Before humans, Australia was colonized by rats

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.

So how many rafted onto the continent in each wave?

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.

To survive, the rats evolved to fill roles in the ecosystem, resulting in a diversity of distinct rodent groups across the continent, she explained.

"It wasn't until the arrival of Europeans into Australia that many native rodent species started suffering massive declines in population numbers," she said. "Since about 1850, at least 14 native species of Australian rodents have gone extinct." Predators and competitors -- foxes, cats and the "black rat," "brown rat" and "house mouse" -- along with urbanization took a toll.

Today, there are about 50 native species found nowhere else in the world but Australia, Roycroft said. And more than 90 species are native to New Guinea that similarly originate from Southeast Asia.

Karen Roberts, collection manager of vertebrate zoology at Museums Victoria, said the native Australian rodents "make up the vast majority of rodent species here. Genetically, the rodents found in Australia today are most closely related to the rodents of Indonesia."

With their Asian ancestry, these native species are related to rodents throughout the world, a significant detail for those who study them.

One big happy 'superfamily'

The natives are a subgroup of the muroids, the most diverse group of mammals in terms of numbers of species, Steppan said: "Almost one out of every three species of mammal is a member of this one group, which is a superfamily."

This "unusually diverse" family includes mice, rats, hamsters, gerbils and "a whole host of other specialized forms that people are not familiar with," he said.

Among the unfamiliar forms are the native Australian rodents, such as the fish-eating rats that can swim, two-legged hoppers that look like little kangaroos and tree-dwelling rodents that chew on leaves all day long.

Overall, the muroids are a "group of mammals that, relative to their age, is the most successful from an evolutionary standpoint: They have the greatest amount of diversification going on," Steppan said, adding that this is the reason they are studied more than other mammals.

If we want to explain life on Earth, we need to focus on rapidly evolving groups, such as the muroid rodents and the species within this superfamily, including distant rodent "cousins" scattered throughout Indonesia and Australia, Steppan said.

"Whatever is exceptional to make them so diverse can give us insight into how everything else is diversified," he said.

Australia is a unique example of an isolated continent that didn't have any rat-like species until the first rodent drifted in from Asia. In essence, then, it's a laboratory for evolutionary biologists to see what happens when you don't have competition, Steppan said. And what happened was very rapid diversification into a lot of very different species, large and small, able to live in deserts and treetops.

"Moving into a new area with less competition doesn't actually explain as much as we thought it would," Steppan said. "There's clearly just many things going on that can cause one group to diversify quickly, and it's actually really hard to tease apart which factor in which case is the actual explanation.

"On a more practical level, understanding the evolution of this group also can flag a lot of useful information about disease," he said. "Some species of rodents are hosts for diseases, and many of those diseases actually co-evolved with their hosts." So as the rodent host diversified, the disease diversified with it. Understanding species evolution helps us better understand how to respond to diseases, he said.

On a basic science level, they "can tell us the most," Steppan said.