Researchers get a grip on the mystery of the panda’s false thumb

Ancient fossils discovered in China have helped researchers get a grip on the enduring mystery of the panda's false thumb.

Scientists believe that the transition from omnivores to bamboo-munching vegetarians was the result of a thumb-like sixth digit on their wrists.

While the stubby extra digit, known as a radial sesamoid, is not as versatile as the human thumb, it allows pandas to hold and crush bamboo stems into bite-size pieces and fuel their formidable appetite.

For more than 100 years, the false thumb of the panda has been known, but a lack of fossil evidence has left researchers puzzled as to when the digit evolved.

The new fossils of an ancient panda found near the city of Zhaotong in the northern Yunnan province of China suggest it was once larger than those seen in pandas today.

The fossils, which are 6 to 7 million years old, belong to an extinct panda relative called Ailurarctos and are believed to be the oldest known evidence for the unusual digit.

Denise Su, an associate professor at Arizona State University and co-leader of the project that recovered the panda specimens, said modern pandas had had enough time to evolve longer false thumbs, but the evolutionary pressure of needing to walk on their hands as well as handle bamboo had apparently kept them short and strong.

Wang said that the panda had to overcome a lot of obstacles when evolving from a carnivorous ancestor to a bamboo-feeder. He said that an opposable thumb from a wrist bone may be the most amazing development against these hurdles.

The researchers found an Ailurarctos arm bone in 2010 and discovered teeth and a false thumb in 2015. The oldest-known evidence of the thumb-like structure came from fossils of modern panda species that were dated to about 100,000 years ago. As well as being shorter than its ancestor, the modern panda s false thumb has a hook on the end that the authors believe might help it grasp bamboo.

The panda's diet is 99% vegetarian, but they occasionally eat small animals. Pandas eat for up to 14 hours a day, consuming nearly 40 kg of bamboo a day as adults.