Can you imagine the tiny, eeeky, dirty rodent rat growing into the size of a sheep or worse a cow? A scientist from the UK said it is highly possible. We humans may ought to brace for the onslaught of rat armageddon.

Dr Jan Zalasiewicz, a geologist from the University of Leicester, said the seeming reality has occurred before ages ago when a much smaller animal lorded over the earth once the giant creatures have gone extinct.

"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," he said.

"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."

What's more, the idea of a giant rodent crawling on top of the earth isn't really new. Josephoartegasia monesi, the largest known extinct rodent that lived three million years ago, was larger than a bull. It weighed over a tonne.

"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," Mr Zalasiewicz said.

Animals will evolve, over time, into whatever designs will enable them to survive and to produce offspring, he pointed out.