For the last 2.5 million years, our planet has experienced cold and warm, millennia-long cycles that collectively have become known as the Ice Age. During cold periods, continental-scale ice sheets blanketed large tracts1 of the northern hemisphere. As the climate warmed up, these colossal2 glaciers3 receded4, leaving Yo百度竞价推广ite-like valleys and other majestic5 geologic6 features behind. The advance and retreat of the ice sheets also had a profound influence in the evolution and geographic7 distribution of many animals, including those that live today in the Arctic regions. A new study published in the Proceedings8 of the Royal Society B: Biological Sciences identifies a newly discovered 3- to 5-million-year-old Tibetan fox from the Himalayan Mountains, Vulpes qiuzhudingi, as the likely ancestor of the living Arctic fox , lending support to the idea that the evolution of present-day animals of the Arctic region is intimately connected to ancestors that first became adapted for life in cold regions in the high altitude environments of the Tibetan Plateau.The papers lead author is Xiaoming Wang, of the Natural History Museum of Los Angeles County . Co-authors are Zhijie Jack9 Tseng , Qiang Li , Gary T. Takeuchi , and Guangpu Xie . These writers, on a team with other geologists12 and paleontologists and led by Wang, uncovered the fossil specimens13 in the Zanda Basin in southern Tibet in 2010.In addition to the arctic fox, the team also uncovered extinct species of a wooly14 rhino15, three-toed horse, Tibetan bharal , chiru , snow leopard16, badger17, as well as 23 other mammals.The origin of the cold-adapted Pleistocene megafauna（巨型动物） has usually been sought either in the arctic tundra18（北极冰原） or in the cool steppes elsewhere. But the teams new fossil as百度竞价推广blage boosts an alternative scenario19, which the authors call the out of Tibet hypothesis. It argues that some of the Ice Age megafauna used ancient Tibet as a training ground for developing adaptations that allowed them to cope with the severe climatic conditions. These Tibetan ancestors were thus pre-adapted to cold climates during the Ice Age .Tibet, according to Wang, is a rich but grueling location for paleontological fieldwork. Fifteen summer seasons, and a good deal of luck, have honed his teams success. The expeditions involve a one-week journey to Lhasa, then a four-day drive into the remote layer cake sediments22 of the Zanda Basin -- a drive made in old model Land Cruisers known to get stuck in streams.At more than 14,000-foot elevation23, its difficult to breathe, water freezes overnight in camps, and the team members disband every morning to walk alone in search of fossils. Wang and his team have trained their eyes to search for ancient lake margins24, where the megafauna theyre interested in are often found. They alternate camp nights with nights in town, so they keep their strength up for a couple of weeks. There are a lot of challenges, Wang said, but in paleontological terms, it is a relatively25 unexplored environment. Our efforts are rewriting a significant chapter of our planets recent geological history.
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1 tracts 大片土地； 地带； （体内的）道； （尤指宣扬宗教、伦理或政治的）短文 参考例句： vast tracts of forest 大片大片的森林 There are tracts of desert in Australia. 澳大利亚有大片沙漠。
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