Giant rats may one day rule the Earth, according to a surprising new thought experiment out of Britain.

Jan Zalasiewicz, from Leicester University, states many of his colleagues believe the beasts could roam much of the Earth, after a cataclysmic extinction. The researchers believe human influence on global climate change could soon bring about the end of much of the life on planet, leading to a planet without people.

Rats may best be able to thrive in such a world, the scientists speculate. They base this statement on the ability of the small animals to adjust to life in a human-dominated environment. The wide range of conditions under which the animals live may provide them the best opportunity to take over the planet.

Species tend to grow larger once they have established over vast terrains, in a process called gigantism.

"[I]t's a guess; it's a thought experiment. But it is based on the way that geology has operated in the past and the types of creatures that have been successful," Zalasiewicz told Live Science.

The geological historian believes the rats could evolve to weigh as much as 20 pounds, three to ten million years after humans are gone. A few, living in areas without competitors, may even grow as large as modern cows, and weigh 175 pounds. While dogs would likely disappear soon after humans were unable to provide for them, cats would likely thrive, along with pigs and rabbits.

At least five major extinctions have occurred over the course of the history of the Earth. The most recent of these was 65 million years ago, when dinosaurs were suddenly wiped from the planet, after ten million years of struggle for survival. Those with the most bird-like features evolved into the birds we see around us today. Small mammals took over the land, and eventually led to human beings.

Hundreds of animals species have become extinct, and thousands endangered during the last few centuries, many due to human activity. Many observers believe these losses may herald the beginning of a new mass extinction.

"It's an illustration that shines light on the larger issue: that we are very quickly, as humans, altering conditions on Earth. And that is remarkable and without precedent in Earth history," Zalasiewicz said.

Such a change is already taking place on "rat islands," areas where the small mammals have taken control of former human habitations. The largest extinct species of rodent, Josephoartegasia monesi, weighed more than a ton and was larger than a modern cow.

Terrifying as thoughts of rats the size of a donkey may be, we may be fortunate that such animals aren't likely to evolve until humans are gone.

If you don't believe in this rodent of unusual size, it may not be a good idea to say it out loud.