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The Pleistocene was the heyday of megafauna, a span of geologic time when big mammals like mammoths, saber-toothed cats, woolly rhinos and giant ground sloths roamed the continents. The epoch lasted over one million years during which glaciers plowed the planet's surface, stretching and retracting across vast expanses. Near the beginning of the end of the Pleistocene some 50,000 years ago, much of the megafauna disappeared in synchrony with the spread of modern humans. This loss left the Holocene, the current geologic epoch (which began about 11,000 years ago) with a much impoverished megafauna. Species such as the American mastodon, dire wolf and giant deer are long gone, but some species, or at least their close relatives, have persisted into the present, giving scientists hope that Pleistocene-like megafauna and their ecosystems can be re-created.

A few years ago, a group of scientists conceived a "re-wilding" plan aimed at restoring North America's lost Pleistocene ecosystems. The purpose: to restore lost ecological processes and evolutionary potential as well as provide a safe haven for megafauna barely surviving in conflict-ridden, unstable or densely populated regions elsewhere. Since that time, much of the discussion about re-wilding has remained focused on North America. Meanwhile, other candidates for re-wilding have been largely overlooked, although there is a major effort underwayin Siberia to preserve and extend Pleistocene-like grasslands at northern latitudes as well as initiatives in Europe, the continent that may hold the greatest promise for bringing the Pleistocene back to life.

In many ways, Europe is a more obvious candidate for re-wilding than North America. The reason: a large portion of species lost in the Americas do not have any close living relatives. Europe has also seen its share of extinctions, including the scimitar cat, cave bear, woolly mammoth, woolly rhinoceros, steppe rhinoceros and giant deer, but many of Europe's lost species still survive or have close wild or domestic relatives elsewhere in the world. Europe also has a historical advantage: The disappearance of its megafauna to a large extent occurred more recently than in North America, with many species persisting well into the Holocene.

Europe has already succeeded in reintroducing some previously extinct species. The bison, which was extinct in the wild in the early 20th century, has now been reestablished in scattered populations across eastern Europe. Small populations of musk ox that lived in Europe in cold climates until the late glacial period have been successfully reintroduced in Scandinavia's mountains. The fallow deer, the closest relative of the now extinct giant deer, survived marginally into Europe's Holocene, but persisted in Asia Minor. After several millennia of reintroductions, the animal now prospers in most European countries. The successful re-wilding of these species bodes well for larger scale projects.

But re-wilding initiatives in Europe must also include reinvigoration of megafauna populations already there that have suffered severe range constriction. Among them: the wolf, brown bear, lynx and moose. Scientists should also consider reintroducing 11 additional megafauna species: the Asiatic lion, leopard, spotted hyena, dhole, horse, cattle, Asiatic wild ass, Asiatic elephant, hippopotamus, water buffalo and hairy rhinoceros.

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First, the predators such as lions must be brought back. The extinct spelaea subspecies of that great cat were widespread in Europe until the end of the last glacial period. The Asiatic lion is an obvious candidate to replace this missing feline predator; it actually flourished in the Balkans as far north as Hungary just a few thousand years ago but now lives only in a limited forest area in India thanks to overhunting. Other predators to consider are the leopard and spotted hyena, along with the dhole, also known as the Asiatic wild dog, all of which were widespread in Europe during the late Pleistocene and could be transplanted from Africa and Asia.

There must also be grazing animals to serve both as prey and to fill the ecological void left by extinct herbivores. Horse and cattle flourished in late-Pleistocene and Holocene Europe, but became extinct in the wild there over the past 400 years as their habitat shrank. Both species still exist on that continent in domesticated forms and as escaped domesticated animals that have turned feral, and are already being used in local re-wilding projects. The Asiatic wild ass could also be introduced in Europe's drier regions as a proxy for the closely related, extinct European wild ass, which prospered well into the Holocene, possibly surviving in Spain as late as A.D. 1540.

Other large mammals could be used as stand-ins for closely related but extinct relatives, such as the Asiatic elephant for the straight-tusked elephant, which thrived in Europe's warm to cool temperate climates during the late Pleistocene. The Asiatic elephant could do well in Europe, because it is not an exclusively tropical species and, in fact, was widespread in temperate China early in the Holocene.

And let's not forget water-loving megafauna like the modern hippopotamus (which lived in warm, temperate conditions in Europe during the late Pleistocene) and the Asian water buffalo, which could serve as a proxy for its extinct European cousin. Finally, there's the Sumatran, or hairy rhinoceros, which, if saved from the brink of extinction in Southeast Asia, perhaps could substitute for the extinct Merck's rhinoceros, a related temperate forest species that became extinct during the last glacial period.

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