Forget Planet of the Apes; in the distant future, the Earth will be populated by giant rats.
Some of them could grow to weigh as much as people.
“Given enough time, rats could probably grow to be at least as large as the capybara. The world’s largest rodent that lives today can reach 80kg (175lbs),” said Dr Jan Zalasiewicz, a paelobiologist at Leicester University. “If the ecospace was sufficiently empty, then they could get larger still.”
The largest extinct rodent known was Josephoartigasia monesi, which lived in South America during the Pliocene and Pleistocene eras (between 5.3 million years ago and the end of the last ice age 12,000 years ago) and weighed an estimated 1 tonne.
It was discovered in Uruguay in 1987 but was only described scientifically in 2008.
Present-day rats look set to repeat that feat of evolution as climate change, habitat loss and poaching drive larger mammals to extinction.
“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,” Dr Zalasiewicz said.
The animals have a versatility that is perhaps second only to that of humans.
“They 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 Dr Zalasiewicz.
“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.”
In a process known as “adaptive radiation”, single species can quickly evolve to take advantage of vacant ecological niches. The classic example are Darwin’s finches, which had split into numerous species, each with distinctive physical characteristics, to take advantage of opportunities on the Galapagos islands.
Further back in time, mammals were, with a few exceptions, tiny creatures, a few centimetres long and weighing less than a kilogram.
It was only after the dinosaurs died out 65 million years ago that mammals came to dominate the world.
Ancestors of the blue whale, the largest animal alive today, were about the size of a wolf 50 million years ago.
Gigantism, the tendency for species to grow larger, is part of an evolutionary arms race between predator and prey.
But not all species follow that route. “Animals can evolve to smaller as well as larger sizes,” said Dr Zalasiewicz. “This will depend on what particular circumstances they find themselves in and what the selective pressures on them are.”
