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 devoted vegetarian in the bear family.
Fossils dating back about six million years, found in southwestern China’s Yunnan province, include a greatly enlarged wrist bone called a radial sesamoid.
It’s the oldest known evidence of the modern giant panda’s false thumb that allows it to grip and break heavy bamboo stems, scientists wrote in a research paper published in the latest edition of the Scientific Reports.
The fossils belong to the panda’s now-extinct ancient relative called an Ailura arcto, 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 devoted 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 likely timing and steps in the evolution of bamboo feeding in pandas.”
Researchers had known about the panda’s false thumb, which is 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 isn’t the most elegant or nimble…even a small, protruding lump at the wrist can be a modest helper to keep bamboo from slipping off bent fingers,” Wang wrote.
The fossils found near the town of Zhaotong in northern Yunnan contained a false thumb that was longer than those of modern pandas, but without an inward-facing hook at the end.
The hook and a fleshy pad around the base of the thumb evolved over time because it had to “carry the burden of considerable body weight,” according to the paper.
Pandas traded their ancestors’ high-protein, omnivorous diet for bamboo, which millions of years ago in southern China is low in year-round nutrients.
They eat up to 15 hours a day and an adult panda can consume 45 kg of bamboo per day. Although their diet is largely vegetarian, wild pandas are known to occasionally prey on small animals.
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