At the meeting on the New Mexico ranch, which is owned by media mogul Ted Turner, the scientists decided on a first step. They would reintroduce the Bolson tortoise, which is about the size of a coffee table, to part of the ranch that would be carefully controlled.
That first experiment should begin soon, and it addresses one fundamental goal of the project. The Bolson tortoise, now found only in northern Mexico, will likely become extinct in the near future if it cannot be reintroduced in a protected environment.
Likewise, many animals that are close relatives of those that roamed North America during the Pleistocene are endangered. Donlan cites the Bactrian camel, now endangered in the Gobi desert, as a replacement for the extinct camels of North America. It's not exactly the same animal, but pretty close.
These repatriated animals would be free to roam within a protected environment, but that doesn't mean they could wander freely into our cities to terrorize the locals.
Greene sees fences. Miles and miles of fences. Keeping animals in, and keeping humans at a safe distance.
So who's going to do all this, and where's the money going to come from? Greene sees some help coming from ranchers, which is pretty surprising given the turmoil from the reintroduction of wolves in the Yellowstone area. He has met with many ranchers in Arizona, he says, and some are beginning to see a marriage of very different partners.
Rewilding, Greene notes, could be very good for ranching, and for tourism.
Some ranchers he has worked with "are interested in combining ranching with science and conservation as a means of saving that way of life," Greene says.
An elephant could clear a lot of brush on an Arizona ranch, he adds, paving the way for grasses that can feed a herd of cattle. So an elephant could become sort of a ranch hand, and a draw for tourists who, presumably, would stay around for a hamburger.
Meanwhile, other extinct species could be left to prey on animals that have flourished since their predators disappeared. That, the researchers contend, would enhance bio-diversity.
They point to the reintroduction of wolves as evidence of that.
"That has had a spectacular effect," Greene says. Wolves have thinned the ranks of elk, and made the survivors more cautious, so vegetation has rebounded in many areas near Yellowstone, providing food for other animals.
"Beavers have come back," Greene says. "I think that's just spectacular."
Various studies have shown that the reintroduction of the wolf has altered the landscape remarkably, returning it more and more to what it was like before humans decided to eliminate this top predator.
Maybe the same reintroduction could be done for other species. Very slowly. Rebuilding the ecology just one brick at a time.