Alfred Wallace, who co-discovered natural selection with Charles Darwin, noted a division running through the islands of South East Asia. To the west of the line, animals and plants resembled those of mainland Asia. To the east, they had more in common with Australia. Christmas Island, well to the west of Wallace's Line, has been revealed to host species of predominantly Australian origin, revealing something new about the way islands are colonized.
Christmas Island is most famous as the site of an annual land crab migration and as Australia's offshore prison. However, Dr Jason Ali of the University of Hong Kong told IFLScience it's also a fascinating geological rarity. Between 40 and 17 million years ago, it was another coral atoll atop a submarine volcanic peak.
The collision with the Asian tectonic plate has buckled the Australian plate, which Ali compares to pushing on both ends of a piece of paper laid flat on a table. The seafloor south of Java has risen sufficiently to bring the long-lost island to the surface. Ali told IFLScience only “about five” places worldwide have experienced something similar.
Ali is part of a team that have recently published two papers on Christmas Island. In Palaeogeography, Palaeoclimatology, Palaeoecology, one dates the re-emergence to 4.4-5.66 million years ago. In Biological Journal of the Linnean Society, the authors identify the origins of the island's animals.
Very few land animals have reached Christmas Island, which is why the crabs have become so dominant. Two rats, two skinks, and a gecko, half of the island's land vertebrate species, are more closely related to counterparts from east of the Wallace Line than from the west, despite the enormously longer journey. Others' origins are unclear.
“The ancestors of these species would have most likely have been washed over on uprooted trees of vegetation mats and transported in by a major oceanic current known as the Indonesian Throughflow,” senior author Professor Jonathan Aitchison of the University of Queensland said in a statement.
Pressure from the Pacific Ocean forces the throughflow into the deep water channels between certain Indonesian islands. The channels are so deep they barred passage even during the peak of the Ice Age, explaining Wallace's observations.
Ali told IFLScience that recent advances in genetic testing were necessary to determine whether some of the closest relatives of the island's inhabitants are Australian or Asian, thus identifying their origins.
Intriguingly, many of Christmas Islands' abundant birds are also from the east. “A lot of bird species straddle the boundary,” Ali said to IFLScience. “But if you look at wind directions, they blow predominantly... from Australia.” He thinks more birds may have been blown off Australia and continued flying for hundreds of kilometers than made the small hop against the wind from Java.
