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Recent evidence, primarily from exploration for oil deposits, shows that the main landmass of the Philippines originated more than 50 million years ago as a series of "island arcs" far out in the Pacific Ocean. As the rocks beneath the sea were gradually squeezed between the Asian continent and the northward-moving Australian continent, which was then much farther south than it is today, parts of the sea-floor were uplifted, and others were thrust beneath the crust of the earth. The pressure and friction generated by this plate-tectonic movement produced undersea volcanoes that gradually rose above the waves. By about 30 million years ago some small but permanent islands protruded above sea level, and by 25 million years ago, several islands of at least 1,000 square kilometers had been established.
Australia continued to move northward and westward, with the pressure on the region between it and Asia forcing the precursors to the Philippine Islands to move toward Asia, resulting in still more volcanic activity. By 15 million years ago, this led to the creation of a large island of 25,000 to 50,000 square kilometers, with extensive highlands that included parts of what is presently northern Luzon. The modern southern Philippines still lay far to the south at this point, and included only a few, much smaller islands.
Southeast Asia assumed much of its current shape only about five million years ago. By this time, the modern highlands of northern Luzon were well-established. Another island reached from southern Luzon nearly to Mindanao, and Mindanao itself consisted of several separate islands that progressively merged. Small islands continued to appear throughout the archipelago, including the Sulu Islands. Palawan and Mindoro, the only parts of the Philippines that had originated as pieces of the Asian mainland, became isolated at this time as well, with Mindoro probably dropping entirely below sea level for a time.
Although many of the details in this story remain unclear, the broad picture has become evident for the first time. We now believe that the ancient geological history of the Philippines is largely responsible for its exceptional array of biological diversity. Because the islands arose many millions of years ago, independently of the Asian mainland, with the exception of Palawan and Mindoro, they have had adequate time and space to receive and shelter rare, over-water animal and plant pioneers. But why, in comparison to nearby countries with similar climates, are levels of diversity so high in the Philippines? And why do such small islands as Sibuyan have such extraordinary numbers of unique species, while some other islands of the same size in the Philippines have none? To answer these questions, we must turn to the more recent geological history of the archipelago.