The invisible line that stops kangaroos from migrating to the Indonesian islands is pretty mindboggling

When you think of kangaroos, your mind automatically associates them with Australia. Despite staying close to the Indonesian archipelago, these animals have never crossed the Australian landmass and now, we may know why. It may seem absurd to think an invisible line could stop the kangaroos from leaving the Australian landmass but that's exactly what the Wallace Line does.

The Wallace Line is an invisible line drawn between Australia and Southeast Asia. This line was named after scientist Alfred Russel Wallace, who, alongside Charles Darwin, drew an imaginary line specifically between Borneo and Sulawesi to distinguish between creatures on either side. In a recent study conducted by researchers from the Australian National University and ETH Zurich, there have been more insights into why there is an uneven distribution of animals across this boundary. The boundary has a west side and an east side. The west side of the boundary has elephants, tigers and rhinoceroses. The east boundary has marsupials (kangaroos) and monotremes (duck-billed platypuses).

After thorough research conducted by ANU and ETH Zurich, researchers have been able to come up with a possible explanation for this phenomenon. Their observations state that the geological history of the region when traced back approximately 45 million years ago, witnessed a significant plate tectonic movement that reshaped the Earth's surface. Before the major shift, Australia was located further south, closer to Antarctica. However, as tectonic plates shifted, Australia gradually drifted north and collided with the Asian landmass, the Indonesian archipelago to be specific. The consequence of this geographical shift led to the creation of the volcanic archipelago of Indonesia, creating the Wallace Line.

Dr. Alex Skeels, a part of ANU, says that this geological shift influenced Earth’s climate. Following the shift, the separation of Australia from Antarctica helped in the formation of the Antarctic Circumpolar Current, inducing global cooling. Consequently, the climate in the Indonesian islands remained warm, wet and tropical, making it a helpful process for the migration of fauna from Asia to Australia. Crucially, the Asian fauna was already adapted to these tropical conditions which made it easier for them to shift to Australia.

The Australian side of things is a different story, they evolved in a cooler and arid climate over time. As a consequence, they were less suited to the warm and humid conditions prevalent in the Indonesian islands. This disruption was largely responsible for the failure of Australian fauna to cross the Wallace Line, a divide that has persisted with time. The Wallace Line plays an important role in understanding the fauna divide and is an excellent phenomenon involving climate, geology and biodiversity. With the help of this intangible line, a major chunk of the mysterious divide has been solved.