During what geologists call the Pleistocene epoch (approximately 1,800,000 to 8000 B.C.), much of the earth is drastically affected by glacial cycles. These are intermittent periods of about 100,000 years (punctuated by shorter interglacial periods), when so much of the earth's water is locked up in ice sheets that it exposes large areas of land today totally under water or protruding as islands. In particular, the alternately rising and lowering sea level covers or exposes the continental shelf (known as the Sunda Shelf) that connects western Indonesia to the Malay peninsula. During those phases when there are land bridges, many animals—including early hominids (possibly the first Homo erectus in Asia) and the orangutan—are able to make their way down into territory as remote as Java and Sumatra. By about 18,000 B.C. the sea level begins to rise, until by about 6000 B.C. it has left the islands and land configurations of Southeast Asia much as they are today. Meanwhile, Homo sapiens has emerged in Indonesia about 40,000 B.C., and during the next several thousand years these humans pass through several cultural stages, distinguished primarily by their various stone tools. By about 3000 B.C. the complex of achievements known as the Neolithic culture begins to emerge in Indonesia.
1,800,000–1,200,000B.C.: A few scholars assign to this time the earliest hominid fossils (probably Homo erectus) found at Mojokerto (Perning), eastern Java, and Sangiran, eastcentral Java, but most do not accept such an early date.
1,200,000–750,000B.C.: Most scholars date to this time the earliest fossil hominid finds in Java. Included among them is the first Homo erectus fossil, found in Java in 1891 by Eugene Dubois, a Dutch anatomist, and dated to about 800,000 B.C. The Homo erectus whose fossils are found on Java over the years is often known as Java Man.
750,000–100,000B.C.: Some experts believe that simple stone tools found in several of the Indonesian islands were made by Homo erectus, at least during the latter part of this period. (Archaeologists call the time from about 500,000 to 15,000 B.C. the Paleolithic or Old Stone Age.)
100,000–40,000B.C.: Fossil finds (known as