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Wallace's Line: An Invisible Line That Animals Usually Don't Cross

The animal kingdoms of Asia and Australia are worlds apart, thanks to an invisible line that runs right between the two neighboring continents.

Most wildlife never cross this imaginary boundary, not even birds.

And so it has been for tens of millions of years, shaping animal evolution in different ways on each side.

It all started about 30 million years ago, when the Australian tectonic plate bashed up against the Eurasian tectonic plate and created an archipelago, rerouting ocean currents and creating new regional climates.

On one side of the map, in Indonesia and Malaysia, monkeys, apes, elephants, tigers, and rhinos evolved; while on the other side, in New Guinea and Australia, marsupials, monotremes, rodents, and cockatoos flourish. Very few species are abundant on both sides.

The curious faunal divide is named Wallace's Line – after the naturalist Alfred Russel Wallace, who first noticed the stark difference in animal life (mostly mammals) while exploring the region in the mid-19th century.

An Invisible Barrier Keeping Two Worlds Apart

Generally speaking, Wallace's line separates a shelf of the Asian continent from a shelf of the Australian tectonic plate. It is a geological line, but it is also a climatic and biological one.

While Wallace's invisible line is most obvious when comparing mammals in Asia and Australia, it also exists for birds, reptiles, and other animals.

Even creatures with wings don't typically make the trip across Wallace's line, and in the ocean, some types of fish and microbes show genetic differences on one side of the border compared to the other, indicating very little mixing between populations.

Scientists have yet to figure out what invisible barriers are holding these species back. Habitat and climate, however, are probably factors accentuating the evolutionary divide.

Wallace's divide isn't an absolute border, but more of a gradient, scientists say. Even still, the blurry line helps us make sense of animal evolution for thousands of species.