These fossils, about six million years old, were discovered in Yunnan province in southwestern China. Among them is a particularly large wrist bone, called the radial sesamoid.

This is the oldest evidence for the existence of a “sixth finger” in the giant panda, which allows it to grasp and break thick bamboo stalks, the researchers pointed out in the latest edition of the Scientific Reports magazine.

This fossil belongs to a now extinct panda ancestor called Ailurarctos, who lived in China between six and eight million years ago.

“The giant panda is…a rare case of a large carnivore…turned herbivore,” said Wang Xiaoming, curator of vertebrate paleontology at the Los Angeles County Museum of Natural History.

“The false thumb of Ailurarctos shows (…) for the first time the chronology and probable stages of the evolution of bamboo feeding in pandas,” he added.

While the existence of the “false thumb” had already been known to researchers for about a century, the fossil evidence of this bone sheds light on several questions that have long remained unanswered, including how and when this extra finger, which does not exist in no other bears, evolved.

Millions of years ago, pandas swapped the omnivorous, protein-rich diet of their ancestors for the nutrient-poor, year-round bamboo in southern China.

They eat for up to 15 hours a day and an adult panda can consume 45 kg of bamboo daily. Although their diet is primarily vegetarian, giant pandas are also known to occasionally hunt small animals.