ROBERT SIEGEL, HOST:
For the first time in more than 2,000 years, packs of wild horses are once again galloping free in western Spain. Ancient wild horses once roamed the Iberian Peninsula, but when the Romans arrived they domesticated the animals.
Now, as Lauren Frayer reports, a breed closely related to those ancient horses is being re-wilded into the Spanish countryside.
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LAUREN FRAYER, BYLINE: This empty scrubland on the Spanish-Portuguese border used to be populated by farmers, plowing the land and grazing their livestock. It was only a generation ago that most Spaniards up and left for jobs in the city. Now the land they've left behind is reverting to a landscape unseen for perhaps thousands of years.
BENIGNO VARILLAS: In the last 40 years, the bush increased in four million hectares and now probably five. This is nearly 10 percent of the territory, became bush land because they lost the human population - they go to the city.
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FRAYER: Benigno Varillas tilts his cowboy hat and looks out over empty, overgrown hills that his relatives once farmed. These cork oaks and brush are increasingly vulnerable to wildfires. And Varillas is fighting them but not with water. He's re-wilding the land with its natural protectors, animals.
VARILLAS: To control the bush, you need to eat the new plants, but also to be big animals. What do you say in English? To trample.
FRAYER: Yeah.
VARILLAS: And this is what Spain don't have no more in a lot of parts. The people take their cows or their horses out.
FRAYER: Conservationists say people aren't part of the natural landscape here but their livestock are.
STAFFAN WIDSTRAND: If the domesticated herbivores are not anymore, then we need to bring in those who were there before.
FRAYER: Staffan Widstrand is with a nonprofit group called Rewilding Europe, which sees the decline of farming as a chance to restore biodiversity.
WIDSTRAND: We had domestic horses here while previously there were wild horses.
FRAYER: Widstrand and Varillas converted this abandoned land into a nature reserve and brought in endangered native horses, millennia after the Romans first came here and domesticated the wild horses that once roamed free.
WIDSTRAND: Its one specific race called Retuertas, which is a very ancient horse breed, that the final little herd lives in the south of Spain from Andalusia. But previously, it most possibly was a horse breed - a horse, you know, sub-species that was all over the Iberian Peninsula. So it has ancient traits. And when they checked the DNA, it stands very, very close to the DNA of those ancient wild horses that were here once.
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FRAYER: On a sunny cold afternoon, a ranch hand opens a gate
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FRAYER: And two dozen Retuerta horses gallop out into the wild. These horses are one example of the type of re-wilding that could take place amid an unprecedented global migration to cities. The U.N. forecasts that 85 percent of humans in the developed world will live in cities by the year 2050. Conservationists are looking at what all those people leave behind: animals, agriculture and ways of life and how to preserve it.
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FRAYER: Forestry engineer Diego Benito, his wife and new baby, are the only full-time residents of the nature reserve where the Retuerta horses now roam. He says he considers his family pioneers, moving to the countryside while most longtime residents are abandoning it. But he says it's worth it for this view alone: wild horses galloping free.
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DIEGO BENITO: The last horses here maybe were thousands of years ago. It's kind of strange. But it also is quite beautiful to see all the whole landscape. And also, animals that you know will be wild. I'm sure that in 5 years or 10 years, this will be really a wild ark of nature.
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FRAYER: A Noah's ark for the future.
For NPR News, I'm Lauren Frayer in Spain.