structure that emerged from these processes later became the foundation of modern human diversity in Southeast Asia.
Shrinking Land Forced Human Migration
When more than 50% of the Sundaland landmass was submerged, the space available for human habitation contracted dramatically. Yet during the same period, population size increased four- to sevenfold compared to the peak of the Ice Age.
In other words, as habitable land diminished, far more people were forced to share increasingly limited space.
Across insular Southeast Asia, land area declined by about 45% between 20,000 and 11,000 years ago. As a result, population density rose by at least 8.6 times. This pressure intensified after the two fastest phases of sea-level rise recorded during that period.
It was this ecological strain that drove human groups to migrate out of the former Sundaland region toward mainland Southeast Asia and South Asia between roughly 12,000 and 8,000 years ago.
In this sense, human dynamics in Southeast Asia at the end of the Ice Age cannot be separated from global climate change. The submergence of Sundaland was not merely a geological event, but a pivotal moment that helped shape the demographic map of Asia as we know it today.
