As large mammals continue to become extinct, rats may evolve to fill the emptying ecospace.

"Rats are one of the best examples of a species that we have helped spread around the world, and that have successfully adapted to many of the new environments that they found themselves in," said Jan Zalasiewicz from the Department of Geology at the University of Leicester.

"They are now on many, if not most, islands around the world - and once there, have proved extraordinarily hard to eradicate. They're often there for good, essentially. Once there, they have out-competed many native species and at times have driven them to extinction.

"As a result, ecospace is being emptied - and rats are in a good position to re-fill a significant chunk of it, in the mid to far geological future."

As rats outlast current environmental competitors, they will eventually experience evolutionary adaptation. Zalaiewicz said these adaptations can include, as history has shown, gigantism.

"Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring," Zalaiewicz said. "For instance, in the Cretaceous Period, when the dinosaurs lived, there were mammals - but these were very small, rat and mouse-sized, because dinosaurs occupied the larger ecological niches."

"Only once the dinosaurs were out of the way did these tiny mammals evolve into many different forms, including some very large and impressive ones: brontotheriums, horses, mastodons, mammoths, rhinoceri and more.

"Given enough time, rats could probably grow to be at least as large as the capybara, the world's largest rodent, that lives today - that can reach 80 kilos [176 pounds]. If the ecospace was sufficiently empty, then they could get larger still," said Zalasiewicz

Rats of the future will not all be of massive proportions. Zalasiewicz predicts many types of evolutionary adaptations in rats over time.

"Animals can evolve to smaller as well as larger sizes. This will depend on what particular circumstances they find themselves in and what the selective pressures on them are, Zalasiewicz said.

"Each island that rats are now present on is in effect a laboratory of future evolution - and each will produce different results," the researcher continued.

"So there will be future thin rats, future fat rats, slow and heavy rats, fast and ferocious rats, probably future aquatic rats - the list goes on. Other animals will likely follow the same pattern, such as domestic cats, rabbits, goats and more," he said.

While Zalasiewicz acknowledges difficulty in predicting the future, he anticipates that the highly adaptive animals will create remarkable descendants and become major players in the future of our planet.