The Case of the False Thumb: Giant Panda’s “Amazing” Feature Developed at Least Six Million Years Ago

Bamboo for food? It all starts with the wrist.

When does a thumb stop being a thumb? When a big panda's extended wrist bone is utilized to grip bamboo The panda's hand has never acquired a completely opposable thumb in its long evolutionary history. Instead, the radial sesamoid, a thumb-like finger, emerged from a wrist bone. Despite being bears, this remarkable adaption allows these bears to live completely on bamboo (members of the order Carnivora, or meat-eaters).

Scientists claim the finding of the oldest bamboo-eating ancestral panda with this "thumb" in a new study released today (June 30, 2022). Surprisingly, it outlives its modern offspring. Xiaoming Wang, Curator of Vertebrate Paleontology at the Natural History Museum of Los Angeles County, and colleagues performed the study.

While the famed false thumb of current giant pandas (Ailuropoda melanoleuca) has been recognized for more than a century, the evolution of this wrist bone remained unknown due to a near-complete lack of fossil records. A 6-7 million-year-old fossil false thumb from an ancestor giant panda, Ailurarctos, was discovered in the Shuitangba site in Zhaotong, Yunnan Province, China. It provides us with the first glimpse at the early use of this extra (sixth) digit, as well as the earliest indication of a bamboo diet in ancestral pandas, allowing us to better understand the development of this unique structure.

“Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries to quietly consuming bamboos, a plant plentiful in the subtropical forest but of low nutrient value,” explains Dr. Xiaoming Wang of the NHM Vertebrate Paleontology Department. “Tightly holding bamboo stems in order to crush them into bite sizes is perhaps the most crucial adaptation to consuming a prodigious quantity of bamboo.” 

How to Walk and Chew Bamboo at the Same Time

This revelation may also help to answer a long-standing panda mystery: why are their fake thumbs so underdeveloped? The fossil Wang and his colleagues uncovered revealed a longer false thumb with a straighter end than its modern successors' shorter, hooked digit. So, why did pandas' artificial thumbs stop developing to get longer?

“Panda’s false thumb must walk and ‘chew’,” Wang explains. “Such a dual function serves as the limit on how big this ‘thumb’ can become.”

Panda gripping vs walking (white bone is the false thumb). Credit: Courtesy of the Natural History Museum of L.A. County

Wang and his colleagues believe that the shorter fake thumbs of modern pandas represent an evolutionary compromise between the requirement to handle bamboo and the necessity to walk. A contemporary panda's hooked second thumb allows them to grasp bamboo while carrying their considerable weight to the next bamboo feast. After all, the "thumb" serves as the radial sesamoid, a bone in the animal's wrist.

“Five to six million years should be enough time for the panda to develop longer false thumbs, but it seems that the evolutionary pressure of needing to travel and bear its weight kept the ‘thumb’  short–strong enough to be useful without being big enough to get in the way,” says Denise Su, associate professor at the School of Biological Sciences.

“Evolving from a carnivorous ancestor and becoming a pure bamboo-feeder, pandas must overcome many obstacles,” Wang says. “An opposable ‘thumb’ from a wrist bone may be the most amazing development against these hurdles.”

Reference: “Earliest giant panda false thumb suggests conflicting demands for locomotion and feeding” by Xiaoming Wang, Denise F. Su, Nina G. Jablonski, Xueping Ji, Jay Kelley, Lawrence J. Flynn and Tao Deng, 30 June 2022, Scientific Reports.

DOI: 10.1038/s41598-022-13402-y

The authors of this article are affiliated with the Natural History Museum of Los Angeles County, Los Angeles, CA, USA; Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Arizona State University, Tempe, Arizona, USA; Pennsylvania State University, University Park, Pennsylvania, USA; Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; Yunnan Institute of Cultural Relics and Archaeology, Kunming, Yunnan, China; Harvard University, Cambridge, Massachusetts, USA.

Funding was provided by the U.S.A. National Science Foundation, Yunnan Natural Science Foundation, National Natural Science Foundation of China, the Governments of Zhaotong and Zhaoyang, Institute of Vertebrate Paleontology and Paleoanthropology.