During the ice age, when glaciers blanketed northern Europe and a wind-ripped tundra stretched down to the Mediterranean countryside, thick forests and fertile meadows gave refuge to the wild horses that roamed the Central European lowlands, browsed the East European steppes and galloped across Asia and the Americas. In the fifth century B.C., Herodotus wrote of how much he enjoyed watching herds grazing in the bogs and marshes of what is now Poland. For ages, these horses, called tarpans, outwitted hunters and survived, but by the 18th century not many remained, in part because diners prized tarpan meat—it was sweet, but more appealingly, it was scarce—and in part because tarpans had interbred with farm horses to produce fertile offspring. In 1879, pursued by humans, the last wild, pure tarpan mare fell down a crevasse in Ukraine and died; the last captive died eight years later in a Moscow zoo.

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At that point the species officially became extinct, just one more chapter in the annals of life on earth. Yet in Bialowieza, a forest straddling the border between Belarus and Poland, tarpans apparently still graze.

Bialowieza (pronounced bya-woe-VYE-zha) is the only fragment of primeval lowland forest preserved in all of Europe, an ecosystem Poles call a puszcza, a word evoking ancient woodlands undefiled by man or woman. Historically, it served as a hunting retreat for kings and czars (who kept an ornate lodge there), but by World War II, it had fallen under the purview of scientists, politicians and poachers. The rapid decline of the largest land animals in Europe—European (or "forest") bison—helped to kindle Poland's conservation movement after the war.

In a small reserve at the edge of the woods, time seems to evaporate as a herd of horses graze on marsh grass beneath colossal pine trees and a dazzling blue sky. On frosty mornings, they browse inside bubbles of steam and leave a sweet leathery odor behind them, invisible clouds above jumbled hoofprints. Spring through fall, the horses live unaided by humans, wading in the ponds and grazing on bushes, tree branches, algae and grass. In winter, they paw the snow to find dry grass or rotting apples, and rangers of the mounted Horse Guard sometimes provide hay and salt; well-muscled, the horses have little fat to insulate them, so they grow shaggy, easily matted coats. It is then that they most resemble the horses painted on the cave walls at prehistoric sites throughout France's Loire Valley.

They're strikingly beautiful creatures: dun with a black stripe down the back and a dark mane. Although they have long ears and large, thick necks, they are lightly built and fast. Unlike more domestic horses, their coats lighten in winter, just as those of ermine and arctic hares do, blending in with the landscape. Then ice clots like marbles in their manes and tails. Still, they thrive on harsh weather and poor diet; and, although the stallions battle fiercely, with bared teeth and thwacking necks, they heal quickly.

How did these lost-in-time animals get here?

For years leading up to World War II, German zoologists pursued a fantastic goal: the resurrection of extinct species. Genetic technologies wouldn't emerge until the 1970s (and in any event remain insufficient for this purpose), but one such zoologist, Lutz Heck, decided to use a traditional method of breeding animals to emphasize specific traits. Heck's reasoning went like this: even an extinct animal's genes remain in the gene pool of closely related living species, so if he concentrated the genes by breeding animals that most resembled their extinct antecedents, in time he would re-create their ancestral forms. He was wrong—not all the genes survive, so extinct species cannot be revived through breeding—but the war gave him an excuse to loot East European zoos for the best specimens to mate with several wild strains, hoping to breed back to pure "Aryan" animals the fierce creatures painted in ocher on Cro-Magnon caves. What better totems for the Third Reich?

Before the war ended, Heck shipped back many of his back-bred, look-alike tarpans to idyllic Bialowieza, where he pictured Adolf Hitler's inner circle hunting in the new millennium. After the war, the care and breeding of the animals, and the stewardship of Poland's part of the forest, returned to Polish hands.

What is so awe-inspiring about this landscape that it could bewitch people from many cultures and eras? For starters, it contains 500-year-old oak trees, as well as soaring pine, spruce and elm rising like citadels hundreds of feet tall. In addition to its throwback tarpans, it boasts a multitude of other species, from one-celled protozoans to boar, elk, lynx, wolf, moose and bison. Beavers, martens, weasels, badgers and ermine glide through the marshes and woods, while Pomeranian eagles share the skies with bats, goshawks, tawny owls and black storks. The air smells of balsam and pine needles, sphagnum moss and heather, berries and mushrooms, marshy meadows and peat bogs. Small wonder the preserve has been named a World Heritage site.

Because it is closed to hunters, loggers and motorized vehicles of any sort, the preserve is the last refuge of unique flora and fauna. Park rangers guide tiny groups of hikers along designated paths, where they're forbidden to litter, smoke or even speak above a whisper. Nothing may be removed. If a ranger needs to carry something into the park, he transports it by rubber-tired horse cart; if he needs to move a fallen tree, he uses a handsaw and workhorses.

Ironically, the breeding experiments that thrived with Heck's ambitions helped to save scores of rare plants and endangered animals. But understand-ably bitter about Heck's Nazi ties and motives, Polish patriots were (and still are) quick to point out that these "tarpans" are technically counterfeits, like the descendants of the Heck-bred aurochsen, or wild oxen, on display elsewhere in Europe. Some zoologists, who prefer to speak of "near-tarpans" and "near-aurochsen," associate the animals with political agendas. They paint Heck as a con man who staged a colossal Nazi hoax by creating new breeds, not resurrecting extinct species. But Herman Reichenbach, in International Zoo News, envisions an important role for the pseudo-throwbacks: "They can still help preserve a natural environment of mixed forest and meadows.... And as a feral type of cattle, the aurochsen may also be able to enhance the gene pool of a domestic animal that has become impoverished genetically."

There are many forms of obsession, some diabolical, some fortuitous. Strolling through Bialowieza's mass of life, one would never guess its political dramas, including the role it played in Nazi ambitions.

Diane Ackerman has published several nonfiction books, including the best seller A Natural History of the Senses.

Adapted from The Zookeeper's Wife, by Diane Ackerman. Copyright © 2007 by Diane Ackerman. With the permission of the publisher, W.W. Norton & Co., Inc.