Sundaland, also known as Sundaica or the Sundaic region, is an area in Southeast Asia that was connected to a larger landmass during times when sea levels were lower over the past 2.6 million years. This region includes the islands of Bali, Borneo, Java, and Sumatra in Indonesia, along with nearby smaller islands. It also includes the Malay Peninsula on the mainland of 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 covers an area similar to the 120-meter isobath, which is a line showing where the ocean is 120 meters deep. It includes 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 its largest size during the past.
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 animal life of Asia from that of Australasia. The islands east of the Wallace Line are called Wallacea, a region that belongs to Australasia. The Wallace Line runs through a deep ocean channel that has never been connected by land bridges. The northern edge of Sundaland is harder to define because a change in plant life patterns around 9ºN is considered its northern boundary.
Most of Sundaland was last exposed during the last ice age, from about 110,000 to 12,000 years ago. When sea levels were 30–40 meters lower, 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 much lower, so Borneo, Java, and Sumatra were often connected to the mainland rather than being islands. During the late Pliocene, sea levels were higher, and Sundaland’s exposed area was smaller than it is today. Sundaland began to sink underwater around 18,000 years ago and remained partially submerged until about 5000 BC. During the Last Glacial Maximum, sea levels dropped by about 120 meters, exposing the entire Sunda Shelf.
Modern climate
Sundaland is located entirely within the tropics, and the equator passes through the central parts of Sumatra and Borneo. In the tropics, rainfall is the main factor that causes differences in climate between regions, rather than temperature. Most of Sundaland is described as perhumid, or everwet, because it receives more than 2,000 millimeters of rain each year. Rainfall in this area is greater than the amount of water lost through plants and soil throughout the year, and there are no dry seasons that can be predicted, unlike other parts of Southeast Asia.
The warm and 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 provide a major heat source to the atmosphere. ENSO also greatly affects the climate of Sundaland. Strong ENSO events lead to droughts across Sundaland and tropical Asia.
Modern ecology
The heavy rainfall in Sundaland supports dense evergreen forests across its islands. As the islands move farther from the equator, these forests change to ones with trees that lose their leaves seasonally and open areas with grasses and shrubs. Most of the remaining untouched lowland forests are home to large dipterocarp trees and orangutans. After logging, the forest structure changes, with trees that do not need much shade becoming more common. Dipterocarps are known for producing large amounts of fruit at the same time, which helps protect the fruit from being eaten too quickly. At higher elevations, forests are shorter and mainly made up of trees from the oak family. Scientists often group Sundaland, the Philippines, Wallacea, and New Guinea into one region called Malesia because their plant life is similar and mostly from Asia.
During the last ice age, sea levels were lower, and all of Sundaland was connected to the Asian continent. This connection allowed many Asian animals, such as elephants, monkeys, apes, tigers, tapirs, and rhinoceroses, to live in Sundaland. When the sea levels rose and flooded the area, these animals became separated into different islands. 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 the Musi and Batanghari rivers in Sumatra. Different environmental factors on each island have caused the animals living there to change over time. The animals now found on each island are not just a small group from a larger Sundaland or Asian animal group, because not all animals that lived in Sundaland before the flooding were found across the entire region. Larger islands, like Borneo and Sumatra, have more types of land mammals than smaller islands.
History
The name "Sunda" has been known for a long time. It appears in Ptolemy's Geography, a book written around 150 AD. In 1852, English navigator George Windsor Earl suggested the idea of a "Great Asiatic Bank," based on similarities among mammals found in Java, Borneo, and Sumatra.
In the 1870s, explorers and scientists began measuring and mapping the seas of Southeast Asia, mainly using depth sounding. In 1921, Dutch geologist Gustaaf Molengraaff proposed that the nearly uniform sea depths of the shelf showed an ancient flat area, called a peneplain, formed by repeated flooding as ice caps melted. Each flooding event made the peneplain flatter. Molengraaff also found evidence of old river systems that drained the area when sea levels were lower.
The name "Sundaland" for the shelf was first suggested by Reinout Willem van Bemmelen in his Geography of Indonesia in 1949. His research was based on work he did during World War II. The ancient river systems described by Molengraaff were later mapped by Tjia in 1980 and studied in more detail by Emmel and Curray in 1982, who identified river deltas, floodplains, and backswamps.
Scientists have studied Sundaland's climate and ecology by analyzing tiny sea creatures (foraminifera), oxygen and carbon levels in cave deposits, pollen from ocean cores, and bat guano. They also used models of species distribution, genetic relationships, and community structures.
