Fossil find solves mystery of how pandas became vegetarian

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

About six-million-year-old fossils found in southwest China’s Yunnan province included a greatly enlarged wrist bone called a radial sesamoid.

It’s the oldest known evidence of the modern giant panda’s false thumb, which allows it to grasp and snap heavy bamboo stems, scientists wrote in a research paper published in the latest issue of Scientific Reports.

The fossils belong to a now-extinct ancient relative of the panda called Ailurarcto, which 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 paleontology at the Natural History Museum of Los Angeles County, said.

“The false thumb in Ailurarctos shows… for the first time the probable timing and steps in the evolution of bamboo feeding in pandas.”

The panda’s false thumb, which functions similarly to the human thumb, had been known to researchers for about a century. But the lack of fossil evidence left questions about how and when the extra finger — not seen in any other bear — evolved.

“Although the giant panda’s false thumb isn’t the most elegant or dexterous… even a small, protruding lump on the wrist can be a modest aid in preventing bamboo from slipping off bent fingers,” Wang wrote.

Fossils found near the northern Yunnan city of Zhaotong included a false thumb that was longer than those found in modern pandas but had no inward hook at the end.

The hook and a fleshy padding around the base of the thumb evolved over time as they had to “carry the burden of considerable body weight,” the paper said.

Millions of years ago, pandas traded their ancestors’ high-protein, omnivorous diet for bamboo, which is perennially poor in southern China.

They eat for up to 15 hours a day and an adult panda can consume 45kg of bamboo per day. While their diet is primarily vegetarian, wild pandas occasionally hunt small animals.