Sundaland, also known as Sundaica or the Sundaic region, is an area in Southeast Asia that was part of a larger landmass that was above water when sea levels were lower over the past 2.6 million years. It includes the islands of Bali, Borneo, Java, and Sumatra in Indonesia, along with nearby smaller islands, as well as the Malay Peninsula in Mainland Southeast Asia.
Extent
The area of Sundaland includes the Sunda Shelf, a stable part of Southeast Asia’s continental shelf that was above water during ice ages in the last 2 million years. It is also about the same size as the Sunda Plate.
The Sunda Shelf’s size is similar to the line showing the depth of the sea at 120 meters. It covers the Malay Peninsula, the islands of Borneo, Java, and Sumatra, as well as the Java Sea, the Gulf of Thailand, and parts of the South China Sea. Sundaland’s total area is about 1,800,000 square kilometers. Over the past 2 million years, the amount of land in Sundaland has changed greatly. Today, the land area is about half of what it was at its largest.
The western and southern edges of Sundaland are marked by the deep Sunda Trench and the Indian Ocean. The eastern edge is the Wallace Line, a boundary discovered by Alfred Russel Wallace that separates the land mammal species of Asia from those of Australia. Islands east of the Wallace Line, called Wallacea, are part of the Australasian region. The Wallace Line is a deep underwater area that has never been connected by land bridges. The northern edge of Sundaland is harder to define, but a change in plant life around 9ºN is considered its northern boundary.
Most of Sundaland was most recently above water during the last ice age, about 110,000 to 12,000 years ago. When sea levels dropped by 30 to 40 meters or more, land bridges connected Borneo, Java, and Sumatra to the Malay Peninsula and mainland Asia. For most of the past 800,000 years, sea levels were lower, so Borneo, Java, and Sumatra were often connected to land, not islands. In contrast, during the late Pliocene, sea levels were higher, and Sundaland’s exposed area was smaller than it is today. Sundaland began to be covered by water around 18,000 years ago and remained submerged until about 5000 BC. During the Last Glacial Maximum, sea levels dropped by about 120 meters, and the entire Sunda Shelf was exposed.
Modern climate
Sundaland is located entirely within the tropics, where the equator passes through central parts of Sumatra and Borneo. In the tropics, rainfall, not temperature, is the main factor that causes differences between regions. Most of Sundaland is classified as very wet, receiving more than 2,000 millimeters of rain each year. Rainfall in these areas is greater than the amount of water lost through evaporation and plant transpiration throughout the year, and there are no dry seasons like those found in other parts of Southeast Asia.
The warm, shallow seas of the Sunda Shelf (averaging 28 °C or higher) are part of the Indo-Pacific Warm Pool and Western Pacific Warm Pool. These seas play an important role in the Hadley circulation and the El Niño-Southern Oscillation (ENSO), especially in January when they add significant heat to the atmosphere. ENSO also strongly influences Sundaland's climate. Strong ENSO events cause droughts across Sundaland and tropical Asia.
Modern ecology
The heavy rainfall in Sundaland supports dense evergreen forests across its islands. As latitude increases, these forests change to deciduous forests and savanna woodlands. The remaining untouched lowland forests are famous for large dipterocarp trees and orangutans. After logging, these forests become dominated by trees and shrubs that do not need much shade. Dipterocarps are known for producing large amounts of fruit at the same time, which happens unpredictably and helps trees avoid being eaten too much by animals. Higher elevation forests are shorter and mostly made up of trees from the oak family. Botanists often group Sundaland, the Philippines, Wallacea, and New Guinea into one plant region called Malesia because their plant life is similar and mostly from Asia.
During the last glacial period, sea levels were lower, and Sundaland was connected to the Asian continent. This connection allowed Asian mammals like elephants, monkeys, apes, tigers, tapirs, and rhinoceroses to live there. When the area flooded and became islands, these species were separated. For example, the river threadfin fish (Polydactylus macrophthalmus, Bleeker 1858) once lived in a river system now called the "North Sunda River" or "Molengraaff River." Today, it is found in the Kapuas River on Borneo and in the Musi and Batanghari rivers on Sumatra. Different environmental pressures on each island caused species to change over time, leading to different groups of mammals on each island. However, the current animals on each island are not just a small part of a larger Sundaland or Asian group, because not all species that lived in Sundaland before flooding were found across the entire Sunda Shelf. The number of land mammal species on each island is related to its size, with the largest islands, Borneo and Sumatra, having the most diverse groups of mammals.
History
The name "Sunda" has been known for a very long time, first appearing in a book called Geography written by Ptolemy around 150 AD. In 1852, an English navigator named George Windsor Earl suggested the idea of a "Great Asiatic Bank" based on similarities among mammals found in Java, Borneo, and Sumatra.
Starting in the 1870s, explorers and scientists began measuring and mapping the seas of Southeast Asia, mainly by using a method called depth sounding. In 1921, a Dutch geologist named Gustaaf Molengraaff proposed that the nearly flat sea depths of the shelf showed evidence of an ancient land surface shaped by repeated flooding as ice caps melted. He also identified old river systems that once drained the area during times when sea levels were lower.
The name "Sundaland" for the shelf was first used by Reinout Willem van Bemmelen in his 1949 book Geography of Indonesia, based on his research during World War II. Molengraaff’s findings about ancient river systems were confirmed and mapped by Tjia in 1980 and described in more detail by Emmel and Curray in 1982, including features like river deltas, floodplains, and backswamps.
