Sundaland

Why this is extraordinary

The Sunda Shelf is the shallow continental floor under the Java Sea and the southern South China Sea. When the sea stands about 120 metres lower than today, most of that floor is dry. A 2000 map study, using present-day depth contours as stand-ins for old shores, draws that low at about 17,000 years ago. Picture the water pulled back. Sumatra, Java, and Borneo connect to each other and to mainland Southeast Asia. The newly connected shelf, including those three islands, exceeded 3.2 million square kilometres. South and east of the Isthmus of Kra alone, the study counts an additional 1.53 million square kilometres. Sulawesi stays separated by a narrow, very deep trench. The paper is explicit that the maps are approximations. Uplift, sinking, tides, and new sediment can move a contour, and over longer spans those movements are not small. Uplift cited for longer spans reaches 1.5 metres per thousand years.

At 100 metres and at 75 metres below present, the same study finds the shelf still largely joined. Major land connections are not lost until the sea has risen much further. The low stands, it says, were short compared with the long stretches when the sea was only moderately down, for example at or below 40 metres for more than half of the periods it considers. A drowned shelf is not only a picture of the coldest century.

A second 2000 paper, on the shelf’s own sediments, is a clock rather than an area. Hanebuth, Stattegger, and Grootes put the sea near 116 metres down by about 21,000 calendar years ago, and then a rise of as much as 16 metres within about 300 years, from 14.6 to 14.3 thousand calendar years ago. A later area count, using those stages, puts about 2.37 million square kilometres of shelf exposed at 116 metres down, and about 1.5 million still exposed at 50 metres down. That 2.37 million is the exposed shelf. The 3.2 million in the map study includes the three great islands. They are not the same rectangle, and this room does not add them together.

People on the islands today, and the Homo erectus beds at Sangiran, are the reason the shelf matters. A walk to Java needs dry ground. Whether camps stood on the plain that is now the Java Sea is mostly untested, because that plain is underwater. No source used here reports a dug settlement on the open shelf dated to the 120 metre low. The pin is that water, not a site. Wallace’s line still held. The shelf did not build a road to New Guinea. That crossing is the Sahul shelf.

The low sea is a geography people could have walked. It is not a dug camp on the floor of the Java Sea. If people walked that plain, the islands were one country for a while. If they did not, the geography is still real and the camps are untested. Ancient genomes document more than one movement into Southeast Asia, which can fill the islands without a single exodus off the glacial floor. The current narrative can teach the drowned land. It cannot yet draw a village on it.