WITH an average altitude of over 4000 metres, Tibet 鈥� the 鈥渞oof of the world鈥� 鈥� towers over its neighbours. So massive is the plateau that it alters weather patterns, bringing India its monsoons. But it was not always so. The region went through a growth spurt only 7 million years ago, much more recently than previously thought.
Yang Wang of Florida State University, Tallahassee, and her colleagues collected fossil horse and rhino teeth in 7-million-year-old sediments in the high Himalayas of southern Tibet. Carbon isotope analysis of the teeth showed that the animals dined on grasses containing carbon 4, which are found in warm climates (Geology, vol 34, p 309). 鈥淭his kind of grass can鈥檛 be found on today鈥檚 Tibetan plateau,鈥� Wang says. The only way this grass could have grown on the plateau was if it was at a lower altitude and therefore warmer.
Wang鈥檚 team calculates that the average altitude of this particular part of Tibet was between 2900 and 3400 metres at that time. The conventional view is that Tibet鈥檚 growth spurt happened over 8 million years ago, so the new findings may mean that the geological start date of the monsoon needs to be revised.
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