Bizarre reason why animals don't cross this mysterious line on the planet
The Wallace Line shows a marked difference in the evolutionary origins of the animals on each side of it
Scientists have come up with a theory for why the animals on each side of a 'line' are so different from one another.
There are plenty of boundaries in nature that might not be visible, but can still have a huge impact on the wildlife in the area.
For example, in waters off the Cape of Good Hope off Africa's southern tip, the Atlantic and Indian Oceans meet.
This creates a distinct enough boundary underwater that some animals don't cross between the two, while others do.
Islands are also fascinating for biologists as species evolve in relative isolation, which is why Charles Darwin was able to observed such different characteristics in animals from different parts of the Galapagos.
But this particular 'boundary' was observed by Darwin's contemporary Alfred Russel Wallace while he was travelling in the Malay Archipelago.
This is a chain of over 25,000 islands including modern day countries Indonesia, the Philippines, Malaysia, Singapore, and Papua New Guinea.
So why is it so interesting?
It's because Wallace observed that the creatures on each side of the 'Wallace Line' showed a lot of differences.
One side of the line is closer to Asia, while the other is closer to Australasia, a part of the world known for its unique wildlife.
And on the side of the Wallace Line which is closer to Australasia there are many animals like those found in Australasia, as well as those found in Asia.
But on the Asian side of the line there are only animals like those found in Asia.
Wallace concluded that this meant Asian animals were able to cross over this biogeographical boundary, while Australasian ones were not.
On the Australasian side you will find things like marsupials - such as the native kangaroo - and monotremes, the group which includes echidnas and platypuses.
Meanwhile on the Asian side there are no marsupials but there elephants, rhinos, monkeys and apes.
One explanation for the distinction between the two sides is the effect of differing climates from geological events a long time ago.
Study lead author Alex Skeels said in a statement: "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.
"This dramatically changed Earth's climate as a whole; it made the climate much cooler."
The extreme climate change impacted the species on each side of the line in different ways, leading to the distinction we see today.
