Fossils dating back about 6 million years found in southwest China’s 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 the Scientific Reports.

The fossils belong to the now-extinct ancient relative of the panda called an Ailurarcto that lived in China 6 to 8 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 similar 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 base of the thumb evolved over time since it had to “bear the burden of considerable body weight”, the paper said.

Pandas traded the high-protein, omnivorous diet of their ancestors for bamboo, that is low in nutrients available year-round in South China millions of years ago.

They eat for up to 15 hours a day and an adult panda can consume 45kg of bamboo a day.

While their diet is mostly vegetarian, wild panda are known to occasionally hunt small animals.