Ancient fossils found in China have helped researchers understand the enduring mystery of the panda’s fake thumb.
Modern giant pandas have a sixth finger-like figure on their wrists, which scientists say is key to their transition from omnivores to vegetarians who bite bamboo.
The bones of the hands of the modern giant panda. Illustration: Los Angeles County Museum of Natural History / Reuters
While the thin extra figure known as the radial sesamoid is not as flexible as the human thumb, it allows pandas to hold and crush bamboo stems into bite-sized pieces and feed their great appetite.
The fake panda’s thumb has been known for more than 100 years, but the almost complete lack of fossil evidence has left researchers puzzled as to when the figure evolved.
Writing in scientific papers, Xiaoming Wang, curator of vertebrate paleontology at the Los Angeles County Museum of Natural History, sheds light on the mystery. New fossils of an ancient panda found near the city of Jaotong in the northern Yunnan province of China not only carry the fake thumb, but suggest that it was once larger than those observed in pandas today.
The fossils, which are 6 to 7 million years old, belong to an extinct relative of a panda called Ailurarctos, and are thought to be the oldest known evidence of the unusual figure.
Denise Soo, an associate professor at Arizona State University and co-leader of the panda specimen restoration project, said modern pandas have had enough time to develop longer fake thumbs, but the evolutionary pressure of having to walk on their hands as and the bamboo on the handle had apparently kept them short and strong.
Evolving from a carnivorous ancestor to a bamboo feeder, the panda had to overcome many obstacles, Wang added. “The opposite ‘thumb’ of the wrist bone may be the most amazing development against these obstacles,” he said.
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Researchers discovered a bone in the hand of Ailurarctos in 2010 and discovered teeth and a false thumb in 2015. So far, the oldest known evidence of a thumb-like structure comes from fossils of modern panda species dating back about 100,000 years. In addition to being shorter than its predecessor, the fake thumb of the modern panda has a hook at the end that the authors say can help it catch bamboo.
While the panda’s diet is 99% vegetarian, they sometimes eat small animals. To meet their nutritional needs, pandas eat up to 14 hours a day, consuming nearly 40 kg of bamboo per day as adults.
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