Since early humans did not possess watercraft or the ability to cross large bodies of water, they had to rely on dry land or shallow water to move from one area to the next. A lowered global sea level caused by massive amounts of water being locked up in glaciers produced the corridors needed for human dispersal. Islands in southeastern Asia opened up to early Homo erectus as the shallow continental shelf extending from mainland Asia was exposed. Exposure of the so-called Sunda Shelf would have formed "Sundaland," a large extension of the Malay Peninsula that linked the islands of today's Indonesian Archipelago (including the islands of Java and Borneo) with the Southeast Asian mainland. With present ocean floor conditions, a 30-meter drop in sea level would have linked Java with the Southeast Asian mainland.
The ancient geographical connection of Java and mainland Asia. Left: "Sundaland" or Sunda is the shallow seabed off the coast of mainland Southeast Asia that would have been dry land when lowered sea level exposed it about 1.8 million years ago. Arrows indicate the probable migration route of Homo erectus and other terrestrial animals. Right: Southeast Asia in modern times. Much of Sunda today lies under the shallow seas of island Southeast Asia.
Java is a part of an extended Southeast Asian peninsula that has a rich fossil record of Homo erectus. Sundanese Asian Homo erectus localities differ from their African and Eurasian counterparts in three significant respects. Stone tools, one of the hallmarks of Homo erectus in Africa and Eurasia were rare in Sundaland. This suggests that the tool assemblage used by Sundan Homo erectus probably centered on perishable materials and differed from that used in Africa and Eurasia. Secondly, the large mammal fauna of Sundaland on which Homo erectus preyed or scavenged had limited diversity, in stark contrast to the broad variety of large mammals known to have lived with Homo erectus in East Africa. Finally, Sundaland, a terrain of extensive seacoasts and muddy shorelines, presented very different habitats and resources from the inland areas occupied by Homo erectus in Africa and Eurasia.25 Central Javan localities such as the Sangiran Dome, Trinil, Kendungbrubus, and Perning (Mojokerto) preserve a range of lowland estuarine, deltaic, and riverine environments. The occurrence of volcanic rocks also points to the presence of nearby volcanic highlands. It is likely that if Homo erectus arrived in the area when sea level was 75 meters or more lower than it is today, a large east-flowing river system, the East Sunda River System, would have provided a resource-rich corridor from western Sundaland to the southern coastal region. Such a varied physiography would have sustained a diverse patchwork of plant and small animal communities that presented Homo erectus in Java with a range of ecological opportunities.
As the Pleistocene progressed there are indications that Southeast Asian Homo erectus became more distinct and isolated from populations to the west and north. They may have intermittently been cut off from global hominid gene flow by sea level changes for about a million years. This isolation may well have fostered the survival of Homo erectus or its little-changed descendant populations on Java much longer than in mainland Asia or the rest of the world.26 In the greater Africa-western Eurasia homi-nid population, a new species of hominid (Homo heidelbergensis) evolved, and this species moved throughout mainland Asia, replacing Homo erectus about a half a million years ago. This is the hypothesis of the "Out of Africa"27 replacement theory as applied to Homo erectus. In Java, Homo erectus may well have held on much longer, until as late as fifty thousand years ago, when the Ngandong hominids lived.28
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