Sulawesi is not an island many people have heard of. Lying to the east of Borneo in the Indonesian archipelago, it is off the main tourist trail and often overshadowed by its larger neighbour.

This Indonesian island may not have the diversity of Borneo, but it has some of the most unusual animal life in the world. What's more, it was the contrasting fauna here that paved the way for some of the greatest scientific discoveries of all time.

Sulawesi is the largest island in the "Wallacea" region, named after British naturalist Alfred Russel Wallace, who explored it in the 1850s. Wallace famously discovered evolution by natural selection, independently of Charles Darwin.

Wallacea is a transition zone between Asian wildlife in the west and Australian in the east. It is delimited by the Wallace Line in the west and Lydekker's Line (named by another British naturalist) in the east.

These naturalists drew their lines because the islands on either side were inhabited by very different mammals and birds.

The islands to the west of the Wallace Line, like Borneo, have mammals similar to that of East Asia. These include tigers, rhinoceroses and apes. Sea levels were lower during the ice ages, so some of the islands were connected to Asia by land bridges.

On the other side, the islands to the east of Lydekker's Line are populated mostly by marsupial mammals and birds similar to those found in Australia. Again, during times of lower sea level most of these islands were connected to Australia via land bridges.

In between the lines lies Sulawesi and the region of Wallacea, inhabited by a mixture of animals from both regions. The islands in Wallacea are the only islands in the world where you will find monkeys and marsupials living together.

Wallacea is isolated by deep oceanic trenches, so only the animals that could swim or fly far enough made it there. That means that the islands of Wallacea have fewer land mammals and land birds than the surrounding islands – but many bird, reptile, and insect species crossed the deep seas by flying or swimming.

The region is one of the most biodiverse on the planet, partly because is made up of many islands where species have evolved in isolation.

This means it has a lot of "endemic species": species that are unique to a particular island or region, and found nowhere else in the world.

Sulawesi is an incredible place to watch wildlife, simply because you are aware that the animals you are seeing are only found there. There are bright green frogs and bold toads, birds waking up the forest with their invigorating chorus, insects in every nook and cranny, and primates and marsupials like nowhere else in the world.

Sulawesi's most interesting species are marsupials called cuscus. Marsupials are usually thought to be confined to Australia and South America, but these are the exception. They are the only marsupials in Asia and some of the most primitive possums in the world.

On Sulawesi there are two species; the bear cuscus and the dwarf cuscus. They live in the upper canopy eating young leaves, alongside monkeys and tarsiers, primates from Asia.

There is nowhere else like this on Earth. To the naturalists who first visited Sulawesi and Wallacea, the strange mix of animals hinted at a long, strange history. The theory of evolution by natural selection, along with the idea of island endemism, soon supplied the explanation.

The region was originally almost completely forested, but today much of the forest is being cut down and many of the islands' rare endemic species are being lost.

Sulawesi has one of the largest areas of lowland tropical forests in the world, and much of it is still unexplored. Some of its unique species could disappear before anyone even discovers them.

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