Ancient fossils discovered in China have helped researchers get a grip on the enduring mystery of the panda’s false thumb.
Modern giant pandas sport a thumb-like sixth digit on their wrists, which scientists believe was pivotal in their transition from omnivores to bamboo-munching vegetarians.
While the stubby extra digit, known as a radial sesamoid, is not as versatile as the human thumb, it enables pandas to hold and crush bamboo stems into bite-size pieces and fuel their formidable appetite.
The panda’s false thumb has been known for more than 100 years, but a near-total lack of fossil evidence has left researchers puzzled as to when the digit evolved.
Writing in Scientific Reports, Xiaoming Wang, curator of vertebrate palaeontology at the Natural History Museum of Los Angeles County, sheds light on the mystery. New fossils of an ancient panda discovered near the city of Zhaotong in the northern Yunnan province of China not only sport the false thumb, but 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 on the project that recovered the panda specimens, said modern pandas have 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.
In evolving from a carnivorous ancestor to a bamboo-feeder, the panda had to overcome plenty of obstacles, Wang added. “An opposable ‘thumb’ from a wrist bone may be the most amazing development against these hurdles,” he said.
The researchers found an Ailurarctos arm bone in 2010 and discovered teeth and the false thumb in 2015. Until now, the oldest-known evidence of the thumb-like structure came from fossils of modern panda species dated to about 100,000 years ago. As well as being shorter than its ancestor’s, the modern panda’s false thumb has a hook on the end, which the authors believe might help it to grasp bamboo.
While the panda’s diet is 99% vegetarian, they occasionally eat small animals. To meet their nutritional needs, pandas eat for up to 14 hours a day, consuming nearly 40kg of bamboo a day as adults.