About 10,000 years ago, when the million-year-long Pleistocene epoch gave way to the ongoing Holocene epoch, much of the world's ecosystems changed. In what is now northern Siberia, vast numbers of large animals, among them mammoths, woolly rhinoceroses, and yaks, both thrived on and nurtured the steppes that, compared to other northern regions of the world, remained relatively unscathed from the repeated advances and retreats of ice sheets. Even so, the steppes there gave way to silt, dust, and ice-based tundra landscapes dominated in some places by forests and in others by mosses. The large animals disappeared. [Sergey Zimov][1], director of the Northeast Science Station in Cherskii in the Republic of Sakha (Yakutia), argues that climate change may not have been the primary reason behind the demise of this Pleistocene ecosystem. Instead, he says changing hunting practices wiped out the large animals whose absence led to the ecosystem shifts. He and his colleagues now are reintroducing bison, Yakutian horses, and other animals, eventually even tigers, in an attempt to reconstitute the Pleistocene ecosystem. The experiment will test the hypothesis that humans, rather than climate change, caused the ecosystem shift at the beginning of the Holocene. The stabilization of the northern tundra soils that this reconstitution could bring also could prevent the release of vast amount of carbon now sequestered in the Siberian soils but in danger of being released in the warmer times projected for the future. All essays appearing in this series can be found online at [www.sciencemag.org/sciext/globalvoices/][2] * * * [1]: http://www.sciencemag.org/cgi/content/full/308/5723/796 [2]: http://www.sciencemag.org/sciext/globalvoices/