Scientists have studied the climate and environment of Sundaland by analyzing clues from ocean bed cores, cave deposits, and other methods. These include studying tiny ocean creatures called foraminifera, oxygen and carbon levels in cave formations, and bat guano. They also use models of species distribution and genetic relationships to understand changes over time.
A very wet climate has existed in Sundaland since the early Miocene, though some periods were drier. Fossil coral reefs from the late Miocene and early Pliocene suggest that the Indian monsoon became stronger, increasing seasonal changes in parts of Sundaland. Pollen evidence from Sumatra shows that temperatures were cooler during the late Pleistocene, possibly 5°C cooler than today at high elevations.
Most recent research shows that sea surface temperatures in the Indo-Pacific were at most 2-3°C lower during the Last Glacial Maximum. Snow levels were much lower than today, about 1,000 meters lower, and glaciers may have existed on Borneo and Sumatra around 10,000 years ago. However, scientists disagree about how rainfall patterns changed over time. Some say rainfall decreased as sea levels dropped, while others argue that changes were minimal and not enough to reduce rainfall.
One reason for disagreement is that the climate during the Last Glacial Maximum may have varied greatly across Indonesia. Other researchers suggest that methods used to study past rainfall may not have worked the same way in the past. Some scientists also note that in very wet environments like Sundaland, water is not a limiting factor for plant life, making it hard to detect rainfall changes using pollen records.
Sundaland, especially Borneo, has been a key area for the evolution of many species since the early Miocene. The modern islands of Borneo, Java, and Sumatra have acted as safe areas for plants and animals during past ice ages and continue to do so today.
Dipterocarp trees, common in modern Southeast Asian rainforests, existed in Sundaland before the Last Glacial Maximum. Evidence also shows that savanna-like vegetation was present in now-submerged areas during the last glacial period. However, scientists disagree about how much savanna existed. Two main theories exist: (1) a continuous savanna corridor connected modern mainland Asia to Java and Borneo, and (2) tropical rainforests dominated Sundaland, with only small savanna patches.
If a savanna corridor existed, it would have allowed savanna-dwelling animals and early humans to move between Sundaland and the Indochinese region. This could have led to new species forming through isolation and dispersal. Scientists like Morley and Flenley (1987) and Heaney (1991) first suggested a savanna corridor existed during the last glacial period based on pollen evidence. Other researchers, using modern species distribution, suggest that rainforests shrank and were replaced by savanna and open forest during this time. Climate models show varying degrees of forest reduction, with some studies suggesting a 50–150 km wide savanna corridor ran from the Malay Peninsula through Sumatra and Java to Borneo. Evidence from bat guano and fossil mammal teeth also supports the existence of this corridor.
In contrast, other scientists argue that Sundaland was mostly covered by rainforest. Studies using species distribution models suggest that Dipterocarp rainforests remained throughout the last glacial period. Some evidence, like river meanders, suggests trees grew along riverbanks, maintaining open areas. Pollen records from sediment cores are conflicting: some show forests persisted, while others show savanna plants increased during glacial periods. More recent studies using bat guano isotopes found that rainforests remained in some areas during the last glacial period. Some researchers suggest differences in soil types, like sandy seabed soils, may explain variations in species distribution.
Before Sundaland emerged during the late Pliocene and early Pleistocene (~2.4 million years ago), there were no mammals on Java. As sea levels dropped, species like the dwarf elephant Sinomastodon bumiajuensis moved from mainland Asia into Sundaland. Later, animals such as tigers, Sumatran rhinoceros, and Indian elephants lived across Sundaland, along with smaller species that could move freely across the region.
Human migrations
According to the most widely accepted theory, the ancestors of modern Austronesian people in Maritime Southeast Asia and nearby areas are believed to have moved south from East Asia to Taiwan and then to other parts of Maritime Southeast Asia. Another theory suggests that the now-submerged landmass called Sundaland might be the origin of Austronesian languages, known as the "Out of Sundaland" theory. However, most experts in archaeology, linguistics, and genetics do not support this idea. The "Out of Taiwan" model, though not always the "Express Train Out of Taiwan" model, is supported by the majority of researchers.
A study from Leeds University, published in Molecular Biology and Evolution, examined genetic information passed from mothers to children. It found that shared ancestry between Taiwan and Southeast Asia likely resulted from earlier migrations. These movements happened around the same time that sea levels rose, possibly causing people to move from the Philippine Islands to as far north as Taiwan over the last 10,000 years.
Population migrations were likely caused by climate change, including the flooding of an ancient continent. Rising sea levels occurred in three major waves, which may have submerged the Sunda continent, forming the Java and South China Seas and the many islands in Indonesia and the Philippines today. These changes in sea levels forced people to leave their coastal homes and move inland across Southeast Asia. This migration led them to adapt to new environments, such as forests and mountains, by starting to farm and raise animals, becoming ancestors of future populations in these regions.
Stephen Oppenheimer believes the origin of the Austronesians is in Sundaland and its surrounding areas. From the perspective of historical linguistics, the main island of Taiwan, also known as Formosa, is considered the home of Austronesian languages. This island has the most distinct language groups among the native Formosan languages.