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 based 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.