A very wet climate, called perhumid, has existed in Sundaland since the early Miocene. Although there were periods of drier conditions, Borneo remained wet. Fossil coral reefs from the late Miocene and early Pliocene suggest that the Indian monsoon became stronger, increasing seasonality in some areas. Pollen evidence from Sumatra shows that temperatures were cooler during the late Pleistocene, with high-elevation sites possibly 5°C cooler than today.
Most recent research agrees 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 (about 1,000 meters lower) than today, and glaciers existed on Borneo and Sumatra around 10,000 years ago. However, scientists disagree on how rainfall changed during the Quaternary. Some say rainfall decreased as sea levels dropped, while others argue that changes were small and not enough to affect precipitation.
One reason for disagreement is that climate conditions during the Last Glacial Maximum varied greatly across Indonesia. Another possibility is that methods used to study past rainfall, like analyzing oxygen levels, may not work the same way today. Some researchers also note that in very wet areas, water is not a limiting factor for plant life, making it hard to detect changes in rainfall using pollen records.
Sundaland, especially Borneo, has been a hotspot for biodiversity since the early Miocene due to repeated immigration and separation of species. The modern islands of Borneo, Java, and Sumatra have acted as refuges for Sundaland's plants and animals during past glacial periods and continue to do so today.
Dipterocarp trees, common in modern Southeast Asian rainforests, were present in Sundaland before the Last Glacial Maximum. Evidence also shows that savanna vegetation existed in now-submerged areas of Sundaland during the last glacial period. However, scientists disagree on how much of Sundaland was covered by savanna. Two main theories exist: (1) a continuous savanna corridor connected modern mainland Asia to Java and Borneo, and (2) Sundaland was mostly rainforest with only small savanna patches.
If a savanna corridor existed, it would have allowed savanna animals and early humans to move between Sundaland and the Indochinese region. This would have helped species evolve through separation and dispersal. Morley and Flenley (1987) and Heaney (1991) first suggested a savanna corridor through Sundaland based on pollen evidence. Other researchers, using modern species distributions, think that rainforests shrank and were replaced by savanna and open forest during glacial periods. Climate models show varying degrees of forest shrinkage, with some predicting a 50–150 kilometer wide savanna corridor running from the Malay Peninsula through Sumatra and Java to Borneo. Wurster et al. (2010) found evidence of savanna expansion using carbon isotope analysis in bat guano, and fossil mammal teeth also support the corridor theory.
In contrast, other scientists argue that Sundaland was mostly rainforest. Raes et al. (2014) used species models to suggest that Dipterocarp rainforests remained throughout the last glacial period. Others point to river systems with incised meanders, which would have been maintained by trees on riverbanks. Pollen records from sediment cores are conflicting: some show forest cover persisted, while others show increased savanna pollen. Wurster et al. (2017) found evidence that rainforest cover remained in some areas during the last glacial period. Some researchers suggest that soil type, not a savanna corridor, explains differences 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 elephantoid Sinomastodon bumiajuensis moved from mainland Asia to Sundaland. Later, tigers, Sumatran rhinoceroses, and Indian elephants lived across Sundaland, along with smaller animals that dispersed widely.
Human migrations
According to the most widely accepted theory, the ancestors of today's Austronesian people in Maritime Southeast Asia and nearby areas are believed to have moved south from East Asia's mainland to Taiwan and then to other parts of Maritime Southeast Asia. Another theory suggests that the now-underwater region called Sundaland might have been the birthplace of Austronesian languages, known as the "Out of Sundaland" theory. However, most experts in archaeology, linguistics, and genetics do not support this view. Instead, the "Out of Taiwan" model (but not necessarily 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, looked at DNA from the maternal side of families. It found that shared ancestry between Taiwan and Southeast Asia likely came from earlier migrations. These movements happened around the same time sea levels rose, possibly causing people to move from the Philippine Islands as far north as Taiwan within the last 10,000 years.
These migrations were likely caused by climate changes, including the flooding of an ancient landmass. Three major rises in sea levels may have submerged the Sunda continent, forming the Java and South China Seas and the many islands of Indonesia and the Philippines. As sea levels changed, people moved inland from their coastal homes. This forced movement led them to adapt to new environments, such as forests and mountains, by farming and raising animals. These early people became the ancestors of future populations in the region.
Stephen Oppenheimer believes the Austronesians originated in Sundaland and its surrounding areas. From the perspective of historical linguistics, the main island of Taiwan, also known as Formosa by Portuguese explorers, is considered the home of Austronesian languages. This island has the greatest differences among the native languages of the region, called Formosan languages.