Rewilding is here to stay. The term broadly refers to restoring areas of wilderness to their former glory, but it is the reintroduction of large mammals, from wolves to beavers, that has captured the popular imagination, and come to define this ambitious conservation strategy.

Such projects are not without controversy. Some ecologists worry that reintroducing extinct animals to our radically changed modern ecosystems might have unforeseen impacts.

Farmers and landowners, meanwhile, express concern about the effect interlopers like wolves or lynxes might have on their livelihoods.

Just imagine how they might react to the ideas proposed by a small but dedicated subset of extreme rewilders. In their vision, the plains of North America and Europe would become home to an even wilder array of species, including lions, elephants and cheetahs.

“In the beginning when we told people about this project they just laughed at us,” says Ole Sommer Bach, curator ofRanders Rainforest zoo, referring to his institution’s plan to introduce a population of Asian elephants in northern Denmark. “I think most people thought it was some kind of provocation, or a practical joke; but it really wasn’t.”

This is Pleistocene rewilding. Advocates want to set the clock back not hundreds, but thousands of years. Around 13,000 years in fact, to when the Pleistocene era was drawing to a close: an almost incomprehensible length of time for us mortals, but the mere blink of an eye for Earth’s ecosystems.

One correspondent labelled Greene a “goofball, dipwad, doofus with a scrambled brain”

Today, the planet’s remaining ‘megafauna’ are largely restricted to Africa and Asia. But during the Pleistocene every continent was populated with enormous mammals, from the giant wombats of Australia to the various species of elephant that roamed North America and Europe.

The animals themselves are now gone. But the ecosystems that evolved with them remain, and their function is severely reduced in the absence of such keystone species.

But help could be at hand. Pleistocene rewilders suggest that some animals still found in Africa and Asia, many of which are on the verge of extinction themselves, are similar enough to their extinct counterparts to serve as effective proxies.

These ideas were first proposed by geoscientist Paul Martin as part of his Pleistocene overkill hypothesis, which holds that humanity was the key instigator of global megafauna extinctions. However, they were crystallised in their current form a decade ago by a team led by two researchers from Cornell University in Ithaca, New York: Josh Donlan and Harry Greene.