1) The Philippine islands were formed by tectonic pressure from the collision of Australia and Asia, which caused parts of the seafloor to uplift and form volcanoes. 2) Evidence from vertebrate species shows similarities between islands that were likely once connected during lower sea levels, and differences between islands that have been isolated longer. 3) A radiolarian biostratigraphic study of an ophiolite in Luzon provides evidence that the Philippines were built upon ancient oceanic crust, supporting the land bridge theory of the islands' origins.

1. The Origin of the
Philippine Islands

2. Tectonic and Volcanic Origin of
the Philippines
As Australia drifted northward and began to collide with Asia, tectonic
pressure caused parts of the Pacific seafloor to uplift, producing
volcanoes that gave rise to the Philippine islands.

3. Evidence for Volcanic and Land
Bridge Theories
 Palawan became contiguous with Borneo when sea level dropped repeatedly
during the Pleistocene, accounting for Palawan’s biotic similarity to the
former.
 The isolation of the Philippines from the rest of Asia allowed it to develop
its unique flora and fauna, thus explaining its staggering levels of endemism.
Moreover, many species are not only endemic to the Philippines but unique
to individual Philippine islands.
 Other islands, like Negros and Panay, share faunal species which surprisingly
are different from those on other adjacent Visayan islands such as Leyte and
Samar, whose fauna are, in turn, more similar to those of Mindanao.
Masbate’s fauna are also more related to Negros and Panay’s than they are
to the more proximal Luzon.
 These patterns were observed in many vertebrates including birds (Peterson
et al. 2000), amphibians (Evans et al. 2003), mammals (Heaney and
Regalado 1998; Steppan et al 2003; Roberts 2006; Esselstyn and Brown
2009), reptiles (McGuire & Heang 2001), and fishes (Carpenter and Springer
2005).

4. Land Bridge theory evidence:
Ophiolites are pieces of oceanic plate that have been thrusted
(obducted) onto the edge of continental plates. They provide
models for processes at mid-ocean ridges.
Radiolarian biostratigraphic study conducted in Luzon
Radiolarians are protozoans found in the
upper layers of all oceans. The skeletal
remains of radiolarians settle to the ocean
floor and form radiolarian ooze. When the
ocean bottom is lifted and transformed into
land, the ooze becomes sedimentary rock.
Silica deposits, such as flint, chert, and the
abrasive tripoli, originate from radiolarian
skeletons. Fossil radiolarians have been found
that date to Precambrian Time (3.96 billion to
540 million years ago).

5. Radiolarian biostratigraphic study
conducted in Luzon
Radiolarian biostratigraphic
results from the Casiguran
Ophiolite provide additional
evidence for the existence of
Mesozoic oceanic substratum
upon which Luzon and
neighboring regions within the
Philippine archipelago were
likely built. Interestingly, the
result closely resembles those
reported for the ophiolite in
southeastern Luzon as well as
the oceanic crust of the
Huatung Basin situated east of
Taiwan and the ophiolites in
eastern Indonesia.