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The discovery of an ancient giant panda skull has confirmed its bamboo diet dates back more than 2 million years and may have played a key part in its survival.

A Chinese-US research team reports its results today following studies on a fossil skull found in South China's Guangxi Zhuang Autonomous Region in 2001.

The six fossils unearthed in Jinyin Cave are dated between 2.4 and 2 million years ago, according to the report in Proceedings of the National Academy of Sciences, a prestigious US journal.

Jin Changzhu, of the Chinese Academy of Sciences (CAS) and lead author of the paper, said the smaller fossil skull indicates the giant pandas were about a third smaller than today's pandas.

Researchers knew the panda reached its maximum size about 500,000 years ago, when it peaked, and then gradually became smaller.

Jin, a paleontologist at the Institute of Vertebrate Paleontology and Paleonanthropology affiliated to the CAS, said the size variation was a basic rule of evolution.

"A species tends to grow bigger when it reaches the peak of its population, but becomes smaller when numbers decline," he said.

The dental remains of the skull, which is the oldest giant panda skull ever found, are similar to today's pandas, indicating the type of teeth that could munch mountains of bamboo. A panda can eat up to 40 kg of bamboo per day.

Paleoanthropologist Russell Ciochon, the US co-author at the University of Iowa, said the panda's focus on bamboo could have helped it survive all these years.

"Once an animal begins to rely on a prevalent and stable food source, such as bamboo, it tends to evolve a larger body size," he said. "As individuals of the evolving species grow bigger, they have a better chance not to be eaten by predators due to their larger body size."

(China Daily 06/19/2007 page1)