The discovery of panda fossils in China has helped researchers solve the mystery of how the giant species developed a "false thumb" and became the only dedicated vegetarian in the bear family.

Fossils dating back about six million years found in Yunnan province included a greatly enlarged wrist bone called a radial sesamoid.

It is the oldest known evidence of the modern giant panda's false thumb that allows it to grip and break heavy bamboo stems, scientists wrote on a research paper published in the latest edition of Scientific Reports.

The fossils belong to a now-extinct ancient relative of the panda called an Ailurarcto that lived in China six to eight million years ago.

"The giant panda is... a rare case of a large carnivore with a short, carnivorous digestive tract... that has become a dedicated herbivore," Wang Xiaoming, curator of vertebrate palaeontology at the Natural History Museum of Los Angeles County, said.

"The false thumb in Ailurarctos shows... for the first time, the likely timing and steps in the evolution of bamboo feeding in pandas."

Researchers had known about the panda's false thumb, which works in a similar way to a human thumb, for about a century. But the lack of fossil evidence had left unanswered questions about how and when the extra digit – not seen in any other bear – evolved.

"While the giant panda's false thumb is not the most elegant or dexterous... even a small, protruding lump at the wrist can be a modest help in preventing bamboo from slipping off bent fingers," Wang wrote.

The fossils found near Zhaotong city, in the north of Yunnan, included a false thumb that was longer than that found in modern pandas, but without an inward hook on the end.

The hook and a fleshy pad around the based of the thumb evolved over time since it had to "bear the burden of considerable body weight", the paper said. (AFP)