Continental crash 35 million years ago likely created an invisible barrier on Earth

An imaginary line runs through Indonesia and most animals and birds living on either side of it do not cross. This is the Wallace Line, a biogeographical barrier first mapped out by British naturalist Alfred Russel Wallace in 1863. This is a transitional zone between Asia and Australia and the organisms living on either side of it are extremely different.

There is an uneven distribution of animal species and birds on both sides of the mysterious boundary, and scientists haven't been able to figure out why for a long time. To the west of the line reside Asiatic species, and to the east, a mix of some Asian and Australian species.

Wallace travelled through a chain of more than 25,000 islands between Southeast Asia and Australia and noticed a drastic change in species after a certain point. This is where the boundary of the Wallace Line was drawn.

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Ecologists were stumped to see the asymmetric distribution of species across the Wallace Line. They concluded that something happened that allowed Asian species to move in one direction, but didn't let the Australian species move in the reverse direction.

A study revealed last year that the strange division came into being after a continental collision triggered extreme climate change. This impacted the species on each side of the divide in different ways, it says.

A continental crash and climate change

This tectonic activity happened around 35 million years ago. Australia broke away from Antarctica and crashed into Asia, leading to the creation of the Malay Archipelago.

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The continental mash-up triggered a change in the climate. Scientists studied more than 20,000 species found on either side of the Wallace Line to figure out their dispersal ability and ecological preferences. They found that the living conditions in the Malay Archipelago suited Asian species a lot better.

When the continents moved, climate change happened in relation to the oceans. "When Australia drifted away from Antarctica, it opened up this area of deep ocean surrounding Antarctica which is now where the Antarctic Circumpolar Current (ACC) is," lead author of the study, Alex Skeels, an evolutionary biologist at the Australian National University, said in a statement.

This made the overall climate much cooler. But it affected the species in these regions differently. The Malay Archipelago remained warm and wet, while the conditions in Australia had become cold and dry.

This meant that the animals living in the Malay region were well-adapted to their area and some of them were suited to move towards Australia. However, Australian species evolved in a cooler and drier climate and couldn't adapt to the tropical islands, Skeels said.