Rats... As big as sheep?

That’s what some of the papers back in 2014 would have you believe. The story came about after Dr Jan Zalasiewicz, a palaeobiologist at the University of Leicester, suggested that rats could evolve to fill gaps in the Earth's ecosystem as larger animals became extinct.

“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,” he said. “If the ecospace was sufficiently empty, then they could get larger still.”

So what’s the likelihood of our future planet being populated by giant rodents?

“The sort of evolution scenarios that Dr Zalasiewicz is speculating about would occur over huge swathes of time – tens of millions of years,” says Dr Victoria Herridge, a palaeobiologist at London’s Natural History Museum.

The last major extinction event – the one that wiped out all non-avian dinosaurs - occurred around 65 million years ago. Since then, a huge diversity of mammals has evolved, but it’s been a very slow process.

“We know from the fossil record that it takes, on average, ten million years for life on Earth to bounce back from a mass extinction event,” says Herridge.

This means that if another mass extinction occurred today – and if we humans survived it – it might be millions of years before we saw any giant rats.

And that's only one side of the problem. When it comes to evolution, the existence or extinction of certain animals is only a tiny part of a delicately woven web. As the rats evolved, they’d face a huge number of challenges.

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“Continental drift; new mountain ranges; significant changes in climate; unpredictable catastrophic events like a meteor hit,” lists Dr Herridge. “And set against all this is a continuous, complex evolutionary interplay between individuals competing for space to live, eat and breed.”

Herridge does agree that rats are a shrewd species. Flexible in their diet, able to live in different environments and already exhibiting niche-filling behaviour, they’d be the perfect candidate to take advantage of a dying species. It’s just a shame that we probably won't be around to see them.

“While it is fun to think about the wondrous new forms that might arise, it is far more chilling to think of the species we are losing right now, and how long it will take to replace them,” says Herridge. “Certainly, given the average duration of a species, no human would get to see this imagined future ecosystem.”

So if you do find some unusually large bite marks in your cheese, they're more likely to have been left by a hungry sheep than a sheep-sized rat.