Woolly rhinos and many other large, shaggy prehistoric animals first evolved their cold tolerance in Tibet, which served as the evolutionary cradle for Ice Age mega plant-eaters, according to a new paper.
The study helps explain why so many different species roamed North America, Europe and Asia during the last Ice Age beginning about 2.8 million years ago. They had previously adapted to cold environments in the western Himalayas before later expanding to other regions.
Several were big with long hair.
"There is a general principle, called Bergmann's Rule, that suggests animals tend to increase their body size in colder environments," said Xiaoming Wang, co-author of the study which appeared in this week's Science. "Large-bodied animals have relatively smaller surface areas to lose heat and thus conserve heat better -- it's a matter of physics."
Wang and his colleagues identified Tibet as the mega herbivore cradle after discovering a new woolly rhino, Coelodonta thibetana, dating to 3.7 million years ago. As its name suggests, this animal was about as furry as a beast can be, and it had a head that functioned like a snow shovel.
"The extinct Tibetan woolly rhino had developed special adaptations for sweeping snow using its flattened, forward-leaning horn to reveal vegetation, a useful behavior for survival in the harsh Tibetan climate," Wang explained.
He and his team suspect that the rhino evolved from Stephanorhinus, a genus of Eurasian-wide distribution. C. thibetana then lived during the Pliocene (5.3 to 2.6 million years ago) at a time, Wang said, "when global climate was much warmer and the northern continents were free of the massive ice sheets seen in the Ice Age later. The Tibetan rhino likely was thus able to accustom itself to cold conditions in high elevations and become pre-adapted for the future Ice Age climate."
Tibet also gave rise to other cold-adapted animals. While woolly mammoths and woolly rhinos bit the proverbial dust some time ago, other species with Tibetan ancestry survived to modern times.
The Tibetan wild yak, for example, is a sister species to European and North American bison. During the Pleistocene, the Tibetan wild ass expanded its population to northern Pakistan and possibly even to Alaska. Snow leopards and blue sheep are two other examples.
"Like the woolly rhino, blue sheep descended down from high Tibet during the Ice Age, presumably for the same reason as the rhinos, but somehow the blue sheep survived to the present day in the high plateau," Wang said.
The Tibetan antelope is now confined to the highest elevations of Tibet and adapted "by evolving one of the finest under furs," often used by local people to weave highly prized shawls.
The Tibetan antelope and virtually all other modern, cold-hardy species are now under threat due to climate change and over-hunting. According to Wang, "The polar bear is a poster child example" of such animals that are presently at risk.
Animal experts largely agree that Tibet was indeed the birthplace for many species that later survived through the Ice Age and beyond.
"We know from today's species that they move up and down mountains in accordance with climate change," said Adrian Lister, a professor in the Department of Paleontology at The Natural History Museum in London. Lister added that "many are now moving upwards" in an attempt "to escape global warming. It seems perfectly reasonable that a similar thing could have happened over longer time scales in the past."
Anthony Barnosky, a professor in the Department of Integrative Biology at UC Berkeley and a curator at the university's Museum of Paleontology, said the study demonstrates "the importance of isolated areas as cradles of evolution."
Wang and his team are already planning future studies in Tibet, where they "have barely scratched the surface."
"Cold places such as Tibet, the Arctic and Antarctic are where the most unexpected discoveries will be made in the future," Wang predicts. "These are the remaining frontiers that are still largely unexplored."