Sundaland, also known as Sundaica or the Sundaic region, is an area in Southeast Asia that was connected to a larger landmass 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 small 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 over the past 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 on a map 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. The total area of Sundaland 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.

The western and southern edges of Sundaland are marked by the deep Sunda Trench, one of the deepest parts of the ocean, and the Indian Ocean. The eastern edge is the Wallace Line, a boundary discovered by Alfred Russel Wallace that separates the land animals of Asia from those of Australasia. Islands east of the Wallace Line, called Wallacea, are part of Australasia. The Wallace Line follows a deep ocean channel that never had land bridges. The northern edge of Sundaland is harder to define using ocean depth measurements, but a change in plant life patterns near 9ºN is considered its northern boundary.

Most of Sundaland was recently 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 much of the past 800,000 years, sea levels were lower, so Borneo, Java, and Sumatra were often connected to land. In contrast, during the late Pliocene, sea levels were higher, and Sundaland had less exposed land than it does 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, and the entire Sunda Shelf was exposed above water.

Modern climate

Sundaland is located entirely within the tropics, where the equator passes through central Sumatra and Borneo. In the tropics, rainfall, not temperature, is the main factor that causes differences in climate across regions. Most of Sundaland has a perhumid, or everwet, climate, receiving more than 2,000 millimeters of rain each year. Rainfall in this area is greater than the amount of water lost through evapotranspiration 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, which average 28°C or higher, are part of the Indo-Pacific Warm Pool and the Western Pacific Warm Pool. These seas play an important role in driving the Hadley circulation and the El Niño-Southern Oscillation (ENSO), especially in January when they act as a major heat source for the atmosphere. ENSO also strongly affects the climate of Sundaland. Strong positive ENSO events lead to droughts across Sundaland and tropical Asia.

Modern ecology

The heavy rainfall in Sundaland supports dense forests with tall, leafy trees 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 scattered trees and grasses. Most of the remaining untouched lowland forests are known for very large dipterocarp trees and orangutans. After logging, the forest structure and types of plants change, with more trees and shrubs that cannot grow in shaded areas. Dipterocarps are famous for sudden, large fruiting events that happen at unexpected times, which helps the trees avoid being eaten by animals. At higher elevations, forests are shorter and mostly made up of trees from the oak family. Botanists group Sundaland with nearby regions like the Philippines, Wallacea, and New Guinea into one plant region called Malesia because their plant life shares many similarities, 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 there. When the area flooded and became islands, these animals 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 changes in the types of animals living there, leading to unique groups of mammals on each island. However, the animals found on each island are not just a smaller group of animals from a larger Sundaland or Asian group, because not all animals that lived in Sundaland before it flooded were found across the entire Sunda Shelf. Larger islands, like Borneo and Sumatra, have more types of land mammals than smaller islands.

• Sumatran tropical pine forests (Sumatra)
• Kinabalu montane alpine meadows (Borneo)
• Sunda Shelf mangroves (Borneo, Sumatra, Riau Islands)

History

The name "Sunda" has been used for a 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.

In the 1870s, explorers and scientists began measuring and mapping the seas of Southeast Asia, often using a method called depth sounding. In 1921, a Dutch geologist named Gustaaf Molengraaff proposed that the flat, uniform sea depths of the shelf showed an ancient flat area called a peneplain. He believed this flat area formed over time as ice caps melted and flooded the region repeatedly. Molengraaff also found evidence of old rivers and drainage systems that once drained the area when sea levels were lower.

The name "Sundaland" for the shelf was first suggested in 1949 by Reinout Willem van Bemmelen in his book Geography of Indonesia. His research was based on studies he did during World War II. Later, in 1980, a scientist named Tjia confirmed and mapped the ancient drainage systems Molengraaff described. In 1982, researchers Emmel and Curray provided more details about these systems, including river deltas, floodplains, and backswamps.

Scientists have studied the climate and environment of Sundaland by analyzing clues from ocean bed cores, cave formations, and bat guano. These clues include chemical markers from tiny ocean creatures, pollen from plants, and data about species distribution and genetic relationships.

Since the early Miocene, Sundaland has had a very wet climate. Even though there were periods of drier weather, Borneo remained wet. Fossil coral reefs from the late Miocene and early Pliocene show that as the Indian monsoon became stronger, some parts of Sundaland had more seasonal weather. Pollen evidence from Sumatra suggests 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 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 changed during the Quaternary. Some believe rainfall decreased as sea levels dropped with ice sheets expanding, while others argue that rainfall changes were small and not enough to affect the region.

One reason for disagreement about climate changes is that the Last Glacial Maximum had different climates across Indonesia. Another reason is 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 areas, it is hard to use plant records to detect changes in rainfall because water is not a limiting factor for plant life.

Sundaland, especially Borneo, has been a hotspot for biodiversity since the early Miocene due to repeated migrations and changes in land connections. Modern islands like 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, which are common in modern Southeast Asian rainforests, existed in Sundaland before the Last Glacial Maximum. Evidence also shows that savanna vegetation, such as grasslands, was present in now-submerged areas of Sundaland during the last glacial period. However, scientists disagree about how much savanna covered Sundaland. 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 patches of savanna.

If a savanna corridor existed, it would have allowed savanna animals and early humans to move between Sundaland and the Indochinese region. This movement could have led to new species forming through separation and spreading. 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 use modern species distribution to infer that forests shrank and were replaced by savanna and open forest during the last glacial period. Climate models show varying degrees of forest shrinkage, with some studies suggesting a 50–150 km wide savanna corridor ran from the Malay Peninsula to Borneo. Stable carbon isotope analysis of bat guano and fossil mammal teeth also supports the existence of this corridor.

In contrast, some scientists argue that Sundaland was mostly covered by tropical rainforest. Studies using species distribution models suggest that Dipterocarp rainforests remained during the last glacial period. Others point to river systems on the Sunda Shelf, which would have been shaped by trees along their banks, as evidence of rainforests. Pollen records from sediment cores are mixed: some show forests remained, while others show savanna pollen increased. More recent studies using bat guano isotopes found evidence of rainforest cover in some areas during the last glacial period. Some scientists suggest that differences in soil types, like sandy soils on the now-submerged seabed, 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. Smaller animals also spread 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 have been the origin of Austronesian languages, known as the "Out of Sundaland" theory. However, most experts, including archaeologists, linguists, and geneticists, do not support this view. Instead, the "Out of Taiwan" model is widely accepted by researchers, though not all versions of this model are equally supported.

A study by Leeds University, published in Molecular Biology and Evolution, examined DNA patterns and found that shared ancestry between Taiwan and Southeast Asia likely resulted from earlier migrations. These population movements may have happened at the same time as sea levels rose, possibly leading to migrations from the Philippine Islands to as far north as Taiwan within the last 10,000 years.

These migrations were likely caused by climate change, specifically the flooding of an ancient continent due to rising sea levels. Sea levels rose in three major waves, which may have submerged the Sunda continent and created the Java and South China Seas, as well as the many islands in Indonesia and the Philippines. As sea levels changed, people were forced to move inland from their coastal homes, adapting to new environments like forests and mountains. This shift led to the development of farming and domestication of plants and animals, forming the basis for future populations in the region.

Stephen Oppenheimer believes the origin of the Austronesian people was 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 shows the greatest differences among the native Formosan language families, which are part of the Austronesian language group.