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The revelation of panda fossils in China has aided researchers in solving the mystery of how the giant species evolved a “false thumb” and became the bear family’s only dedicated vegetarian.
A radial sesamoid, a greatly enlarged wrist bone found in southwest China’s Yunnan province, dates back about six million years.
Scientists wrote in a research paper published in the newest issue of the Scientific Reports that it is the oldest evidence presented of the modern giant panda’s false thumb, which allows it to grip and break heavy bamboo stems.
The fossils belonged to an Ailurarcto, a now-extinct ancient relative of the panda that lived in China six to eight million years old.
“The giant panda is a rare case of a large carnivore with a short, carnivorous digestive tract… that has evolved into a dedicated herbivore,” said Wang Xiaoming, curator of tetrapods paleontology at the Natural History Museum of Los Angeles County.
“For the first time, the false thumb in Ailurarctos reveals… the likely timing and steps in the evolution of wood feeding in pandas.”
For nearly a century, scientists had known about the panda’s false thumb, which functions similarly to a human thumb. However, the lack of fossil evidence had left unresolved questions about how and when the extra digit, which had never been seen in another bear, evolved.
“While the giant panda’s false thumb isn’t the most graceful or dexterous… even a small, protruding lump at the forearm can be of modest assistance in preventing bamboo from slipping off bent fingers,” Wang wrote.
The fossils discovered near Zhaotong city in Yunnan’s northwestern region included a falsified thumb that was longer than that discovered in modern pandas but lacked an inward hook at the end.
According to the paper, the hook and a fleshy pad around the base of the thumb evolved over time because it had to “bear the burden of substantial body weight.”
Pandas traded their ancestors’ high-protein, omnivorous diet for bamboo, which is low in nutrients and available year-round in South China millions of years ago.
An adult panda can devour 45kg of bamboo per day and can eat for up to 15 hours per day. While wild pandas eat mostly vegetables, they are known to hunt small animals on occasion.
