Most of the rats and rodents in Australia today are descendants of a single species that arrived from Asia in two waves, the first 6 million years ago and the second about a million years ago. Like colonists of previous centuries, one or multiple rats arrived by way of the sea, scientists believe, clinging to floating foliage set adrift from the islands of Indonesia.
"They didn't have to float all the way from Indonesia to Australia in one voyage," said Scott Steppan, a professor of biology at Florida State University. The sea level was lower at that time, and Australia and New Guinea were part of the same land mass, so only a set of straits had to be crossed.
Floating part of the way, walking at other times, the rats probably made the journey after a storm, just as in modern times, mammals are sometimes found aboard floating vegetation.
No one knows, but it could be "as little as one pregnant female," Steppan said. If this Founding Mother survived with at least some of her offspring, there may have been some inbreeding at first, but eventually, separate lineages would develop, allowing for a greater diversity of potential mates.
This evolution, both rapid and extensive, is of interest to scientists because of its potential for practical applications in medicine.
Australia before people
Before the rats came over, "Australia had a diverse animal community of lots of different mammals, but they were all marsupials, so you had kangaroos and wombats and possums and all of their relatives in a variety of different shapes and sizes," Steppan said.
Placental mammals, including humans, develop embryos inside their bodies, but marsupials have an external pouch for this purpose.
An assemblage of reptiles and birds also greeted the newly arriving rodents, said Emily Roycroft, a Ph.D. candidate at the University of Melbourne who also works with the mammal collection at Museums Victoria.