Continental Shelves and Their Significance for Human Migration

Like desert greening, sea level change has undoubtedly been a major factor in allowing (or perhaps even forcing) humans to migrate. But how major? Were the Americas settled because the ice retreated, or because the falling sea level exposed the Beringian land bridge? Probably, both factors were important in Arctic circumstances. But sea level changes have been invoked in the past as major reasons for migration in warmer latitudes, especially during the Holocene drowning of the very extensive Sundaland continental shelf in Southeast Asia. During the last glacial maximum, Sundaland formed a 2.2 million km2 exposed subcontinent. The sea level rise of 120 m between 15,000 and 7000 years ago reduced this land mass to its present large but separate islands (Borneo, Sumatra, and Java being the largest), and in the process roughly doubled the extent of coastline (Dunn and Dunn 1977).

Several authors have regarded this relatively rapid drowning of Sundaland as a fundamental driver in the Palaeolithic colonization of other nearby regions such as the Philippines and western Oceania.32 Some have even related it to the human colonization of much of the world during the Holocene. For instance, Stephen Oppenheimer (1998) and Peter Watson (2012) hypothesize mass emigration due to rising sea levels out of Sundaland about 8000 years ago, giving rise to many significant populations, including even the early Sumerians of the Ubaid period in southern Iraq (5000-3000 Bc). According to such cataclysmic scenarios, people literally became washed off the land and conveniently invented boat technology as a result. But could it really have been as dramatic as this (Bellwood 2000)?

In fact, there is no archaeological, linguistic, or genetic evidence at all for such a major dispersal out of Sundaland during the postglacial period of sea level rise.33 It is quite true that the early Holocene rises in worldwide sea level were sometimes very rapid, particularly that of 60 m between 9650 and 5000 bc (Smith et al. 2011). But, by 4000 Bc, sea levels had become virtually stable, simply fluctuating within a few meters of the present. These were the conducive conditions that stimulated Pacific ancestors to invent efficient boats in coastal waters off southern China and in Southeast Asia. During Pleistocene periods of rapid sea level change, coastal hunters and gatherers would have found it very difficult to exploit coastal resources successfully Rising sea levels would have drowned such resources; falling sea levels would have exposed salt flats or coral cliffs. At times, the rates of change would have been so rapid that people would have been forced to move. But because the major oceans are linked worldwide their levels would have changed everywhere in relative unison, so coastlines everywhere would have been equally affected, allowing for localized factors such as isostasy and tectonic movement. There would be no reason in such circumstances to expect significant migration, except for moving further inland, nor indeed is there evidence for any.

Human groups on Sundaland presumably moved and adapted locally during such difficult times by emphasizing the exploitation of terrestrial resources, although it is rather difficult to demonstrate this archaeologically since sites close to the coastline that were occupied before 4000 Bc are naturally now under the sea. This problem applies in all continental shelf situations that were not glaciated. In glaciated regions,

Isostatic uplift kept pace with sea level rise as the ice melted, as discussed for Arctic North America in Chapter 4. Coastal archaeological sites there have remained above sea level. But these very cold regions played little role in the great demographic increases in late Holocene agricultural latitudes.