"Africa and parts of Asia are now the only places where megafauna are relatively intact, and the loss of many of these species within this century seems likely," the team, led by Josh Donlan from New York's Cornell University, said.

"Given this risk of further extinction, re-wilding of North American sites carries global conservation implications," the team wrote in Wednesday's issue of the science journal Nature.

It said large mammals were common across all continents until the Late Pleistocene wipeout that hit North America hardest and handed the world to smaller species. The largest mammals in the United States today are bison.

The Pleistocene epoch lasted from about 1.65 million years ago to 10,000 years ago.

"Large carnivores and herbivores often play important roles in the maintenance of biodiversity, and thus many extinct mammals must have shaped the evolution of the species we know today," the scientists wrote.

They said the pronghorn antelope's remarkable turn of speed must be due at least in part to the presence of the now extinct predatory American cheetah alongside it on North America's grasslands.

Reintroducing the modern relatives of the Late Pleistocene losers to North America could spark fresh interest in conservation, contribute to biodiversity and begin to put right some of the wrongs caused by human activities.

"Establishing Asian asses and Przewalski's horse in North America might help prevent the extinction of these endangered species and would restore equid species to their evolutionary homeland," the scientists wrote.

They proposed a second phase that would include reintroducing African cheetahs, lions and Asian and African elephants to large private parks.

"Free-roaming, managed cheetahs in the southwestern United States could save the fastest carnivore from extinction, restore what must have been strong interactions with pronghorn and facilitate ecotourism as an alternative for ranchers.

"Managed elephant populations could similarly benefit ranchers through grassland maintenance and ecotourism," they wrote, adding that reintroducing lions would represent the pinnacle of the Pleistocene re-wilding of North America.

They admitted the plan would be controversial but said it was a far better option than simply accepting the terminal decline of some of the world's most impressive species due to human encroachment and global warming.